@settitle Bison @value{VERSION}
@setchapternewpage odd
-@iftex
@finalout
-@end iftex
@c SMALL BOOK version
@c This edition has been formatted so that you can format and print it in
@c the smallbook format.
@c @smallbook
-@c Set following if you have the new `shorttitlepage' command
-@c @clear shorttitlepage-enabled
-@c @set shorttitlepage-enabled
+@c Set following if you want to document %default-prec and %no-default-prec.
+@c This feature is experimental and may change in future Bison versions.
+@c @set defaultprec
-@c ISPELL CHECK: done, 14 Jan 1993 --bob
-
-@c Check COPYRIGHT dates. should be updated in the titlepage, ifinfo
-@c titlepage; should NOT be changed in the GPL. --mew
-
-@iftex
+@ifnotinfo
@syncodeindex fn cp
@syncodeindex vr cp
@syncodeindex tp cp
-@end iftex
+@end ifnotinfo
@ifinfo
@synindex fn cp
@synindex vr cp
@end ifinfo
@comment %**end of header
-@ifinfo
-@format
-START-INFO-DIR-ENTRY
-* bison: (bison). GNU Project parser generator (yacc replacement).
-END-INFO-DIR-ENTRY
-@end format
-@end ifinfo
-
-@ifinfo
-This file documents the Bison parser generator.
+@copying
-Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, 1999,
-2000, 2001
-Free Software Foundation, Inc.
+This manual (@value{UPDATED}) is for GNU Bison (version
+@value{VERSION}), the GNU parser generator.
-Permission is granted to make and distribute verbatim copies of
-this manual provided the copyright notice and this permission notice
-are preserved on all copies.
+Copyright @copyright{} 1988-1993, 1995, 1998-2012 Free Software
+Foundation, Inc.
-@ignore
-Permission is granted to process this file through Tex and print the
-results, provided the printed document carries copying permission
-notice identical to this one except for the removal of this paragraph
-(this paragraph not being relevant to the printed manual).
+@quotation
+Permission is granted to copy, distribute and/or modify this document
+under the terms of the GNU Free Documentation License,
+Version 1.3 or any later version published by the Free Software
+Foundation; with no Invariant Sections, with the Front-Cover texts
+being ``A GNU Manual,'' and with the Back-Cover Texts as in
+(a) below. A copy of the license is included in the section entitled
+``GNU Free Documentation License.''
+
+(a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
+modify this GNU manual. Buying copies from the FSF
+supports it in developing GNU and promoting software
+freedom.''
+@end quotation
+@end copying
-@end ignore
-Permission is granted to copy and distribute modified versions of this
-manual under the conditions for verbatim copying, provided also that the
-sections entitled ``GNU General Public License'' and ``Conditions for
-Using Bison'' are included exactly as in the original, and provided that
-the entire resulting derived work is distributed under the terms of a
-permission notice identical to this one.
-
-Permission is granted to copy and distribute translations of this manual
-into another language, under the above conditions for modified versions,
-except that the sections entitled ``GNU General Public License'',
-``Conditions for Using Bison'' and this permission notice may be
-included in translations approved by the Free Software Foundation
-instead of in the original English.
-@end ifinfo
+@dircategory Software development
+@direntry
+* bison: (bison). GNU parser generator (Yacc replacement).
+@end direntry
-@ifset shorttitlepage-enabled
-@shorttitlepage Bison
-@end ifset
@titlepage
@title Bison
-@subtitle The YACC-compatible Parser Generator
+@subtitle The Yacc-compatible Parser Generator
@subtitle @value{UPDATED}, Bison Version @value{VERSION}
@author by Charles Donnelly and Richard Stallman
@page
@vskip 0pt plus 1filll
-Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998,
-1999, 2000, 2001
-Free Software Foundation, Inc.
-
+@insertcopying
@sp 2
Published by the Free Software Foundation @*
-59 Temple Place, Suite 330 @*
-Boston, MA 02111-1307 USA @*
+51 Franklin Street, Fifth Floor @*
+Boston, MA 02110-1301 USA @*
Printed copies are available from the Free Software Foundation.@*
ISBN 1-882114-44-2
-
-Permission is granted to make and distribute verbatim copies of
-this manual provided the copyright notice and this permission notice
-are preserved on all copies.
-
-@ignore
-Permission is granted to process this file through TeX and print the
-results, provided the printed document carries copying permission
-notice identical to this one except for the removal of this paragraph
-(this paragraph not being relevant to the printed manual).
-
-@end ignore
-Permission is granted to copy and distribute modified versions of this
-manual under the conditions for verbatim copying, provided also that the
-sections entitled ``GNU General Public License'' and ``Conditions for
-Using Bison'' are included exactly as in the original, and provided that
-the entire resulting derived work is distributed under the terms of a
-permission notice identical to this one.
-
-Permission is granted to copy and distribute translations of this manual
-into another language, under the above conditions for modified versions,
-except that the sections entitled ``GNU General Public License'',
-``Conditions for Using Bison'' and this permission notice may be
-included in translations approved by the Free Software Foundation
-instead of in the original English.
@sp 2
Cover art by Etienne Suvasa.
@end titlepage
@ifnottex
@node Top
@top Bison
-
-This manual documents version @value{VERSION} of Bison, updated
-@value{UPDATED}.
+@insertcopying
@end ifnottex
@menu
* Introduction::
* Conditions::
-* Copying:: The GNU General Public License says
- how you can copy and share Bison
+* Copying:: The GNU General Public License says
+ how you can copy and share Bison.
Tutorial sections:
-* Concepts:: Basic concepts for understanding Bison.
-* Examples:: Three simple explained examples of using Bison.
+* Concepts:: Basic concepts for understanding Bison.
+* Examples:: Three simple explained examples of using Bison.
Reference sections:
-* Grammar File:: Writing Bison declarations and rules.
-* Interface:: C-language interface to the parser function @code{yyparse}.
-* Algorithm:: How the Bison parser works at run-time.
-* Error Recovery:: Writing rules for error recovery.
+* Grammar File:: Writing Bison declarations and rules.
+* Interface:: C-language interface to the parser function @code{yyparse}.
+* Algorithm:: How the Bison parser works at run-time.
+* Error Recovery:: Writing rules for error recovery.
* Context Dependency:: What to do if your language syntax is too
- messy for Bison to handle straightforwardly.
-* Debugging:: Debugging Bison parsers that parse wrong.
-* Invocation:: How to run Bison (to produce the parser source file).
-* Table of Symbols:: All the keywords of the Bison language are explained.
-* Glossary:: Basic concepts are explained.
-* Copying This Manual:: License for copying this manual.
-* Index:: Cross-references to the text.
-
-@detailmenu --- The Detailed Node Listing ---
+ messy for Bison to handle straightforwardly.
+* Debugging:: Understanding or debugging Bison parsers.
+* Invocation:: How to run Bison (to produce the parser implementation).
+* Other Languages:: Creating C++ and Java parsers.
+* FAQ:: Frequently Asked Questions
+* Table of Symbols:: All the keywords of the Bison language are explained.
+* Glossary:: Basic concepts are explained.
+* Copying This Manual:: License for copying this manual.
+* Bibliography:: Publications cited in this manual.
+* Index:: Cross-references to the text.
+
+@detailmenu
+ --- The Detailed Node Listing ---
The Concepts of Bison
-* Language and Grammar:: Languages and context-free grammars,
- as mathematical ideas.
-* Grammar in Bison:: How we represent grammars for Bison's sake.
-* Semantic Values:: Each token or syntactic grouping can have
- a semantic value (the value of an integer,
- the name of an identifier, etc.).
-* Semantic Actions:: Each rule can have an action containing C code.
-* Bison Parser:: What are Bison's input and output,
- how is the output used?
-* Stages:: Stages in writing and running Bison grammars.
-* Grammar Layout:: Overall structure of a Bison grammar file.
+* Language and Grammar:: Languages and context-free grammars,
+ as mathematical ideas.
+* Grammar in Bison:: How we represent grammars for Bison's sake.
+* Semantic Values:: Each token or syntactic grouping can have
+ a semantic value (the value of an integer,
+ the name of an identifier, etc.).
+* Semantic Actions:: Each rule can have an action containing C code.
+* GLR Parsers:: Writing parsers for general context-free languages.
+* Locations:: Overview of location tracking.
+* Bison Parser:: What are Bison's input and output,
+ how is the output used?
+* Stages:: Stages in writing and running Bison grammars.
+* Grammar Layout:: Overall structure of a Bison grammar file.
+
+Writing GLR Parsers
+
+* Simple GLR Parsers:: Using GLR parsers on unambiguous grammars.
+* Merging GLR Parses:: Using GLR parsers to resolve ambiguities.
+* GLR Semantic Actions:: Considerations for semantic values and deferred actions.
+* Semantic Predicates:: Controlling a parse with arbitrary computations.
+* Compiler Requirements:: GLR parsers require a modern C compiler.
Examples
-* RPN Calc:: Reverse polish notation calculator;
- a first example with no operator precedence.
-* Infix Calc:: Infix (algebraic) notation calculator.
- Operator precedence is introduced.
+* RPN Calc:: Reverse polish notation calculator;
+ a first example with no operator precedence.
+* Infix Calc:: Infix (algebraic) notation calculator.
+ Operator precedence is introduced.
* Simple Error Recovery:: Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$.
* Multi-function Calc:: Calculator with memory and trig functions.
- It uses multiple data-types for semantic values.
-* Exercises:: Ideas for improving the multi-function calculator.
+ It uses multiple data-types for semantic values.
+* Exercises:: Ideas for improving the multi-function calculator.
Reverse Polish Notation Calculator
-* Decls: Rpcalc Decls. Prologue (declarations) for rpcalc.
-* Rules: Rpcalc Rules. Grammar Rules for rpcalc, with explanation.
-* Lexer: Rpcalc Lexer. The lexical analyzer.
-* Main: Rpcalc Main. The controlling function.
-* Error: Rpcalc Error. The error reporting function.
-* Gen: Rpcalc Gen. Running Bison on the grammar file.
-* Comp: Rpcalc Compile. Run the C compiler on the output code.
+* Rpcalc Declarations:: Prologue (declarations) for rpcalc.
+* Rpcalc Rules:: Grammar Rules for rpcalc, with explanation.
+* Rpcalc Lexer:: The lexical analyzer.
+* Rpcalc Main:: The controlling function.
+* Rpcalc Error:: The error reporting function.
+* Rpcalc Generate:: Running Bison on the grammar file.
+* Rpcalc Compile:: Run the C compiler on the output code.
Grammar Rules for @code{rpcalc}
-* Rpcalc Input::
-* Rpcalc Line::
-* Rpcalc Expr::
+* Rpcalc Input:: Explanation of the @code{input} nonterminal
+* Rpcalc Line:: Explanation of the @code{line} nonterminal
+* Rpcalc Expr:: Explanation of the @code{expr} nonterminal
Location Tracking Calculator: @code{ltcalc}
-* Decls: Ltcalc Decls. Bison and C declarations for ltcalc.
-* Rules: Ltcalc Rules. Grammar rules for ltcalc, with explanations.
-* Lexer: Ltcalc Lexer. The lexical analyzer.
+* Ltcalc Declarations:: Bison and C declarations for ltcalc.
+* Ltcalc Rules:: Grammar rules for ltcalc, with explanations.
+* Ltcalc Lexer:: The lexical analyzer.
Multi-Function Calculator: @code{mfcalc}
-* Decl: Mfcalc Decl. Bison declarations for multi-function calculator.
-* Rules: Mfcalc Rules. Grammar rules for the calculator.
-* Symtab: Mfcalc Symtab. Symbol table management subroutines.
+* Mfcalc Declarations:: Bison declarations for multi-function calculator.
+* Mfcalc Rules:: Grammar rules for the calculator.
+* Mfcalc Symbol Table:: Symbol table management subroutines.
+* Mfcalc Lexer:: The lexical analyzer.
+* Mfcalc Main:: The controlling function.
Bison Grammar Files
-* Grammar Outline:: Overall layout of the grammar file.
-* Symbols:: Terminal and nonterminal symbols.
-* Rules:: How to write grammar rules.
-* Recursion:: Writing recursive rules.
-* Semantics:: Semantic values and actions.
-* Declarations:: All kinds of Bison declarations are described here.
-* Multiple Parsers:: Putting more than one Bison parser in one program.
+* Grammar Outline:: Overall layout of the grammar file.
+* Symbols:: Terminal and nonterminal symbols.
+* Rules:: How to write grammar rules.
+* Recursion:: Writing recursive rules.
+* Semantics:: Semantic values and actions.
+* Tracking Locations:: Locations and actions.
+* Named References:: Using named references in actions.
+* Declarations:: All kinds of Bison declarations are described here.
+* Multiple Parsers:: Putting more than one Bison parser in one program.
Outline of a Bison Grammar
-* Prologue:: Syntax and usage of the prologue (declarations section).
-* Bison Declarations:: Syntax and usage of the Bison declarations section.
-* Grammar Rules:: Syntax and usage of the grammar rules section.
-* Epilogue:: Syntax and usage of the epilogue (additional code section).
+* Prologue:: Syntax and usage of the prologue.
+* Prologue Alternatives:: Syntax and usage of alternatives to the prologue.
+* Bison Declarations:: Syntax and usage of the Bison declarations section.
+* Grammar Rules:: Syntax and usage of the grammar rules section.
+* Epilogue:: Syntax and usage of the epilogue.
Defining Language Semantics
This says when, why and how to use the exceptional
action in the middle of a rule.
+Tracking Locations
+
+* Location Type:: Specifying a data type for locations.
+* Actions and Locations:: Using locations in actions.
+* Location Default Action:: Defining a general way to compute locations.
+
Bison Declarations
+* Require Decl:: Requiring a Bison version.
* Token Decl:: Declaring terminal symbols.
* Precedence Decl:: Declaring terminals with precedence and associativity.
* Union Decl:: Declaring the set of all semantic value types.
* Type Decl:: Declaring the choice of type for a nonterminal symbol.
-* Expect Decl:: Suppressing warnings about shift/reduce conflicts.
+* Initial Action Decl:: Code run before parsing starts.
+* Destructor Decl:: Declaring how symbols are freed.
+* Expect Decl:: Suppressing warnings about parsing conflicts.
* Start Decl:: Specifying the start symbol.
* Pure Decl:: Requesting a reentrant parser.
+* Push Decl:: Requesting a push parser.
* Decl Summary:: Table of all Bison declarations.
+* %define Summary:: Defining variables to adjust Bison's behavior.
+* %code Summary:: Inserting code into the parser source.
Parser C-Language Interface
-* Parser Function:: How to call @code{yyparse} and what it returns.
-* Lexical:: You must supply a function @code{yylex}
- which reads tokens.
-* Error Reporting:: You must supply a function @code{yyerror}.
-* Action Features:: Special features for use in actions.
+* Parser Function:: How to call @code{yyparse} and what it returns.
+* Push Parser Function:: How to call @code{yypush_parse} and what it returns.
+* Pull Parser Function:: How to call @code{yypull_parse} and what it returns.
+* Parser Create Function:: How to call @code{yypstate_new} and what it returns.
+* Parser Delete Function:: How to call @code{yypstate_delete} and what it returns.
+* Lexical:: You must supply a function @code{yylex}
+ which reads tokens.
+* Error Reporting:: You must supply a function @code{yyerror}.
+* Action Features:: Special features for use in actions.
+* Internationalization:: How to let the parser speak in the user's
+ native language.
The Lexical Analyzer Function @code{yylex}
* Calling Convention:: How @code{yyparse} calls @code{yylex}.
-* Token Values:: How @code{yylex} must return the semantic value
- of the token it has read.
-* Token Positions:: How @code{yylex} must return the text position
- (line number, etc.) of the token, if the
- actions want that.
-* Pure Calling:: How the calling convention differs
- in a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}).
+* Token Values:: How @code{yylex} must return the semantic value
+ of the token it has read.
+* Token Locations:: How @code{yylex} must return the text location
+ (line number, etc.) of the token, if the
+ actions want that.
+* Pure Calling:: How the calling convention differs in a pure parser
+ (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}).
The Bison Parser Algorithm
-* Look-Ahead:: Parser looks one token ahead when deciding what to do.
+* Lookahead:: Parser looks one token ahead when deciding what to do.
* Shift/Reduce:: Conflicts: when either shifting or reduction is valid.
* Precedence:: Operator precedence works by resolving conflicts.
* Contextual Precedence:: When an operator's precedence depends on context.
* Parser States:: The parser is a finite-state-machine with stack.
* Reduce/Reduce:: When two rules are applicable in the same situation.
-* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
-* Stack Overflow:: What happens when stack gets full. How to avoid it.
+* Mysterious Conflicts:: Conflicts that look unjustified.
+* Tuning LR:: How to tune fundamental aspects of LR-based parsing.
+* Generalized LR Parsing:: Parsing arbitrary context-free grammars.
+* Memory Management:: What happens when memory is exhausted. How to avoid it.
Operator Precedence
* Why Precedence:: An example showing why precedence is needed.
-* Using Precedence:: How to specify precedence in Bison grammars.
+* Using Precedence:: How to specify precedence and associativity.
+* Precedence Only:: How to specify precedence only.
* Precedence Examples:: How these features are used in the previous example.
* How Precedence:: How they work.
+Tuning LR
+
+* LR Table Construction:: Choose a different construction algorithm.
+* Default Reductions:: Disable default reductions.
+* LAC:: Correct lookahead sets in the parser states.
+* Unreachable States:: Keep unreachable parser states for debugging.
+
Handling Context Dependencies
* Semantic Tokens:: Token parsing can depend on the semantic context.
* Tie-in Recovery:: Lexical tie-ins have implications for how
error recovery rules must be written.
+Debugging Your Parser
+
+* Understanding:: Understanding the structure of your parser.
+* Tracing:: Tracing the execution of your parser.
+
Invoking Bison
* Bison Options:: All the options described in detail,
- in alphabetical order by short options.
+ in alphabetical order by short options.
* Option Cross Key:: Alphabetical list of long options.
-* VMS Invocation:: Bison command syntax on VMS.
+* Yacc Library:: Yacc-compatible @code{yylex} and @code{main}.
+
+Parsers Written In Other Languages
+
+* C++ Parsers:: The interface to generate C++ parser classes
+* Java Parsers:: The interface to generate Java parser classes
+
+C++ Parsers
+
+* C++ Bison Interface:: Asking for C++ parser generation
+* C++ Semantic Values:: %union vs. C++
+* C++ Location Values:: The position and location classes
+* C++ Parser Interface:: Instantiating and running the parser
+* C++ Scanner Interface:: Exchanges between yylex and parse
+* A Complete C++ Example:: Demonstrating their use
+
+C++ Location Values
+
+* C++ position:: One point in the source file
+* C++ location:: Two points in the source file
+
+A Complete C++ Example
+
+* Calc++ --- C++ Calculator:: The specifications
+* Calc++ Parsing Driver:: An active parsing context
+* Calc++ Parser:: A parser class
+* Calc++ Scanner:: A pure C++ Flex scanner
+* Calc++ Top Level:: Conducting the band
+
+Java Parsers
+
+* Java Bison Interface:: Asking for Java parser generation
+* Java Semantic Values:: %type and %token vs. Java
+* Java Location Values:: The position and location classes
+* Java Parser Interface:: Instantiating and running the parser
+* Java Scanner Interface:: Specifying the scanner for the parser
+* Java Action Features:: Special features for use in actions
+* Java Differences:: Differences between C/C++ and Java Grammars
+* Java Declarations Summary:: List of Bison declarations used with Java
+
+Frequently Asked Questions
+
+* Memory Exhausted:: Breaking the Stack Limits
+* How Can I Reset the Parser:: @code{yyparse} Keeps some State
+* Strings are Destroyed:: @code{yylval} Loses Track of Strings
+* Implementing Gotos/Loops:: Control Flow in the Calculator
+* Multiple start-symbols:: Factoring closely related grammars
+* Secure? Conform?:: Is Bison POSIX safe?
+* I can't build Bison:: Troubleshooting
+* Where can I find help?:: Troubleshouting
+* Bug Reports:: Troublereporting
+* More Languages:: Parsers in C++, Java, and so on
+* Beta Testing:: Experimenting development versions
+* Mailing Lists:: Meeting other Bison users
Copying This Manual
-* GNU Free Documentation License:: License for copying this manual.
+* Copying This Manual:: License for copying this manual.
@end detailmenu
@end menu
@unnumbered Introduction
@cindex introduction
-@dfn{Bison} is a general-purpose parser generator that converts a
-grammar description for an LALR(1) context-free grammar into a C
-program to parse that grammar. Once you are proficient with Bison,
-you may use it to develop a wide range of language parsers, from those
-used in simple desk calculators to complex programming languages.
-
-Bison is upward compatible with Yacc: all properly-written Yacc grammars
-ought to work with Bison with no change. Anyone familiar with Yacc
-should be able to use Bison with little trouble. You need to be fluent in
-C programming in order to use Bison or to understand this manual.
-
-We begin with tutorial chapters that explain the basic concepts of using
-Bison and show three explained examples, each building on the last. If you
-don't know Bison or Yacc, start by reading these chapters. Reference
-chapters follow which describe specific aspects of Bison in detail.
-
-Bison was written primarily by Robert Corbett; Richard Stallman made it
-Yacc-compatible. Wilfred Hansen of Carnegie Mellon University added
-multi-character string literals and other features.
+@dfn{Bison} is a general-purpose parser generator that converts an
+annotated context-free grammar into a deterministic LR or generalized
+LR (GLR) parser employing LALR(1) parser tables. As an experimental
+feature, Bison can also generate IELR(1) or canonical LR(1) parser
+tables. Once you are proficient with Bison, you can use it to develop
+a wide range of language parsers, from those used in simple desk
+calculators to complex programming languages.
+
+Bison is upward compatible with Yacc: all properly-written Yacc
+grammars ought to work with Bison with no change. Anyone familiar
+with Yacc should be able to use Bison with little trouble. You need
+to be fluent in C or C++ programming in order to use Bison or to
+understand this manual. Java is also supported as an experimental
+feature.
+
+We begin with tutorial chapters that explain the basic concepts of
+using Bison and show three explained examples, each building on the
+last. If you don't know Bison or Yacc, start by reading these
+chapters. Reference chapters follow, which describe specific aspects
+of Bison in detail.
+
+Bison was written originally by Robert Corbett. Richard Stallman made
+it Yacc-compatible. Wilfred Hansen of Carnegie Mellon University
+added multi-character string literals and other features. Since then,
+Bison has grown more robust and evolved many other new features thanks
+to the hard work of a long list of volunteers. For details, see the
+@file{THANKS} and @file{ChangeLog} files included in the Bison
+distribution.
This edition corresponds to version @value{VERSION} of Bison.
@node Conditions
@unnumbered Conditions for Using Bison
-As of Bison version 1.24, we have changed the distribution terms for
-@code{yyparse} to permit using Bison's output in nonfree programs.
-Formerly, Bison parsers could be used only in programs that were free
-software.
+The distribution terms for Bison-generated parsers permit using the
+parsers in nonfree programs. Before Bison version 2.2, these extra
+permissions applied only when Bison was generating LALR(1)
+parsers in C@. And before Bison version 1.24, Bison-generated
+parsers could be used only in programs that were free software.
-The other GNU programming tools, such as the GNU C compiler, have never
+The other GNU programming tools, such as the GNU C
+compiler, have never
had such a requirement. They could always be used for nonfree
software. The reason Bison was different was not due to a special
policy decision; it resulted from applying the usual General Public
License to all of the Bison source code.
-The output of the Bison utility---the Bison parser file---contains a
-verbatim copy of a sizable piece of Bison, which is the code for the
-@code{yyparse} function. (The actions from your grammar are inserted
-into this function at one point, but the rest of the function is not
-changed.) When we applied the GPL terms to the code for @code{yyparse},
+The main output of the Bison utility---the Bison parser implementation
+file---contains a verbatim copy of a sizable piece of Bison, which is
+the code for the parser's implementation. (The actions from your
+grammar are inserted into this implementation at one point, but most
+of the rest of the implementation is not changed.) When we applied
+the GPL terms to the skeleton code for the parser's implementation,
the effect was to restrict the use of Bison output to free software.
We didn't change the terms because of sympathy for people who want to
practical conditions for using Bison match the practical conditions for
using the other GNU tools.
-@include gpl.texi
+This exception applies when Bison is generating code for a parser.
+You can tell whether the exception applies to a Bison output file by
+inspecting the file for text beginning with ``As a special
+exception@dots{}''. The text spells out the exact terms of the
+exception.
+
+@node Copying
+@unnumbered GNU GENERAL PUBLIC LICENSE
+@include gpl-3.0.texi
@node Concepts
@chapter The Concepts of Bison
use Bison or Yacc, we suggest you start by reading this chapter carefully.
@menu
-* Language and Grammar:: Languages and context-free grammars,
- as mathematical ideas.
-* Grammar in Bison:: How we represent grammars for Bison's sake.
-* Semantic Values:: Each token or syntactic grouping can have
- a semantic value (the value of an integer,
- the name of an identifier, etc.).
-* Semantic Actions:: Each rule can have an action containing C code.
-* Locations Overview:: Tracking Locations.
-* Bison Parser:: What are Bison's input and output,
- how is the output used?
-* Stages:: Stages in writing and running Bison grammars.
-* Grammar Layout:: Overall structure of a Bison grammar file.
+* Language and Grammar:: Languages and context-free grammars,
+ as mathematical ideas.
+* Grammar in Bison:: How we represent grammars for Bison's sake.
+* Semantic Values:: Each token or syntactic grouping can have
+ a semantic value (the value of an integer,
+ the name of an identifier, etc.).
+* Semantic Actions:: Each rule can have an action containing C code.
+* GLR Parsers:: Writing parsers for general context-free languages.
+* Locations:: Overview of location tracking.
+* Bison Parser:: What are Bison's input and output,
+ how is the output used?
+* Stages:: Stages in writing and running Bison grammars.
+* Grammar Layout:: Overall structure of a Bison grammar file.
@end menu
@node Language and Grammar
@cindex BNF
@cindex Backus-Naur form
The most common formal system for presenting such rules for humans to read
-is @dfn{Backus-Naur Form} or ``BNF'', which was developed in order to
-specify the language Algol 60. Any grammar expressed in BNF is a
-context-free grammar. The input to Bison is essentially machine-readable
-BNF.
-
-Not all context-free languages can be handled by Bison, only those
-that are LALR(1). In brief, this means that it must be possible to
-tell how to parse any portion of an input string with just a single
-token of look-ahead. Strictly speaking, that is a description of an
-LR(1) grammar, and LALR(1) involves additional restrictions that are
-hard to explain simply; but it is rare in actual practice to find an
-LR(1) grammar that fails to be LALR(1). @xref{Mystery Conflicts, ,
-Mysterious Reduce/Reduce Conflicts}, for more information on this.
+is @dfn{Backus-Naur Form} or ``BNF'', which was developed in
+order to specify the language Algol 60. Any grammar expressed in
+BNF is a context-free grammar. The input to Bison is
+essentially machine-readable BNF.
+
+@cindex LALR grammars
+@cindex IELR grammars
+@cindex LR grammars
+There are various important subclasses of context-free grammars. Although
+it can handle almost all context-free grammars, Bison is optimized for what
+are called LR(1) grammars. In brief, in these grammars, it must be possible
+to tell how to parse any portion of an input string with just a single token
+of lookahead. For historical reasons, Bison by default is limited by the
+additional restrictions of LALR(1), which is hard to explain simply.
+@xref{Mysterious Conflicts}, for more information on this. As an
+experimental feature, you can escape these additional restrictions by
+requesting IELR(1) or canonical LR(1) parser tables. @xref{LR Table
+Construction}, to learn how.
+
+@cindex GLR parsing
+@cindex generalized LR (GLR) parsing
+@cindex ambiguous grammars
+@cindex nondeterministic parsing
+
+Parsers for LR(1) grammars are @dfn{deterministic}, meaning
+roughly that the next grammar rule to apply at any point in the input is
+uniquely determined by the preceding input and a fixed, finite portion
+(called a @dfn{lookahead}) of the remaining input. A context-free
+grammar can be @dfn{ambiguous}, meaning that there are multiple ways to
+apply the grammar rules to get the same inputs. Even unambiguous
+grammars can be @dfn{nondeterministic}, meaning that no fixed
+lookahead always suffices to determine the next grammar rule to apply.
+With the proper declarations, Bison is also able to parse these more
+general context-free grammars, using a technique known as GLR
+parsing (for Generalized LR). Bison's GLR parsers
+are able to handle any context-free grammar for which the number of
+possible parses of any given string is finite.
@cindex symbols (abstract)
@cindex token
@cindex syntactic grouping
@cindex grouping, syntactic
-In the formal grammatical rules for a language, each kind of syntactic unit
-or grouping is named by a @dfn{symbol}. Those which are built by grouping
-smaller constructs according to grammatical rules are called
+In the formal grammatical rules for a language, each kind of syntactic
+unit or grouping is named by a @dfn{symbol}. Those which are built by
+grouping smaller constructs according to grammatical rules are called
@dfn{nonterminal symbols}; those which can't be subdivided are called
@dfn{terminal symbols} or @dfn{token types}. We call a piece of input
corresponding to a single terminal symbol a @dfn{token}, and a piece
-corresponding to a single nonterminal symbol a @dfn{grouping}.@refill
+corresponding to a single nonterminal symbol a @dfn{grouping}.
We can use the C language as an example of what symbols, terminal and
-nonterminal, mean. The tokens of C are identifiers, constants (numeric and
-string), and the various keywords, arithmetic operators and punctuation
-marks. So the terminal symbols of a grammar for C include `identifier',
-`number', `string', plus one symbol for each keyword, operator or
-punctuation mark: `if', `return', `const', `static', `int', `char',
-`plus-sign', `open-brace', `close-brace', `comma' and many more. (These
-tokens can be subdivided into characters, but that is a matter of
+nonterminal, mean. The tokens of C are identifiers, constants (numeric
+and string), and the various keywords, arithmetic operators and
+punctuation marks. So the terminal symbols of a grammar for C include
+`identifier', `number', `string', plus one symbol for each keyword,
+operator or punctuation mark: `if', `return', `const', `static', `int',
+`char', `plus-sign', `open-brace', `close-brace', `comma' and many more.
+(These tokens can be subdivided into characters, but that is a matter of
lexicography, not grammar.)
Here is a simple C function subdivided into tokens:
@example
int /* @r{keyword `int'} */
-square (x) /* @r{identifier, open-paren,} */
- /* @r{identifier, close-paren} */
- int x; /* @r{keyword `int', identifier, semicolon} */
+square (int x) /* @r{identifier, open-paren, keyword `int',}
+ @r{identifier, close-paren} */
@{ /* @r{open-brace} */
- return x * x; /* @r{keyword `return', identifier,} */
- /* @r{asterisk, identifier, semicolon} */
+ return x * x; /* @r{keyword `return', identifier, asterisk,}
+ @r{identifier, semicolon} */
@} /* @r{close-brace} */
@end example
a @dfn{Bison grammar} file. @xref{Grammar File, ,Bison Grammar Files}.
A nonterminal symbol in the formal grammar is represented in Bison input
-as an identifier, like an identifier in C. By convention, it should be
+as an identifier, like an identifier in C@. By convention, it should be
in lower case, such as @code{expr}, @code{stmt} or @code{declaration}.
The Bison representation for a terminal symbol is also called a @dfn{token
used in every rule.
@example
-stmt: RETURN expr ';'
- ;
+stmt: RETURN expr ';' ;
@end example
@noindent
@emph{any} integer constant is grammatically valid in that position. The
precise value of the constant is irrelevant to how to parse the input: if
@samp{x+4} is grammatical then @samp{x+1} or @samp{x+3989} is equally
-grammatical.@refill
+grammatical.
But the precise value is very important for what the input means once it is
parsed. A compiler is useless if it fails to distinguish between 4, 1 and
3989 as constants in the program! Therefore, each token in a Bison grammar
-has both a token type and a @dfn{semantic value}. @xref{Semantics, ,Defining Language Semantics},
+has both a token type and a @dfn{semantic value}. @xref{Semantics,
+,Defining Language Semantics},
for details.
The token type is a terminal symbol defined in the grammar, such as
@code{INTEGER}, @code{IDENTIFIER} or @code{','}. It tells everything
you need to know to decide where the token may validly appear and how to
group it with other tokens. The grammar rules know nothing about tokens
-except their types.@refill
+except their types.
The semantic value has all the rest of the information about the
meaning of the token, such as the value of an integer, or the name of an
two subexpressions:
@example
-expr: expr '+' expr @{ $$ = $1 + $3; @}
- ;
+expr: expr '+' expr @{ $$ = $1 + $3; @} ;
@end example
@noindent
The action says how to produce the semantic value of the sum expression
from the values of the two subexpressions.
-@node Locations Overview
+@node GLR Parsers
+@section Writing GLR Parsers
+@cindex GLR parsing
+@cindex generalized LR (GLR) parsing
+@findex %glr-parser
+@cindex conflicts
+@cindex shift/reduce conflicts
+@cindex reduce/reduce conflicts
+
+In some grammars, Bison's deterministic
+LR(1) parsing algorithm cannot decide whether to apply a
+certain grammar rule at a given point. That is, it may not be able to
+decide (on the basis of the input read so far) which of two possible
+reductions (applications of a grammar rule) applies, or whether to apply
+a reduction or read more of the input and apply a reduction later in the
+input. These are known respectively as @dfn{reduce/reduce} conflicts
+(@pxref{Reduce/Reduce}), and @dfn{shift/reduce} conflicts
+(@pxref{Shift/Reduce}).
+
+To use a grammar that is not easily modified to be LR(1), a
+more general parsing algorithm is sometimes necessary. If you include
+@code{%glr-parser} among the Bison declarations in your file
+(@pxref{Grammar Outline}), the result is a Generalized LR
+(GLR) parser. These parsers handle Bison grammars that
+contain no unresolved conflicts (i.e., after applying precedence
+declarations) identically to deterministic parsers. However, when
+faced with unresolved shift/reduce and reduce/reduce conflicts,
+GLR parsers use the simple expedient of doing both,
+effectively cloning the parser to follow both possibilities. Each of
+the resulting parsers can again split, so that at any given time, there
+can be any number of possible parses being explored. The parsers
+proceed in lockstep; that is, all of them consume (shift) a given input
+symbol before any of them proceed to the next. Each of the cloned
+parsers eventually meets one of two possible fates: either it runs into
+a parsing error, in which case it simply vanishes, or it merges with
+another parser, because the two of them have reduced the input to an
+identical set of symbols.
+
+During the time that there are multiple parsers, semantic actions are
+recorded, but not performed. When a parser disappears, its recorded
+semantic actions disappear as well, and are never performed. When a
+reduction makes two parsers identical, causing them to merge, Bison
+records both sets of semantic actions. Whenever the last two parsers
+merge, reverting to the single-parser case, Bison resolves all the
+outstanding actions either by precedences given to the grammar rules
+involved, or by performing both actions, and then calling a designated
+user-defined function on the resulting values to produce an arbitrary
+merged result.
+
+@menu
+* Simple GLR Parsers:: Using GLR parsers on unambiguous grammars.
+* Merging GLR Parses:: Using GLR parsers to resolve ambiguities.
+* GLR Semantic Actions:: Considerations for semantic values and deferred actions.
+* Semantic Predicates:: Controlling a parse with arbitrary computations.
+* Compiler Requirements:: GLR parsers require a modern C compiler.
+@end menu
+
+@node Simple GLR Parsers
+@subsection Using GLR on Unambiguous Grammars
+@cindex GLR parsing, unambiguous grammars
+@cindex generalized LR (GLR) parsing, unambiguous grammars
+@findex %glr-parser
+@findex %expect-rr
+@cindex conflicts
+@cindex reduce/reduce conflicts
+@cindex shift/reduce conflicts
+
+In the simplest cases, you can use the GLR algorithm
+to parse grammars that are unambiguous but fail to be LR(1).
+Such grammars typically require more than one symbol of lookahead.
+
+Consider a problem that
+arises in the declaration of enumerated and subrange types in the
+programming language Pascal. Here are some examples:
+
+@example
+type subrange = lo .. hi;
+type enum = (a, b, c);
+@end example
+
+@noindent
+The original language standard allows only numeric
+literals and constant identifiers for the subrange bounds (@samp{lo}
+and @samp{hi}), but Extended Pascal (ISO/IEC
+10206) and many other
+Pascal implementations allow arbitrary expressions there. This gives
+rise to the following situation, containing a superfluous pair of
+parentheses:
+
+@example
+type subrange = (a) .. b;
+@end example
+
+@noindent
+Compare this to the following declaration of an enumerated
+type with only one value:
+
+@example
+type enum = (a);
+@end example
+
+@noindent
+(These declarations are contrived, but they are syntactically
+valid, and more-complicated cases can come up in practical programs.)
+
+These two declarations look identical until the @samp{..} token.
+With normal LR(1) one-token lookahead it is not
+possible to decide between the two forms when the identifier
+@samp{a} is parsed. It is, however, desirable
+for a parser to decide this, since in the latter case
+@samp{a} must become a new identifier to represent the enumeration
+value, while in the former case @samp{a} must be evaluated with its
+current meaning, which may be a constant or even a function call.
+
+You could parse @samp{(a)} as an ``unspecified identifier in parentheses'',
+to be resolved later, but this typically requires substantial
+contortions in both semantic actions and large parts of the
+grammar, where the parentheses are nested in the recursive rules for
+expressions.
+
+You might think of using the lexer to distinguish between the two
+forms by returning different tokens for currently defined and
+undefined identifiers. But if these declarations occur in a local
+scope, and @samp{a} is defined in an outer scope, then both forms
+are possible---either locally redefining @samp{a}, or using the
+value of @samp{a} from the outer scope. So this approach cannot
+work.
+
+A simple solution to this problem is to declare the parser to
+use the GLR algorithm.
+When the GLR parser reaches the critical state, it
+merely splits into two branches and pursues both syntax rules
+simultaneously. Sooner or later, one of them runs into a parsing
+error. If there is a @samp{..} token before the next
+@samp{;}, the rule for enumerated types fails since it cannot
+accept @samp{..} anywhere; otherwise, the subrange type rule
+fails since it requires a @samp{..} token. So one of the branches
+fails silently, and the other one continues normally, performing
+all the intermediate actions that were postponed during the split.
+
+If the input is syntactically incorrect, both branches fail and the parser
+reports a syntax error as usual.
+
+The effect of all this is that the parser seems to ``guess'' the
+correct branch to take, or in other words, it seems to use more
+lookahead than the underlying LR(1) algorithm actually allows
+for. In this example, LR(2) would suffice, but also some cases
+that are not LR(@math{k}) for any @math{k} can be handled this way.
+
+In general, a GLR parser can take quadratic or cubic worst-case time,
+and the current Bison parser even takes exponential time and space
+for some grammars. In practice, this rarely happens, and for many
+grammars it is possible to prove that it cannot happen.
+The present example contains only one conflict between two
+rules, and the type-declaration context containing the conflict
+cannot be nested. So the number of
+branches that can exist at any time is limited by the constant 2,
+and the parsing time is still linear.
+
+Here is a Bison grammar corresponding to the example above. It
+parses a vastly simplified form of Pascal type declarations.
+
+@example
+%token TYPE DOTDOT ID
+
+@group
+%left '+' '-'
+%left '*' '/'
+@end group
+
+%%
+
+@group
+type_decl: TYPE ID '=' type ';' ;
+@end group
+
+@group
+type:
+ '(' id_list ')'
+| expr DOTDOT expr
+;
+@end group
+
+@group
+id_list:
+ ID
+| id_list ',' ID
+;
+@end group
+
+@group
+expr:
+ '(' expr ')'
+| expr '+' expr
+| expr '-' expr
+| expr '*' expr
+| expr '/' expr
+| ID
+;
+@end group
+@end example
+
+When used as a normal LR(1) grammar, Bison correctly complains
+about one reduce/reduce conflict. In the conflicting situation the
+parser chooses one of the alternatives, arbitrarily the one
+declared first. Therefore the following correct input is not
+recognized:
+
+@example
+type t = (a) .. b;
+@end example
+
+The parser can be turned into a GLR parser, while also telling Bison
+to be silent about the one known reduce/reduce conflict, by adding
+these two declarations to the Bison grammar file (before the first
+@samp{%%}):
+
+@example
+%glr-parser
+%expect-rr 1
+@end example
+
+@noindent
+No change in the grammar itself is required. Now the
+parser recognizes all valid declarations, according to the
+limited syntax above, transparently. In fact, the user does not even
+notice when the parser splits.
+
+So here we have a case where we can use the benefits of GLR,
+almost without disadvantages. Even in simple cases like this, however,
+there are at least two potential problems to beware. First, always
+analyze the conflicts reported by Bison to make sure that GLR
+splitting is only done where it is intended. A GLR parser
+splitting inadvertently may cause problems less obvious than an
+LR parser statically choosing the wrong alternative in a
+conflict. Second, consider interactions with the lexer (@pxref{Semantic
+Tokens}) with great care. Since a split parser consumes tokens without
+performing any actions during the split, the lexer cannot obtain
+information via parser actions. Some cases of lexer interactions can be
+eliminated by using GLR to shift the complications from the
+lexer to the parser. You must check the remaining cases for
+correctness.
+
+In our example, it would be safe for the lexer to return tokens based on
+their current meanings in some symbol table, because no new symbols are
+defined in the middle of a type declaration. Though it is possible for
+a parser to define the enumeration constants as they are parsed, before
+the type declaration is completed, it actually makes no difference since
+they cannot be used within the same enumerated type declaration.
+
+@node Merging GLR Parses
+@subsection Using GLR to Resolve Ambiguities
+@cindex GLR parsing, ambiguous grammars
+@cindex generalized LR (GLR) parsing, ambiguous grammars
+@findex %dprec
+@findex %merge
+@cindex conflicts
+@cindex reduce/reduce conflicts
+
+Let's consider an example, vastly simplified from a C++ grammar.
+
+@example
+%@{
+ #include <stdio.h>
+ #define YYSTYPE char const *
+ int yylex (void);
+ void yyerror (char const *);
+%@}
+
+%token TYPENAME ID
+
+%right '='
+%left '+'
+
+%glr-parser
+
+%%
+
+prog:
+ /* Nothing. */
+| prog stmt @{ printf ("\n"); @}
+;
+
+stmt:
+ expr ';' %dprec 1
+| decl %dprec 2
+;
+
+expr:
+ ID @{ printf ("%s ", $$); @}
+| TYPENAME '(' expr ')'
+ @{ printf ("%s <cast> ", $1); @}
+| expr '+' expr @{ printf ("+ "); @}
+| expr '=' expr @{ printf ("= "); @}
+;
+
+decl:
+ TYPENAME declarator ';'
+ @{ printf ("%s <declare> ", $1); @}
+| TYPENAME declarator '=' expr ';'
+ @{ printf ("%s <init-declare> ", $1); @}
+;
+
+declarator:
+ ID @{ printf ("\"%s\" ", $1); @}
+| '(' declarator ')'
+;
+@end example
+
+@noindent
+This models a problematic part of the C++ grammar---the ambiguity between
+certain declarations and statements. For example,
+
+@example
+T (x) = y+z;
+@end example
+
+@noindent
+parses as either an @code{expr} or a @code{stmt}
+(assuming that @samp{T} is recognized as a @code{TYPENAME} and
+@samp{x} as an @code{ID}).
+Bison detects this as a reduce/reduce conflict between the rules
+@code{expr : ID} and @code{declarator : ID}, which it cannot resolve at the
+time it encounters @code{x} in the example above. Since this is a
+GLR parser, it therefore splits the problem into two parses, one for
+each choice of resolving the reduce/reduce conflict.
+Unlike the example from the previous section (@pxref{Simple GLR Parsers}),
+however, neither of these parses ``dies,'' because the grammar as it stands is
+ambiguous. One of the parsers eventually reduces @code{stmt : expr ';'} and
+the other reduces @code{stmt : decl}, after which both parsers are in an
+identical state: they've seen @samp{prog stmt} and have the same unprocessed
+input remaining. We say that these parses have @dfn{merged.}
+
+At this point, the GLR parser requires a specification in the
+grammar of how to choose between the competing parses.
+In the example above, the two @code{%dprec}
+declarations specify that Bison is to give precedence
+to the parse that interprets the example as a
+@code{decl}, which implies that @code{x} is a declarator.
+The parser therefore prints
+
+@example
+"x" y z + T <init-declare>
+@end example
+
+The @code{%dprec} declarations only come into play when more than one
+parse survives. Consider a different input string for this parser:
+
+@example
+T (x) + y;
+@end example
+
+@noindent
+This is another example of using GLR to parse an unambiguous
+construct, as shown in the previous section (@pxref{Simple GLR Parsers}).
+Here, there is no ambiguity (this cannot be parsed as a declaration).
+However, at the time the Bison parser encounters @code{x}, it does not
+have enough information to resolve the reduce/reduce conflict (again,
+between @code{x} as an @code{expr} or a @code{declarator}). In this
+case, no precedence declaration is used. Again, the parser splits
+into two, one assuming that @code{x} is an @code{expr}, and the other
+assuming @code{x} is a @code{declarator}. The second of these parsers
+then vanishes when it sees @code{+}, and the parser prints
+
+@example
+x T <cast> y +
+@end example
+
+Suppose that instead of resolving the ambiguity, you wanted to see all
+the possibilities. For this purpose, you must merge the semantic
+actions of the two possible parsers, rather than choosing one over the
+other. To do so, you could change the declaration of @code{stmt} as
+follows:
+
+@example
+stmt:
+ expr ';' %merge <stmtMerge>
+| decl %merge <stmtMerge>
+;
+@end example
+
+@noindent
+and define the @code{stmtMerge} function as:
+
+@example
+static YYSTYPE
+stmtMerge (YYSTYPE x0, YYSTYPE x1)
+@{
+ printf ("<OR> ");
+ return "";
+@}
+@end example
+
+@noindent
+with an accompanying forward declaration
+in the C declarations at the beginning of the file:
+
+@example
+%@{
+ #define YYSTYPE char const *
+ static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1);
+%@}
+@end example
+
+@noindent
+With these declarations, the resulting parser parses the first example
+as both an @code{expr} and a @code{decl}, and prints
+
+@example
+"x" y z + T <init-declare> x T <cast> y z + = <OR>
+@end example
+
+Bison requires that all of the
+productions that participate in any particular merge have identical
+@samp{%merge} clauses. Otherwise, the ambiguity would be unresolvable,
+and the parser will report an error during any parse that results in
+the offending merge.
+
+@node GLR Semantic Actions
+@subsection GLR Semantic Actions
+
+The nature of GLR parsing and the structure of the generated
+parsers give rise to certain restrictions on semantic values and actions.
+
+@subsubsection Deferred semantic actions
+@cindex deferred semantic actions
+By definition, a deferred semantic action is not performed at the same time as
+the associated reduction.
+This raises caveats for several Bison features you might use in a semantic
+action in a GLR parser.
+
+@vindex yychar
+@cindex GLR parsers and @code{yychar}
+@vindex yylval
+@cindex GLR parsers and @code{yylval}
+@vindex yylloc
+@cindex GLR parsers and @code{yylloc}
+In any semantic action, you can examine @code{yychar} to determine the type of
+the lookahead token present at the time of the associated reduction.
+After checking that @code{yychar} is not set to @code{YYEMPTY} or @code{YYEOF},
+you can then examine @code{yylval} and @code{yylloc} to determine the
+lookahead token's semantic value and location, if any.
+In a nondeferred semantic action, you can also modify any of these variables to
+influence syntax analysis.
+@xref{Lookahead, ,Lookahead Tokens}.
+
+@findex yyclearin
+@cindex GLR parsers and @code{yyclearin}
+In a deferred semantic action, it's too late to influence syntax analysis.
+In this case, @code{yychar}, @code{yylval}, and @code{yylloc} are set to
+shallow copies of the values they had at the time of the associated reduction.
+For this reason alone, modifying them is dangerous.
+Moreover, the result of modifying them is undefined and subject to change with
+future versions of Bison.
+For example, if a semantic action might be deferred, you should never write it
+to invoke @code{yyclearin} (@pxref{Action Features}) or to attempt to free
+memory referenced by @code{yylval}.
+
+@subsubsection YYERROR
+@findex YYERROR
+@cindex GLR parsers and @code{YYERROR}
+Another Bison feature requiring special consideration is @code{YYERROR}
+(@pxref{Action Features}), which you can invoke in a semantic action to
+initiate error recovery.
+During deterministic GLR operation, the effect of @code{YYERROR} is
+the same as its effect in a deterministic parser.
+The effect in a deferred action is similar, but the precise point of the
+error is undefined; instead, the parser reverts to deterministic operation,
+selecting an unspecified stack on which to continue with a syntax error.
+In a semantic predicate (see @ref{Semantic Predicates}) during nondeterministic
+parsing, @code{YYERROR} silently prunes
+the parse that invoked the test.
+
+@subsubsection Restrictions on semantic values and locations
+GLR parsers require that you use POD (Plain Old Data) types for
+semantic values and location types when using the generated parsers as
+C++ code.
+
+@node Semantic Predicates
+@subsection Controlling a Parse with Arbitrary Predicates
+@findex %?
+@cindex Semantic predicates in GLR parsers
+
+In addition to the @code{%dprec} and @code{%merge} directives,
+GLR parsers
+allow you to reject parses on the basis of arbitrary computations executed
+in user code, without having Bison treat this rejection as an error
+if there are alternative parses. (This feature is experimental and may
+evolve. We welcome user feedback.) For example,
+
+@example
+widget:
+ %?@{ new_syntax @} "widget" id new_args @{ $$ = f($3, $4); @}
+| %?@{ !new_syntax @} "widget" id old_args @{ $$ = f($3, $4); @}
+;
+@end example
+
+@noindent
+is one way to allow the same parser to handle two different syntaxes for
+widgets. The clause preceded by @code{%?} is treated like an ordinary
+action, except that its text is treated as an expression and is always
+evaluated immediately (even when in nondeterministic mode). If the
+expression yields 0 (false), the clause is treated as a syntax error,
+which, in a nondeterministic parser, causes the stack in which it is reduced
+to die. In a deterministic parser, it acts like YYERROR.
+
+As the example shows, predicates otherwise look like semantic actions, and
+therefore you must be take them into account when determining the numbers
+to use for denoting the semantic values of right-hand side symbols.
+Predicate actions, however, have no defined value, and may not be given
+labels.
+
+There is a subtle difference between semantic predicates and ordinary
+actions in nondeterministic mode, since the latter are deferred.
+For example, we could try to rewrite the previous example as
+
+@example
+widget:
+ @{ if (!new_syntax) YYERROR; @}
+ "widget" id new_args @{ $$ = f($3, $4); @}
+| @{ if (new_syntax) YYERROR; @}
+ "widget" id old_args @{ $$ = f($3, $4); @}
+;
+@end example
+
+@noindent
+(reversing the sense of the predicate tests to cause an error when they are
+false). However, this
+does @emph{not} have the same effect if @code{new_args} and @code{old_args}
+have overlapping syntax.
+Since the mid-rule actions testing @code{new_syntax} are deferred,
+a GLR parser first encounters the unresolved ambiguous reduction
+for cases where @code{new_args} and @code{old_args} recognize the same string
+@emph{before} performing the tests of @code{new_syntax}. It therefore
+reports an error.
+
+Finally, be careful in writing predicates: deferred actions have not been
+evaluated, so that using them in a predicate will have undefined effects.
+
+@node Compiler Requirements
+@subsection Considerations when Compiling GLR Parsers
+@cindex @code{inline}
+@cindex GLR parsers and @code{inline}
+
+The GLR parsers require a compiler for ISO C89 or
+later. In addition, they use the @code{inline} keyword, which is not
+C89, but is C99 and is a common extension in pre-C99 compilers. It is
+up to the user of these parsers to handle
+portability issues. For instance, if using Autoconf and the Autoconf
+macro @code{AC_C_INLINE}, a mere
+
+@example
+%@{
+ #include <config.h>
+%@}
+@end example
+
+@noindent
+will suffice. Otherwise, we suggest
+
+@example
+%@{
+ #if (__STDC_VERSION__ < 199901 && ! defined __GNUC__ \
+ && ! defined inline)
+ # define inline
+ #endif
+%@}
+@end example
+
+@node Locations
@section Locations
@cindex location
-@cindex textual position
-@cindex position, textual
+@cindex textual location
+@cindex location, textual
Many applications, like interpreters or compilers, have to produce verbose
-and useful error messages. To achieve this, one must be able to keep track of
-the @dfn{textual position}, or @dfn{location}, of each syntactic construct.
+and useful error messages. To achieve this, one must be able to keep track of
+the @dfn{textual location}, or @dfn{location}, of each syntactic construct.
Bison provides a mechanism for handling these locations.
-Each token has a semantic value. In a similar fashion, each token has an
-associated location, but the type of locations is the same for all tokens and
-groupings. Moreover, the output parser is equipped with a default data
-structure for storing locations (@pxref{Locations}, for more details).
+Each token has a semantic value. In a similar fashion, each token has an
+associated location, but the type of locations is the same for all tokens
+and groupings. Moreover, the output parser is equipped with a default data
+structure for storing locations (@pxref{Tracking Locations}, for more
+details).
Like semantic values, locations can be reached in actions using a dedicated
-set of constructs. In the example above, the location of the whole grouping
+set of constructs. In the example above, the location of the whole grouping
is @code{@@$}, while the locations of the subexpressions are @code{@@1} and
@code{@@3}.
When a rule is matched, a default action is used to compute the semantic value
-of its left hand side (@pxref{Actions}). In the same way, another default
-action is used for locations. However, the action for locations is general
+of its left hand side (@pxref{Actions}). In the same way, another default
+action is used for locations. However, the action for locations is general
enough for most cases, meaning there is usually no need to describe for each
-rule how @code{@@$} should be formed. When building a new location for a given
+rule how @code{@@$} should be formed. When building a new location for a given
grouping, the default behavior of the output parser is to take the beginning
of the first symbol, and the end of the last symbol.
@node Bison Parser
-@section Bison Output: the Parser File
+@section Bison Output: the Parser Implementation File
@cindex Bison parser
@cindex Bison utility
@cindex lexical analyzer, purpose
@cindex parser
-When you run Bison, you give it a Bison grammar file as input. The output
-is a C source file that parses the language described by the grammar.
-This file is called a @dfn{Bison parser}. Keep in mind that the Bison
-utility and the Bison parser are two distinct programs: the Bison utility
-is a program whose output is the Bison parser that becomes part of your
-program.
+When you run Bison, you give it a Bison grammar file as input. The
+most important output is a C source file that implements a parser for
+the language described by the grammar. This parser is called a
+@dfn{Bison parser}, and this file is called a @dfn{Bison parser
+implementation file}. Keep in mind that the Bison utility and the
+Bison parser are two distinct programs: the Bison utility is a program
+whose output is the Bison parser implementation file that becomes part
+of your program.
The job of the Bison parser is to group tokens into groupings according to
the grammar rules---for example, to build identifiers and operators into
expressions. As it does this, it runs the actions for the grammar rules it
uses.
-The tokens come from a function called the @dfn{lexical analyzer} that you
-must supply in some fashion (such as by writing it in C). The Bison parser
-calls the lexical analyzer each time it wants a new token. It doesn't know
-what is ``inside'' the tokens (though their semantic values may reflect
-this). Typically the lexical analyzer makes the tokens by parsing
-characters of text, but Bison does not depend on this. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
-
-The Bison parser file is C code which defines a function named
-@code{yyparse} which implements that grammar. This function does not make
-a complete C program: you must supply some additional functions. One is
-the lexical analyzer. Another is an error-reporting function which the
-parser calls to report an error. In addition, a complete C program must
-start with a function called @code{main}; you have to provide this, and
-arrange for it to call @code{yyparse} or the parser will never run.
-@xref{Interface, ,Parser C-Language Interface}.
+The tokens come from a function called the @dfn{lexical analyzer} that
+you must supply in some fashion (such as by writing it in C). The Bison
+parser calls the lexical analyzer each time it wants a new token. It
+doesn't know what is ``inside'' the tokens (though their semantic values
+may reflect this). Typically the lexical analyzer makes the tokens by
+parsing characters of text, but Bison does not depend on this.
+@xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
+
+The Bison parser implementation file is C code which defines a
+function named @code{yyparse} which implements that grammar. This
+function does not make a complete C program: you must supply some
+additional functions. One is the lexical analyzer. Another is an
+error-reporting function which the parser calls to report an error.
+In addition, a complete C program must start with a function called
+@code{main}; you have to provide this, and arrange for it to call
+@code{yyparse} or the parser will never run. @xref{Interface, ,Parser
+C-Language Interface}.
Aside from the token type names and the symbols in the actions you
-write, all variable and function names used in the Bison parser file
-begin with @samp{yy} or @samp{YY}. This includes interface functions
-such as the lexical analyzer function @code{yylex}, the error reporting
-function @code{yyerror} and the parser function @code{yyparse} itself.
-This also includes numerous identifiers used for internal purposes.
-Therefore, you should avoid using C identifiers starting with @samp{yy}
-or @samp{YY} in the Bison grammar file except for the ones defined in
-this manual.
+write, all symbols defined in the Bison parser implementation file
+itself begin with @samp{yy} or @samp{YY}. This includes interface
+functions such as the lexical analyzer function @code{yylex}, the
+error reporting function @code{yyerror} and the parser function
+@code{yyparse} itself. This also includes numerous identifiers used
+for internal purposes. Therefore, you should avoid using C
+identifiers starting with @samp{yy} or @samp{YY} in the Bison grammar
+file except for the ones defined in this manual. Also, you should
+avoid using the C identifiers @samp{malloc} and @samp{free} for
+anything other than their usual meanings.
+
+In some cases the Bison parser implementation file includes system
+headers, and in those cases your code should respect the identifiers
+reserved by those headers. On some non-GNU hosts, @code{<alloca.h>},
+@code{<malloc.h>}, @code{<stddef.h>}, and @code{<stdlib.h>} are
+included as needed to declare memory allocators and related types.
+@code{<libintl.h>} is included if message translation is in use
+(@pxref{Internationalization}). Other system headers may be included
+if you define @code{YYDEBUG} to a nonzero value (@pxref{Tracing,
+,Tracing Your Parser}).
@node Stages
@section Stages in Using Bison
@enumerate
@item
Formally specify the grammar in a form recognized by Bison
-(@pxref{Grammar File, ,Bison Grammar Files}). For each grammatical rule in the language,
-describe the action that is to be taken when an instance of that rule
-is recognized. The action is described by a sequence of C statements.
+(@pxref{Grammar File, ,Bison Grammar Files}). For each grammatical rule
+in the language, describe the action that is to be taken when an
+instance of that rule is recognized. The action is described by a
+sequence of C statements.
@item
-Write a lexical analyzer to process input and pass tokens to the
-parser. The lexical analyzer may be written by hand in C
-(@pxref{Lexical, ,The Lexical Analyzer Function @code{yylex}}). It could also be produced using Lex, but the use
-of Lex is not discussed in this manual.
+Write a lexical analyzer to process input and pass tokens to the parser.
+The lexical analyzer may be written by hand in C (@pxref{Lexical, ,The
+Lexical Analyzer Function @code{yylex}}). It could also be produced
+using Lex, but the use of Lex is not discussed in this manual.
@item
Write a controlling function that calls the Bison-produced parser.
@example
%@{
-@var{Prologue (declarations)}
+@var{Prologue}
%@}
@var{Bison declarations}
%%
@var{Grammar rules}
%%
-@var{Epilogue (additional code)}
+@var{Epilogue}
@end example
@noindent
The @samp{%%}, @samp{%@{} and @samp{%@}} are punctuation that appears
in every Bison grammar file to separate the sections.
-The prologue may define types and variables used in the actions. You can
+The prologue may define types and variables used in the actions. You can
also use preprocessor commands to define macros used there, and use
@code{#include} to include header files that do any of these things.
+You need to declare the lexical analyzer @code{yylex} and the error
+printer @code{yyerror} here, along with any other global identifiers
+used by the actions in the grammar rules.
The Bison declarations declare the names of the terminal and nonterminal
symbols, and may also describe operator precedence and the data types of
The grammar rules define how to construct each nonterminal symbol from its
parts.
-The epilogue can contain any code you want to use. Often the definition of
-the lexical analyzer @code{yylex} goes here, plus subroutines called by the
-actions in the grammar rules. In a simple program, all the rest of the
-program can go here.
+The epilogue can contain any code you want to use. Often the
+definitions of functions declared in the prologue go here. In a
+simple program, all the rest of the program can go here.
@node Examples
@chapter Examples
@cindex simple examples
@cindex examples, simple
-Now we show and explain three sample programs written using Bison: a
+Now we show and explain several sample programs written using Bison: a
reverse polish notation calculator, an algebraic (infix) notation
-calculator, and a multi-function calculator. All three have been tested
-under BSD Unix 4.3; each produces a usable, though limited, interactive
-desk-top calculator.
+calculator --- later extended to track ``locations'' ---
+and a multi-function calculator. All
+produce usable, though limited, interactive desk-top calculators.
These examples are simple, but Bison grammars for real programming
-languages are written the same way.
-@ifinfo
-You can copy these examples out of the Info file and into a source file
-to try them.
-@end ifinfo
+languages are written the same way. You can copy these examples into a
+source file to try them.
@menu
-* RPN Calc:: Reverse polish notation calculator;
- a first example with no operator precedence.
-* Infix Calc:: Infix (algebraic) notation calculator.
- Operator precedence is introduced.
+* RPN Calc:: Reverse polish notation calculator;
+ a first example with no operator precedence.
+* Infix Calc:: Infix (algebraic) notation calculator.
+ Operator precedence is introduced.
* Simple Error Recovery:: Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$.
-* Multi-function Calc:: Calculator with memory and trig functions.
- It uses multiple data-types for semantic values.
-* Exercises:: Ideas for improving the multi-function calculator.
+* Multi-function Calc:: Calculator with memory and trig functions.
+ It uses multiple data-types for semantic values.
+* Exercises:: Ideas for improving the multi-function calculator.
@end menu
@node RPN Calc
The second example will illustrate how operator precedence is handled.
The source code for this calculator is named @file{rpcalc.y}. The
-@samp{.y} extension is a convention used for Bison input files.
+@samp{.y} extension is a convention used for Bison grammar files.
@menu
-* Decls: Rpcalc Decls. Prologue (declarations) for rpcalc.
-* Rules: Rpcalc Rules. Grammar Rules for rpcalc, with explanation.
-* Lexer: Rpcalc Lexer. The lexical analyzer.
-* Main: Rpcalc Main. The controlling function.
-* Error: Rpcalc Error. The error reporting function.
-* Gen: Rpcalc Gen. Running Bison on the grammar file.
-* Comp: Rpcalc Compile. Run the C compiler on the output code.
+* Rpcalc Declarations:: Prologue (declarations) for rpcalc.
+* Rpcalc Rules:: Grammar Rules for rpcalc, with explanation.
+* Rpcalc Lexer:: The lexical analyzer.
+* Rpcalc Main:: The controlling function.
+* Rpcalc Error:: The error reporting function.
+* Rpcalc Generate:: Running Bison on the grammar file.
+* Rpcalc Compile:: Run the C compiler on the output code.
@end menu
-@node Rpcalc Decls
+@node Rpcalc Declarations
@subsection Declarations for @code{rpcalc}
Here are the C and Bison declarations for the reverse polish notation
calculator. As in C, comments are placed between @samp{/*@dots{}*/}.
+@comment file: rpcalc.y
@example
-/* Reverse polish notation calculator. */
+/* Reverse polish notation calculator. */
%@{
-#define YYSTYPE double
-#include <math.h>
+ #define YYSTYPE double
+ #include <stdio.h>
+ #include <math.h>
+ int yylex (void);
+ void yyerror (char const *);
%@}
%token NUM
-%% /* Grammar rules and actions follow */
+%% /* Grammar rules and actions follow. */
@end example
The declarations section (@pxref{Prologue, , The prologue}) contains two
-preprocessor directives.
+preprocessor directives and two forward declarations.
The @code{#define} directive defines the macro @code{YYSTYPE}, thus
specifying the C data type for semantic values of both tokens and
The @code{#include} directive is used to declare the exponentiation
function @code{pow}.
-The second section, Bison declarations, provides information to Bison about
-the token types (@pxref{Bison Declarations, ,The Bison Declarations Section}). Each terminal symbol that is
-not a single-character literal must be declared here. (Single-character
+The forward declarations for @code{yylex} and @code{yyerror} are
+needed because the C language requires that functions be declared
+before they are used. These functions will be defined in the
+epilogue, but the parser calls them so they must be declared in the
+prologue.
+
+The second section, Bison declarations, provides information to Bison
+about the token types (@pxref{Bison Declarations, ,The Bison
+Declarations Section}). Each terminal symbol that is not a
+single-character literal must be declared here. (Single-character
literals normally don't need to be declared.) In this example, all the
arithmetic operators are designated by single-character literals, so the
only terminal symbol that needs to be declared is @code{NUM}, the token
Here are the grammar rules for the reverse polish notation calculator.
+@comment file: rpcalc.y
@example
-input: /* empty */
- | input line
+@group
+input:
+ /* empty */
+| input line
;
+@end group
-line: '\n'
- | exp '\n' @{ printf ("\t%.10g\n", $1); @}
+@group
+line:
+ '\n'
+| exp '\n' @{ printf ("%.10g\n", $1); @}
;
+@end group
-exp: NUM @{ $$ = $1; @}
- | exp exp '+' @{ $$ = $1 + $2; @}
- | exp exp '-' @{ $$ = $1 - $2; @}
- | exp exp '*' @{ $$ = $1 * $2; @}
- | exp exp '/' @{ $$ = $1 / $2; @}
- /* Exponentiation */
- | exp exp '^' @{ $$ = pow ($1, $2); @}
- /* Unary minus */
- | exp 'n' @{ $$ = -$1; @}
+@group
+exp:
+ NUM @{ $$ = $1; @}
+| exp exp '+' @{ $$ = $1 + $2; @}
+| exp exp '-' @{ $$ = $1 - $2; @}
+| exp exp '*' @{ $$ = $1 * $2; @}
+| exp exp '/' @{ $$ = $1 / $2; @}
+| exp exp '^' @{ $$ = pow ($1, $2); @} /* Exponentiation */
+| exp 'n' @{ $$ = -$1; @} /* Unary minus */
;
+@end group
%%
@end example
The groupings of the rpcalc ``language'' defined here are the expression
(given the name @code{exp}), the line of input (@code{line}), and the
complete input transcript (@code{input}). Each of these nonterminal
-symbols has several alternate rules, joined by the @samp{|} punctuator
+symbols has several alternate rules, joined by the vertical bar @samp{|}
which is read as ``or''. The following sections explain what these rules
mean.
rule are referred to as @code{$1}, @code{$2}, and so on.
@menu
-* Rpcalc Input::
-* Rpcalc Line::
-* Rpcalc Expr::
+* Rpcalc Input:: Explanation of the @code{input} nonterminal
+* Rpcalc Line:: Explanation of the @code{line} nonterminal
+* Rpcalc Expr:: Explanation of the @code{expr} nonterminal
@end menu
@node Rpcalc Input
Consider the definition of @code{input}:
@example
-input: /* empty */
- | input line
+input:
+ /* empty */
+| input line
;
@end example
The parser function @code{yyparse} continues to process input until a
grammatical error is seen or the lexical analyzer says there are no more
-input tokens; we will arrange for the latter to happen at end of file.
+input tokens; we will arrange for the latter to happen at end-of-input.
@node Rpcalc Line
@subsubsection Explanation of @code{line}
Now consider the definition of @code{line}:
@example
-line: '\n'
- | exp '\n' @{ printf ("\t%.10g\n", $1); @}
+line:
+ '\n'
+| exp '\n' @{ printf ("%.10g\n", $1); @}
;
@end example
followed by a plus-sign. The third handles subtraction, and so on.
@example
-exp: NUM
- | exp exp '+' @{ $$ = $1 + $2; @}
- | exp exp '-' @{ $$ = $1 - $2; @}
- @dots{}
- ;
+exp:
+ NUM
+| exp exp '+' @{ $$ = $1 + $2; @}
+| exp exp '-' @{ $$ = $1 - $2; @}
+@dots{}
+;
@end example
We have used @samp{|} to join all the rules for @code{exp}, but we could
equally well have written them separately:
@example
-exp: NUM ;
-exp: exp exp '+' @{ $$ = $1 + $2; @} ;
-exp: exp exp '-' @{ $$ = $1 - $2; @} ;
- @dots{}
+exp: NUM ;
+exp: exp exp '+' @{ $$ = $1 + $2; @};
+exp: exp exp '-' @{ $$ = $1 - $2; @};
+@dots{}
@end example
Most of the rules have actions that compute the value of the expression in
This is what happens in the first rule (the one that uses @code{NUM}).
The formatting shown here is the recommended convention, but Bison does
-not require it. You can add or change whitespace as much as you wish.
+not require it. You can add or change white space as much as you wish.
For example, this:
@example
-exp : NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{}
+exp: NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{} ;
@end example
@noindent
means the same thing as this:
@example
-exp: NUM
- | exp exp '+' @{ $$ = $1 + $2; @}
- | @dots{}
+exp:
+ NUM
+| exp exp '+' @{ $$ = $1 + $2; @}
+| @dots{}
+;
@end example
@noindent
@cindex writing a lexical analyzer
@cindex lexical analyzer, writing
-The lexical analyzer's job is low-level parsing: converting characters or
-sequences of characters into tokens. The Bison parser gets its tokens by
-calling the lexical analyzer. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
+The lexical analyzer's job is low-level parsing: converting characters
+or sequences of characters into tokens. The Bison parser gets its
+tokens by calling the lexical analyzer. @xref{Lexical, ,The Lexical
+Analyzer Function @code{yylex}}.
-Only a simple lexical analyzer is needed for the RPN calculator. This
+Only a simple lexical analyzer is needed for the RPN
+calculator. This
lexical analyzer skips blanks and tabs, then reads in numbers as
@code{double} and returns them as @code{NUM} tokens. Any other character
that isn't part of a number is a separate token. Note that the token-code
represents a token type. The same text used in Bison rules to stand for
this token type is also a C expression for the numeric code for the type.
This works in two ways. If the token type is a character literal, then its
-numeric code is the ASCII code for that character; you can use the same
+numeric code is that of the character; you can use the same
character literal in the lexical analyzer to express the number. If the
token type is an identifier, that identifier is defined by Bison as a C
macro whose definition is the appropriate number. In this example,
The semantic value of the token (if it has one) is stored into the
global variable @code{yylval}, which is where the Bison parser will look
for it. (The C data type of @code{yylval} is @code{YYSTYPE}, which was
-defined at the beginning of the grammar; @pxref{Rpcalc Decls,
+defined at the beginning of the grammar; @pxref{Rpcalc Declarations,
,Declarations for @code{rpcalc}}.)
-A token type code of zero is returned if the end-of-file is encountered.
-(Bison recognizes any nonpositive value as indicating the end of the
-input.)
+A token type code of zero is returned if the end-of-input is encountered.
+(Bison recognizes any nonpositive value as indicating end-of-input.)
Here is the code for the lexical analyzer:
+@comment file: rpcalc.y
@example
@group
-/* Lexical analyzer returns a double floating point
- number on the stack and the token NUM, or the ASCII
- character read if not a number. Skips all blanks
- and tabs, returns 0 for EOF. */
+/* The lexical analyzer returns a double floating point
+ number on the stack and the token NUM, or the numeric code
+ of the character read if not a number. It skips all blanks
+ and tabs, and returns 0 for end-of-input. */
#include <ctype.h>
@end group
@{
int c;
- /* skip white space */
+ /* Skip white space. */
while ((c = getchar ()) == ' ' || c == '\t')
- ;
+ continue;
@end group
@group
- /* process numbers */
+ /* Process numbers. */
if (c == '.' || isdigit (c))
@{
ungetc (c, stdin);
@}
@end group
@group
- /* return end-of-file */
+ /* Return end-of-input. */
if (c == EOF)
return 0;
- /* return single chars */
+ /* Return a single char. */
return c;
@}
@end group
kept to the bare minimum. The only requirement is that it call
@code{yyparse} to start the process of parsing.
+@comment file: rpcalc.y
@example
@group
int
When @code{yyparse} detects a syntax error, it calls the error reporting
function @code{yyerror} to print an error message (usually but not
-always @code{"parse error"}). It is up to the programmer to supply
+always @code{"syntax error"}). It is up to the programmer to supply
@code{yyerror} (@pxref{Interface, ,Parser C-Language Interface}), so
here is the definition we will use:
+@comment file: rpcalc.y
@example
@group
#include <stdio.h>
+@end group
+@group
+/* Called by yyparse on error. */
void
-yyerror (const char *s) /* Called by yyparse on error */
+yyerror (char const *s)
@{
- printf ("%s\n", s);
+ fprintf (stderr, "%s\n", s);
@}
@end group
@end example
cause the calculator program to exit. This is not clean behavior for a
real calculator, but it is adequate for the first example.
-@node Rpcalc Gen
+@node Rpcalc Generate
@subsection Running Bison to Make the Parser
@cindex running Bison (introduction)
Before running Bison to produce a parser, we need to decide how to
arrange all the source code in one or more source files. For such a
-simple example, the easiest thing is to put everything in one file. The
-definitions of @code{yylex}, @code{yyerror} and @code{main} go at the
-end, in the epilogue of the file
+simple example, the easiest thing is to put everything in one file,
+the grammar file. The definitions of @code{yylex}, @code{yyerror} and
+@code{main} go at the end, in the epilogue of the grammar file
(@pxref{Grammar Layout, ,The Overall Layout of a Bison Grammar}).
For a large project, you would probably have several source files, and use
@code{make} to arrange to recompile them.
-With all the source in a single file, you use the following command to
-convert it into a parser file:
+With all the source in the grammar file, you use the following command
+to convert it into a parser implementation file:
@example
-bison @var{file_name}.y
+bison @var{file}.y
@end example
@noindent
-In this example the file was called @file{rpcalc.y} (for ``Reverse Polish
-CALCulator''). Bison produces a file named @file{@var{file_name}.tab.c},
-removing the @samp{.y} from the original file name. The file output by
-Bison contains the source code for @code{yyparse}. The additional
-functions in the input file (@code{yylex}, @code{yyerror} and @code{main})
-are copied verbatim to the output.
+In this example, the grammar file is called @file{rpcalc.y} (for
+``Reverse Polish @sc{calc}ulator''). Bison produces a parser
+implementation file named @file{@var{file}.tab.c}, removing the
+@samp{.y} from the grammar file name. The parser implementation file
+contains the source code for @code{yyparse}. The additional functions
+in the grammar file (@code{yylex}, @code{yyerror} and @code{main}) are
+copied verbatim to the parser implementation file.
@node Rpcalc Compile
-@subsection Compiling the Parser File
+@subsection Compiling the Parser Implementation File
@cindex compiling the parser
-Here is how to compile and run the parser file:
+Here is how to compile and run the parser implementation file:
@example
@group
# @r{List files in current directory.}
-% ls
+$ @kbd{ls}
rpcalc.tab.c rpcalc.y
@end group
@group
# @r{Compile the Bison parser.}
# @r{@samp{-lm} tells compiler to search math library for @code{pow}.}
-% cc rpcalc.tab.c -lm -o rpcalc
+$ @kbd{cc -lm -o rpcalc rpcalc.tab.c}
@end group
@group
# @r{List files again.}
-% ls
+$ @kbd{ls}
rpcalc rpcalc.tab.c rpcalc.y
@end group
@end example
example session using @code{rpcalc}.
@example
-% rpcalc
-4 9 +
-13
-3 7 + 3 4 5 *+-
--13
-3 7 + 3 4 5 * + - n @r{Note the unary minus, @samp{n}}
-13
-5 6 / 4 n +
--3.166666667
-3 4 ^ @r{Exponentiation}
-81
-^D @r{End-of-file indicator}
-%
+$ @kbd{rpcalc}
+@kbd{4 9 +}
+@result{} 13
+@kbd{3 7 + 3 4 5 *+-}
+@result{} -13
+@kbd{3 7 + 3 4 5 * + - n} @r{Note the unary minus, @samp{n}}
+@result{} 13
+@kbd{5 6 / 4 n +}
+@result{} -3.166666667
+@kbd{3 4 ^} @r{Exponentiation}
+@result{} 81
+@kbd{^D} @r{End-of-file indicator}
+$
@end example
@node Infix Calc
@file{calc.y}, an infix desk-top calculator.
@example
-/* Infix notation calculator--calc */
+/* Infix notation calculator. */
+@group
%@{
-#define YYSTYPE double
-#include <math.h>
+ #define YYSTYPE double
+ #include <math.h>
+ #include <stdio.h>
+ int yylex (void);
+ void yyerror (char const *);
%@}
+@end group
-/* BISON Declarations */
+@group
+/* Bison declarations. */
%token NUM
%left '-' '+'
%left '*' '/'
-%left NEG /* negation--unary minus */
-%right '^' /* exponentiation */
+%precedence NEG /* negation--unary minus */
+%right '^' /* exponentiation */
+@end group
-/* Grammar follows */
-%%
-input: /* empty string */
- | input line
+%% /* The grammar follows. */
+@group
+input:
+ /* empty */
+| input line
;
+@end group
-line: '\n'
- | exp '\n' @{ printf ("\t%.10g\n", $1); @}
+@group
+line:
+ '\n'
+| exp '\n' @{ printf ("\t%.10g\n", $1); @}
;
+@end group
-exp: NUM @{ $$ = $1; @}
- | exp '+' exp @{ $$ = $1 + $3; @}
- | exp '-' exp @{ $$ = $1 - $3; @}
- | exp '*' exp @{ $$ = $1 * $3; @}
- | exp '/' exp @{ $$ = $1 / $3; @}
- | '-' exp %prec NEG @{ $$ = -$2; @}
- | exp '^' exp @{ $$ = pow ($1, $3); @}
- | '(' exp ')' @{ $$ = $2; @}
+@group
+exp:
+ NUM @{ $$ = $1; @}
+| exp '+' exp @{ $$ = $1 + $3; @}
+| exp '-' exp @{ $$ = $1 - $3; @}
+| exp '*' exp @{ $$ = $1 * $3; @}
+| exp '/' exp @{ $$ = $1 / $3; @}
+| '-' exp %prec NEG @{ $$ = -$2; @}
+| exp '^' exp @{ $$ = pow ($1, $3); @}
+| '(' exp ')' @{ $$ = $2; @}
;
+@end group
%%
@end example
types and says they are left-associative operators. The declarations
@code{%left} and @code{%right} (right associativity) take the place of
@code{%token} which is used to declare a token type name without
-associativity. (These tokens are single-character literals, which
+associativity/precedence. (These tokens are single-character literals, which
ordinarily don't need to be declared. We declare them here to specify
-the associativity.)
+the associativity/precedence.)
Operator precedence is determined by the line ordering of the
declarations; the higher the line number of the declaration (lower on
the page or screen), the higher the precedence. Hence, exponentiation
has the highest precedence, unary minus (@code{NEG}) is next, followed
-by @samp{*} and @samp{/}, and so on. @xref{Precedence, ,Operator Precedence}.
+by @samp{*} and @samp{/}, and so on. Unary minus is not associative,
+only precedence matters (@code{%precedence}. @xref{Precedence, ,Operator
+Precedence}.
-The other important new feature is the @code{%prec} in the grammar section
-for the unary minus operator. The @code{%prec} simply instructs Bison that
-the rule @samp{| '-' exp} has the same precedence as @code{NEG}---in this
-case the next-to-highest. @xref{Contextual Precedence, ,Context-Dependent Precedence}.
+The other important new feature is the @code{%prec} in the grammar
+section for the unary minus operator. The @code{%prec} simply instructs
+Bison that the rule @samp{| '-' exp} has the same precedence as
+@code{NEG}---in this case the next-to-highest. @xref{Contextual
+Precedence, ,Context-Dependent Precedence}.
Here is a sample run of @file{calc.y}:
@need 500
@example
-% calc
-4 + 4.5 - (34/(8*3+-3))
+$ @kbd{calc}
+@kbd{4 + 4.5 - (34/(8*3+-3))}
6.880952381
--56 + 2
+@kbd{-56 + 2}
-54
-3 ^ 2
+@kbd{3 ^ 2}
9
@end example
@example
@group
-line: '\n'
- | exp '\n' @{ printf ("\t%.10g\n", $1); @}
- | error '\n' @{ yyerrok; @}
+line:
+ '\n'
+| exp '\n' @{ printf ("\t%.10g\n", $1); @}
+| error '\n' @{ yyerrok; @}
;
@end group
@end example
This addition to the grammar allows for simple error recovery in the
-event of a parse error. If an expression that cannot be evaluated is
+event of a syntax error. If an expression that cannot be evaluated is
read, the error will be recognized by the third rule for @code{line},
and parsing will continue. (The @code{yyerror} function is still called
upon to print its message as well.) The action executes the statement
@code{yyerrok}, a macro defined automatically by Bison; its meaning is
that error recovery is complete (@pxref{Error Recovery}). Note the
difference between @code{yyerrok} and @code{yyerror}; neither one is a
-misprint.@refill
+misprint.
This form of error recovery deals with syntax errors. There are other
kinds of errors; for example, division by zero, which raises an exception
@cindex @code{ltcalc}
@cindex calculator, location tracking
-This example extends the infix notation calculator with location tracking.
-This feature will be used to improve error reporting, and provide better
-error messages.
-
-For the sake of clarity, we will switch for this example to an integer
-calculator, since most of the work needed to use locations will be done
-in the lexical analyser.
+This example extends the infix notation calculator with location
+tracking. This feature will be used to improve the error messages. For
+the sake of clarity, this example is a simple integer calculator, since
+most of the work needed to use locations will be done in the lexical
+analyzer.
@menu
-* Decls: Ltcalc Decls. Bison and C declarations for ltcalc.
-* Rules: Ltcalc Rules. Grammar rules for ltcalc, with explanations.
-* Lexer: Ltcalc Lexer. The lexical analyzer.
+* Ltcalc Declarations:: Bison and C declarations for ltcalc.
+* Ltcalc Rules:: Grammar rules for ltcalc, with explanations.
+* Ltcalc Lexer:: The lexical analyzer.
@end menu
-@node Ltcalc Decls
+@node Ltcalc Declarations
@subsection Declarations for @code{ltcalc}
-The C and Bison declarations for the location tracking calculator are the same
-as the declarations for the infix notation calculator.
+The C and Bison declarations for the location tracking calculator are
+the same as the declarations for the infix notation calculator.
@example
/* Location tracking calculator. */
%@{
-#define YYSTYPE int
-#include <math.h>
+ #define YYSTYPE int
+ #include <math.h>
+ int yylex (void);
+ void yyerror (char const *);
%@}
/* Bison declarations. */
%left '-' '+'
%left '*' '/'
-%left NEG
+%precedence NEG
%right '^'
-%% /* Grammar follows */
+%% /* The grammar follows. */
@end example
-In the code above, there are no declarations specific to locations. Defining
-a data type for storing locations is not needed: we will use the type provided
-by default (@pxref{Location Type, ,Data Types of Locations}), which is a four
-member structure with the following integer fields: @code{first_line},
-@code{first_column}, @code{last_line} and @code{last_column}.
+@noindent
+Note there are no declarations specific to locations. Defining a data
+type for storing locations is not needed: we will use the type provided
+by default (@pxref{Location Type, ,Data Types of Locations}), which is a
+four member structure with the following integer fields:
+@code{first_line}, @code{first_column}, @code{last_line} and
+@code{last_column}. By conventions, and in accordance with the GNU
+Coding Standards and common practice, the line and column count both
+start at 1.
@node Ltcalc Rules
@subsection Grammar Rules for @code{ltcalc}
-Whether you choose to handle locations or not has no effect on the syntax of
-your language. Therefore, grammar rules for this example will be very close to
-those of the previous example: we will only modify them to benefit from the new
-informations we will have.
+Whether handling locations or not has no effect on the syntax of your
+language. Therefore, grammar rules for this example will be very close
+to those of the previous example: we will only modify them to benefit
+from the new information.
-Here, we will use locations to report divisions by zero, and locate the wrong
-expressions or subexpressions.
+Here, we will use locations to report divisions by zero, and locate the
+wrong expressions or subexpressions.
@example
@group
-input : /* empty */
- | input line
+input:
+ /* empty */
+| input line
;
@end group
@group
-line : '\n'
- | exp '\n' @{ printf ("%d\n", $1); @}
+line:
+ '\n'
+| exp '\n' @{ printf ("%d\n", $1); @}
;
@end group
@group
-exp : NUM @{ $$ = $1; @}
- | exp '+' exp @{ $$ = $1 + $3; @}
- | exp '-' exp @{ $$ = $1 - $3; @}
- | exp '*' exp @{ $$ = $1 * $3; @}
-@end group
- | exp '/' exp
-@group
- @{
- if ($3)
- $$ = $1 / $3;
- else
- @{
- $$ = 1;
- printf("Division by zero, l%d,c%d-l%d,c%d",
- @@3.first_line, @@3.first_column,
- @@3.last_line, @@3.last_column);
- @}
- @}
+exp:
+ NUM @{ $$ = $1; @}
+| exp '+' exp @{ $$ = $1 + $3; @}
+| exp '-' exp @{ $$ = $1 - $3; @}
+| exp '*' exp @{ $$ = $1 * $3; @}
@end group
@group
- | '-' exp %preg NEG @{ $$ = -$2; @}
- | exp '^' exp @{ $$ = pow ($1, $3); @}
- | '(' exp ')' @{ $$ = $2; @}
+| exp '/' exp
+ @{
+ if ($3)
+ $$ = $1 / $3;
+ else
+ @{
+ $$ = 1;
+ fprintf (stderr, "%d.%d-%d.%d: division by zero",
+ @@3.first_line, @@3.first_column,
+ @@3.last_line, @@3.last_column);
+ @}
+ @}
+@end group
+@group
+| '-' exp %prec NEG @{ $$ = -$2; @}
+| exp '^' exp @{ $$ = pow ($1, $3); @}
+| '(' exp ')' @{ $$ = $2; @}
@end group
@end example
using the pseudo-variables @code{@@@var{n}} for rule components, and the
pseudo-variable @code{@@$} for groupings.
-In this example, we never assign a value to @code{@@$}, because the
-output parser can do this automatically. By default, before executing
-the C code of each action, @code{@@$} is set to range from the beginning
-of @code{@@1} to the end of @code{@@@var{n}}, for a rule with @var{n}
-components.
-
-Of course, this behavior can be redefined (@pxref{Location Default
-Action, , Default Action for Locations}), and for very specific rules,
-@code{@@$} can be computed by hand.
+We don't need to assign a value to @code{@@$}: the output parser does it
+automatically. By default, before executing the C code of each action,
+@code{@@$} is set to range from the beginning of @code{@@1} to the end
+of @code{@@@var{n}}, for a rule with @var{n} components. This behavior
+can be redefined (@pxref{Location Default Action, , Default Action for
+Locations}), and for very specific rules, @code{@@$} can be computed by
+hand.
@node Ltcalc Lexer
@subsection The @code{ltcalc} Lexical Analyzer.
-Until now, we relied on Bison's defaults to enable location tracking. The next
-step is to rewrite the lexical analyser, and make it able to feed the parser
-with locations of tokens, as he already does for semantic values.
+Until now, we relied on Bison's defaults to enable location
+tracking. The next step is to rewrite the lexical analyzer, and make it
+able to feed the parser with the token locations, as it already does for
+semantic values.
-To do so, we must take into account every single character of the input text,
-to avoid the computed locations of being fuzzy or wrong:
+To this end, we must take into account every single character of the
+input text, to avoid the computed locations of being fuzzy or wrong:
@example
@group
yylex (void)
@{
int c;
+@end group
- /* skip white space */
+@group
+ /* Skip white space. */
while ((c = getchar ()) == ' ' || c == '\t')
++yylloc.last_column;
+@end group
- /* step */
+@group
+ /* Step. */
yylloc.first_line = yylloc.last_line;
yylloc.first_column = yylloc.last_column;
@end group
@group
- /* process numbers */
+ /* Process numbers. */
if (isdigit (c))
@{
yylval = c - '0';
@}
@end group
- /* return end-of-file */
+ /* Return end-of-input. */
if (c == EOF)
return 0;
- /* return single chars and update location */
+@group
+ /* Return a single char, and update location. */
if (c == '\n')
@{
++yylloc.last_line;
++yylloc.last_column;
return c;
@}
+@end group
@end example
-Basically, the lexical analyzer does the same processing as before: it skips
-blanks and tabs, and reads numbers or single-character tokens. In addition
-to this, it updates the @code{yylloc} global variable (of type @code{YYLTYPE}),
-where the location of tokens is stored.
+Basically, the lexical analyzer performs the same processing as before:
+it skips blanks and tabs, and reads numbers or single-character tokens.
+In addition, it updates @code{yylloc}, the global variable (of type
+@code{YYLTYPE}) containing the token's location.
-Now, each time this function returns a token, the parser has it's number as
-well as it's semantic value, and it's position in the text. The last needed
-change is to initialize @code{yylloc}, for example in the controlling
-function:
+Now, each time this function returns a token, the parser has its number
+as well as its semantic value, and its location in the text. The last
+needed change is to initialize @code{yylloc}, for example in the
+controlling function:
@example
+@group
int
main (void)
@{
yylloc.first_column = yylloc.last_column = 0;
return yyparse ();
@}
+@end group
@end example
-Remember that computing locations is not a matter of syntax. Every character
-must be associated to a location update, whether it is in valid input, in
-comments, in literal strings, and so on...
+Remember that computing locations is not a matter of syntax. Every
+character must be associated to a location update, whether it is in
+valid input, in comments, in literal strings, and so on.
@node Multi-function Calc
@section Multi-Function Calculator: @code{mfcalc}
It is easy to add new operators to the infix calculator as long as they are
only single-character literals. The lexical analyzer @code{yylex} passes
-back all nonnumber characters as tokens, so new grammar rules suffice for
+back all nonnumeric characters as tokens, so new grammar rules suffice for
adding a new operator. But we want something more flexible: built-in
functions whose syntax has this form:
Here is a sample session with the multi-function calculator:
@example
-% mfcalc
-pi = 3.141592653589
-3.1415926536
-sin(pi)
-0.0000000000
-alpha = beta1 = 2.3
-2.3000000000
-alpha
-2.3000000000
-ln(alpha)
-0.8329091229
-exp(ln(beta1))
-2.3000000000
-%
+@group
+$ @kbd{mfcalc}
+@kbd{pi = 3.141592653589}
+@result{} 3.1415926536
+@end group
+@group
+@kbd{sin(pi)}
+@result{} 0.0000000000
+@end group
+@kbd{alpha = beta1 = 2.3}
+@result{} 2.3000000000
+@kbd{alpha}
+@result{} 2.3000000000
+@kbd{ln(alpha)}
+@result{} 0.8329091229
+@kbd{exp(ln(beta1))}
+@result{} 2.3000000000
+$
@end example
Note that multiple assignment and nested function calls are permitted.
@menu
-* Decl: Mfcalc Decl. Bison declarations for multi-function calculator.
-* Rules: Mfcalc Rules. Grammar rules for the calculator.
-* Symtab: Mfcalc Symtab. Symbol table management subroutines.
+* Mfcalc Declarations:: Bison declarations for multi-function calculator.
+* Mfcalc Rules:: Grammar rules for the calculator.
+* Mfcalc Symbol Table:: Symbol table management subroutines.
+* Mfcalc Lexer:: The lexical analyzer.
+* Mfcalc Main:: The controlling function.
@end menu
-@node Mfcalc Decl
+@node Mfcalc Declarations
@subsection Declarations for @code{mfcalc}
Here are the C and Bison declarations for the multi-function calculator.
-@smallexample
+@comment file: mfcalc.y
+@example
+@group
%@{
-#include <math.h> /* For math functions, cos(), sin(), etc. */
-#include "calc.h" /* Contains definition of `symrec' */
+ #include <stdio.h> /* For printf, etc. */
+ #include <math.h> /* For pow, used in the grammar. */
+ #include "calc.h" /* Contains definition of `symrec'. */
+ int yylex (void);
+ void yyerror (char const *);
%@}
+@end group
+@group
%union @{
-double val; /* For returning numbers. */
-symrec *tptr; /* For returning symbol-table pointers */
+ double val; /* For returning numbers. */
+ symrec *tptr; /* For returning symbol-table pointers. */
@}
-
-%token <val> NUM /* Simple double precision number */
-%token <tptr> VAR FNCT /* Variable and Function */
+@end group
+%token <val> NUM /* Simple double precision number. */
+%token <tptr> VAR FNCT /* Variable and Function. */
%type <val> exp
+@group
%right '='
%left '-' '+'
%left '*' '/'
-%left NEG /* Negation--unary minus */
-%right '^' /* Exponentiation */
-
-/* Grammar follows */
-
-%%
-@end smallexample
+%precedence NEG /* negation--unary minus */
+%right '^' /* exponentiation */
+@end group
+%% /* The grammar follows. */
+@end example
The above grammar introduces only two new features of the Bison language.
These features allow semantic values to have various data types
declarations are augmented with information about their data type (placed
between angle brackets).
-The Bison construct @code{%type} is used for declaring nonterminal symbols,
-just as @code{%token} is used for declaring token types. We have not used
-@code{%type} before because nonterminal symbols are normally declared
-implicitly by the rules that define them. But @code{exp} must be declared
-explicitly so we can specify its value type. @xref{Type Decl, ,Nonterminal Symbols}.
+The Bison construct @code{%type} is used for declaring nonterminal
+symbols, just as @code{%token} is used for declaring token types. We
+have not used @code{%type} before because nonterminal symbols are
+normally declared implicitly by the rules that define them. But
+@code{exp} must be declared explicitly so we can specify its value type.
+@xref{Type Decl, ,Nonterminal Symbols}.
@node Mfcalc Rules
@subsection Grammar Rules for @code{mfcalc}
Most of them are copied directly from @code{calc}; three rules,
those which mention @code{VAR} or @code{FNCT}, are new.
-@smallexample
-input: /* empty */
- | input line
+@comment file: mfcalc.y
+@example
+@group
+input:
+ /* empty */
+| input line
;
+@end group
+@group
line:
- '\n'
- | exp '\n' @{ printf ("\t%.10g\n", $1); @}
- | error '\n' @{ yyerrok; @}
+ '\n'
+| exp '\n' @{ printf ("%.10g\n", $1); @}
+| error '\n' @{ yyerrok; @}
;
+@end group
-exp: NUM @{ $$ = $1; @}
- | VAR @{ $$ = $1->value.var; @}
- | VAR '=' exp @{ $$ = $3; $1->value.var = $3; @}
- | FNCT '(' exp ')' @{ $$ = (*($1->value.fnctptr))($3); @}
- | exp '+' exp @{ $$ = $1 + $3; @}
- | exp '-' exp @{ $$ = $1 - $3; @}
- | exp '*' exp @{ $$ = $1 * $3; @}
- | exp '/' exp @{ $$ = $1 / $3; @}
- | '-' exp %prec NEG @{ $$ = -$2; @}
- | exp '^' exp @{ $$ = pow ($1, $3); @}
- | '(' exp ')' @{ $$ = $2; @}
+@group
+exp:
+ NUM @{ $$ = $1; @}
+| VAR @{ $$ = $1->value.var; @}
+| VAR '=' exp @{ $$ = $3; $1->value.var = $3; @}
+| FNCT '(' exp ')' @{ $$ = (*($1->value.fnctptr))($3); @}
+| exp '+' exp @{ $$ = $1 + $3; @}
+| exp '-' exp @{ $$ = $1 - $3; @}
+| exp '*' exp @{ $$ = $1 * $3; @}
+| exp '/' exp @{ $$ = $1 / $3; @}
+| '-' exp %prec NEG @{ $$ = -$2; @}
+| exp '^' exp @{ $$ = pow ($1, $3); @}
+| '(' exp ')' @{ $$ = $2; @}
;
-/* End of grammar */
+@end group
+/* End of grammar. */
%%
-@end smallexample
+@end example
-@node Mfcalc Symtab
+@node Mfcalc Symbol Table
@subsection The @code{mfcalc} Symbol Table
@cindex symbol table example
definition, which is kept in the header @file{calc.h}, is as follows. It
provides for either functions or variables to be placed in the table.
-@smallexample
+@comment file: calc.h
+@example
@group
-/* Fonctions type. */
+/* Function type. */
typedef double (*func_t) (double);
+@end group
-/* Data type for links in the chain of symbols. */
+@group
+/* Data type for links in the chain of symbols. */
struct symrec
@{
- char *name; /* name of symbol */
+ char *name; /* name of symbol */
int type; /* type of symbol: either VAR or FNCT */
union
@{
- double var; /* value of a VAR */
- func_t fnctptr; /* value of a FNCT */
+ double var; /* value of a VAR */
+ func_t fnctptr; /* value of a FNCT */
@} value;
- struct symrec *next; /* link field */
+ struct symrec *next; /* link field */
@};
@end group
@group
typedef struct symrec symrec;
-/* The symbol table: a chain of `struct symrec'. */
+/* The symbol table: a chain of `struct symrec'. */
extern symrec *sym_table;
-symrec *putsym (const char *, func_t);
-symrec *getsym (const char *);
+symrec *putsym (char const *, int);
+symrec *getsym (char const *);
@end group
-@end smallexample
-
-The new version of @code{main} includes a call to @code{init_table}, a
-function that initializes the symbol table. Here it is, and
-@code{init_table} as well:
-
-@smallexample
-@group
-#include <stdio.h>
+@end example
-int
-main (void)
-@{
- init_table ();
- return yyparse ();
-@}
-@end group
+The new version of @code{main} will call @code{init_table} to initialize
+the symbol table:
+@comment file: mfcalc.y
+@example
@group
-void
-yyerror (const char *s) /* Called by yyparse on error */
-@{
- printf ("%s\n", s);
-@}
-
struct init
@{
- char *fname;
- double (*fnct)(double);
+ char const *fname;
+ double (*fnct) (double);
@};
@end group
@group
-struct init arith_fncts[] =
+struct init const arith_fncts[] =
@{
- "sin", sin,
- "cos", cos,
- "atan", atan,
- "ln", log,
- "exp", exp,
- "sqrt", sqrt,
- 0, 0
+ @{ "atan", atan @},
+ @{ "cos", cos @},
+ @{ "exp", exp @},
+ @{ "ln", log @},
+ @{ "sin", sin @},
+ @{ "sqrt", sqrt @},
+ @{ 0, 0 @},
@};
+@end group
+@group
/* The symbol table: a chain of `struct symrec'. */
-symrec *sym_table = (symrec *) 0;
+symrec *sym_table;
@end group
@group
-/* Put arithmetic functions in table. */
+/* Put arithmetic functions in table. */
+static
void
init_table (void)
@{
int i;
- symrec *ptr;
for (i = 0; arith_fncts[i].fname != 0; i++)
@{
- ptr = putsym (arith_fncts[i].fname, FNCT);
+ symrec *ptr = putsym (arith_fncts[i].fname, FNCT);
ptr->value.fnctptr = arith_fncts[i].fnct;
@}
@}
@end group
-@end smallexample
+@end example
By simply editing the initialization list and adding the necessary include
files, you can add additional functions to the calculator.
The function @code{getsym} is passed the name of the symbol to look up. If
found, a pointer to that symbol is returned; otherwise zero is returned.
-@smallexample
+@comment file: mfcalc.y
+@example
+#include <stdlib.h> /* malloc. */
+#include <string.h> /* strlen. */
+
+@group
symrec *
-putsym (char *sym_name, int sym_type)
+putsym (char const *sym_name, int sym_type)
@{
- symrec *ptr;
- ptr = (symrec *) malloc (sizeof (symrec));
+ symrec *ptr = (symrec *) malloc (sizeof (symrec));
ptr->name = (char *) malloc (strlen (sym_name) + 1);
strcpy (ptr->name,sym_name);
ptr->type = sym_type;
- ptr->value.var = 0; /* set value to 0 even if fctn. */
+ ptr->value.var = 0; /* Set value to 0 even if fctn. */
ptr->next = (struct symrec *)sym_table;
sym_table = ptr;
return ptr;
@}
+@end group
+@group
symrec *
-getsym (const char *sym_name)
+getsym (char const *sym_name)
@{
symrec *ptr;
for (ptr = sym_table; ptr != (symrec *) 0;
ptr = (symrec *)ptr->next)
- if (strcmp (ptr->name,sym_name) == 0)
+ if (strcmp (ptr->name, sym_name) == 0)
return ptr;
return 0;
@}
-@end smallexample
+@end group
+@end example
+
+@node Mfcalc Lexer
+@subsection The @code{mfcalc} Lexer
The function @code{yylex} must now recognize variables, numeric values, and
the single-character arithmetic operators. Strings of alphanumeric
-characters with a leading non-digit are recognized as either variables or
+characters with a leading letter are recognized as either variables or
functions depending on what the symbol table says about them.
The string is passed to @code{getsym} for look up in the symbol table. If
(@code{VAR} or @code{FNCT}) is returned to @code{yyparse}. If it is not
already in the table, then it is installed as a @code{VAR} using
@code{putsym}. Again, a pointer and its type (which must be @code{VAR}) is
-returned to @code{yyparse}.@refill
+returned to @code{yyparse}.
No change is needed in the handling of numeric values and arithmetic
operators in @code{yylex}.
-@smallexample
+@comment file: mfcalc.y
+@example
@group
#include <ctype.h>
+@end group
+@group
int
yylex (void)
@{
int c;
- /* Ignore whitespace, get first nonwhite character. */
- while ((c = getchar ()) == ' ' || c == '\t');
+ /* Ignore white space, get first nonwhite character. */
+ while ((c = getchar ()) == ' ' || c == '\t')
+ continue;
if (c == EOF)
return 0;
/* Char starts an identifier => read the name. */
if (isalpha (c))
@{
- symrec *s;
+ /* Initially make the buffer long enough
+ for a 40-character symbol name. */
+ static size_t length = 40;
static char *symbuf = 0;
- static int length = 0;
+ symrec *s;
int i;
@end group
-
-@group
- /* Initially make the buffer long enough
- for a 40-character symbol name. */
- if (length == 0)
- length = 40, symbuf = (char *)malloc (length + 1);
+ if (!symbuf)
+ symbuf = (char *) malloc (length + 1);
i = 0;
do
-@end group
@group
@{
/* If buffer is full, make it bigger. */
if (i == length)
@{
length *= 2;
- symbuf = (char *)realloc (symbuf, length + 1);
+ symbuf = (char *) realloc (symbuf, length + 1);
@}
/* Add this character to the buffer. */
symbuf[i++] = c;
@}
@end group
@group
- while (c != EOF && isalnum (c));
+ while (isalnum (c));
ungetc (c, stdin);
symbuf[i] = '\0';
return c;
@}
@end group
-@end smallexample
+@end example
+
+@node Mfcalc Main
+@subsection The @code{mfcalc} Main
+
+The error reporting function is unchanged, and the new version of
+@code{main} includes a call to @code{init_table}:
+
+@comment file: mfcalc.y
+@example
+@group
+/* Called by yyparse on error. */
+void
+yyerror (char const *s)
+@{
+ fprintf (stderr, "%s\n", s);
+@}
+@end group
+
+@group
+int
+main (int argc, char const* argv[])
+@{
+ init_table ();
+ return yyparse ();
+@}
+@end group
+@end example
-This program is both powerful and flexible. You may easily add new
-functions, and it is a simple job to modify this code to install predefined
-variables such as @code{pi} or @code{e} as well.
+This program is both powerful and flexible. You may easily add new
+functions, and it is a simple job to modify this code to install
+predefined variables such as @code{pi} or @code{e} as well.
@node Exercises
@section Exercises
Bison takes as input a context-free grammar specification and produces a
C-language function that recognizes correct instances of the grammar.
-The Bison grammar input file conventionally has a name ending in @samp{.y}.
+The Bison grammar file conventionally has a name ending in @samp{.y}.
@xref{Invocation, ,Invoking Bison}.
@menu
-* Grammar Outline:: Overall layout of the grammar file.
-* Symbols:: Terminal and nonterminal symbols.
-* Rules:: How to write grammar rules.
-* Recursion:: Writing recursive rules.
-* Semantics:: Semantic values and actions.
-* Locations:: Locations and actions.
-* Declarations:: All kinds of Bison declarations are described here.
-* Multiple Parsers:: Putting more than one Bison parser in one program.
+* Grammar Outline:: Overall layout of the grammar file.
+* Symbols:: Terminal and nonterminal symbols.
+* Rules:: How to write grammar rules.
+* Recursion:: Writing recursive rules.
+* Semantics:: Semantic values and actions.
+* Tracking Locations:: Locations and actions.
+* Named References:: Using named references in actions.
+* Declarations:: All kinds of Bison declarations are described here.
+* Multiple Parsers:: Putting more than one Bison parser in one program.
@end menu
@node Grammar Outline
@example
%@{
-@var{Prologue}
+ @var{Prologue}
%@}
@var{Bison declarations}
@end example
Comments enclosed in @samp{/* @dots{} */} may appear in any of the sections.
+As a GNU extension, @samp{//} introduces a comment that
+continues until end of line.
@menu
-* Prologue:: Syntax and usage of the prologue.
-* Bison Declarations:: Syntax and usage of the Bison declarations section.
-* Grammar Rules:: Syntax and usage of the grammar rules section.
-* Epilogue:: Syntax and usage of the epilogue.
+* Prologue:: Syntax and usage of the prologue.
+* Prologue Alternatives:: Syntax and usage of alternatives to the prologue.
+* Bison Declarations:: Syntax and usage of the Bison declarations section.
+* Grammar Rules:: Syntax and usage of the grammar rules section.
+* Epilogue:: Syntax and usage of the epilogue.
@end menu
-@node Prologue, Bison Declarations, , Grammar Outline
+@node Prologue
@subsection The prologue
@cindex declarations section
@cindex Prologue
@cindex declarations
-The @var{prologue} section contains macro definitions and
-declarations of functions and variables that are used in the actions in the
-grammar rules. These are copied to the beginning of the parser file so
-that they precede the definition of @code{yyparse}. You can use
-@samp{#include} to get the declarations from a header file. If you don't
-need any C declarations, you may omit the @samp{%@{} and @samp{%@}}
-delimiters that bracket this section.
+The @var{Prologue} section contains macro definitions and declarations
+of functions and variables that are used in the actions in the grammar
+rules. These are copied to the beginning of the parser implementation
+file so that they precede the definition of @code{yyparse}. You can
+use @samp{#include} to get the declarations from a header file. If
+you don't need any C declarations, you may omit the @samp{%@{} and
+@samp{%@}} delimiters that bracket this section.
+
+The @var{Prologue} section is terminated by the first occurrence
+of @samp{%@}} that is outside a comment, a string literal, or a
+character constant.
+
+You may have more than one @var{Prologue} section, intermixed with the
+@var{Bison declarations}. This allows you to have C and Bison
+declarations that refer to each other. For example, the @code{%union}
+declaration may use types defined in a header file, and you may wish to
+prototype functions that take arguments of type @code{YYSTYPE}. This
+can be done with two @var{Prologue} blocks, one before and one after the
+@code{%union} declaration.
+
+@example
+%@{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+ #include "ptypes.h"
+%@}
+
+%union @{
+ long int n;
+ tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%@{
+ static void print_token_value (FILE *, int, YYSTYPE);
+ #define YYPRINT(F, N, L) print_token_value (F, N, L)
+%@}
+
+@dots{}
+@end example
+
+When in doubt, it is usually safer to put prologue code before all
+Bison declarations, rather than after. For example, any definitions
+of feature test macros like @code{_GNU_SOURCE} or
+@code{_POSIX_C_SOURCE} should appear before all Bison declarations, as
+feature test macros can affect the behavior of Bison-generated
+@code{#include} directives.
+
+@node Prologue Alternatives
+@subsection Prologue Alternatives
+@cindex Prologue Alternatives
+
+@findex %code
+@findex %code requires
+@findex %code provides
+@findex %code top
+
+The functionality of @var{Prologue} sections can often be subtle and
+inflexible. As an alternative, Bison provides a @code{%code}
+directive with an explicit qualifier field, which identifies the
+purpose of the code and thus the location(s) where Bison should
+generate it. For C/C++, the qualifier can be omitted for the default
+location, or it can be one of @code{requires}, @code{provides},
+@code{top}. @xref{%code Summary}.
+
+Look again at the example of the previous section:
+
+@example
+%@{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+ #include "ptypes.h"
+%@}
+
+%union @{
+ long int n;
+ tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%@{
+ static void print_token_value (FILE *, int, YYSTYPE);
+ #define YYPRINT(F, N, L) print_token_value (F, N, L)
+%@}
+
+@dots{}
+@end example
+
+@noindent
+Notice that there are two @var{Prologue} sections here, but there's a
+subtle distinction between their functionality. For example, if you
+decide to override Bison's default definition for @code{YYLTYPE}, in
+which @var{Prologue} section should you write your new definition?
+You should write it in the first since Bison will insert that code
+into the parser implementation file @emph{before} the default
+@code{YYLTYPE} definition. In which @var{Prologue} section should you
+prototype an internal function, @code{trace_token}, that accepts
+@code{YYLTYPE} and @code{yytokentype} as arguments? You should
+prototype it in the second since Bison will insert that code
+@emph{after} the @code{YYLTYPE} and @code{yytokentype} definitions.
+
+This distinction in functionality between the two @var{Prologue} sections is
+established by the appearance of the @code{%union} between them.
+This behavior raises a few questions.
+First, why should the position of a @code{%union} affect definitions related to
+@code{YYLTYPE} and @code{yytokentype}?
+Second, what if there is no @code{%union}?
+In that case, the second kind of @var{Prologue} section is not available.
+This behavior is not intuitive.
+
+To avoid this subtle @code{%union} dependency, rewrite the example using a
+@code{%code top} and an unqualified @code{%code}.
+Let's go ahead and add the new @code{YYLTYPE} definition and the
+@code{trace_token} prototype at the same time:
+
+@example
+%code top @{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+
+ /* WARNING: The following code really belongs
+ * in a `%code requires'; see below. */
+
+ #include "ptypes.h"
+ #define YYLTYPE YYLTYPE
+ typedef struct YYLTYPE
+ @{
+ int first_line;
+ int first_column;
+ int last_line;
+ int last_column;
+ char *filename;
+ @} YYLTYPE;
+@}
+
+%union @{
+ long int n;
+ tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%code @{
+ static void print_token_value (FILE *, int, YYSTYPE);
+ #define YYPRINT(F, N, L) print_token_value (F, N, L)
+ static void trace_token (enum yytokentype token, YYLTYPE loc);
+@}
+
+@dots{}
+@end example
+
+@noindent
+In this way, @code{%code top} and the unqualified @code{%code} achieve the same
+functionality as the two kinds of @var{Prologue} sections, but it's always
+explicit which kind you intend.
+Moreover, both kinds are always available even in the absence of @code{%union}.
+
+The @code{%code top} block above logically contains two parts. The
+first two lines before the warning need to appear near the top of the
+parser implementation file. The first line after the warning is
+required by @code{YYSTYPE} and thus also needs to appear in the parser
+implementation file. However, if you've instructed Bison to generate
+a parser header file (@pxref{Decl Summary, ,%defines}), you probably
+want that line to appear before the @code{YYSTYPE} definition in that
+header file as well. The @code{YYLTYPE} definition should also appear
+in the parser header file to override the default @code{YYLTYPE}
+definition there.
+
+In other words, in the @code{%code top} block above, all but the first two
+lines are dependency code required by the @code{YYSTYPE} and @code{YYLTYPE}
+definitions.
+Thus, they belong in one or more @code{%code requires}:
+
+@example
+@group
+%code top @{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+@}
+@end group
+
+@group
+%code requires @{
+ #include "ptypes.h"
+@}
+@end group
+@group
+%union @{
+ long int n;
+ tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+@end group
+
+@group
+%code requires @{
+ #define YYLTYPE YYLTYPE
+ typedef struct YYLTYPE
+ @{
+ int first_line;
+ int first_column;
+ int last_line;
+ int last_column;
+ char *filename;
+ @} YYLTYPE;
+@}
+@end group
+
+@group
+%code @{
+ static void print_token_value (FILE *, int, YYSTYPE);
+ #define YYPRINT(F, N, L) print_token_value (F, N, L)
+ static void trace_token (enum yytokentype token, YYLTYPE loc);
+@}
+@end group
+
+@dots{}
+@end example
+
+@noindent
+Now Bison will insert @code{#include "ptypes.h"} and the new
+@code{YYLTYPE} definition before the Bison-generated @code{YYSTYPE}
+and @code{YYLTYPE} definitions in both the parser implementation file
+and the parser header file. (By the same reasoning, @code{%code
+requires} would also be the appropriate place to write your own
+definition for @code{YYSTYPE}.)
+
+When you are writing dependency code for @code{YYSTYPE} and
+@code{YYLTYPE}, you should prefer @code{%code requires} over
+@code{%code top} regardless of whether you instruct Bison to generate
+a parser header file. When you are writing code that you need Bison
+to insert only into the parser implementation file and that has no
+special need to appear at the top of that file, you should prefer the
+unqualified @code{%code} over @code{%code top}. These practices will
+make the purpose of each block of your code explicit to Bison and to
+other developers reading your grammar file. Following these
+practices, we expect the unqualified @code{%code} and @code{%code
+requires} to be the most important of the four @var{Prologue}
+alternatives.
+
+At some point while developing your parser, you might decide to
+provide @code{trace_token} to modules that are external to your
+parser. Thus, you might wish for Bison to insert the prototype into
+both the parser header file and the parser implementation file. Since
+this function is not a dependency required by @code{YYSTYPE} or
+@code{YYLTYPE}, it doesn't make sense to move its prototype to a
+@code{%code requires}. More importantly, since it depends upon
+@code{YYLTYPE} and @code{yytokentype}, @code{%code requires} is not
+sufficient. Instead, move its prototype from the unqualified
+@code{%code} to a @code{%code provides}:
+
+@example
+@group
+%code top @{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+@}
+@end group
+
+@group
+%code requires @{
+ #include "ptypes.h"
+@}
+@end group
+@group
+%union @{
+ long int n;
+ tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+@end group
+
+@group
+%code requires @{
+ #define YYLTYPE YYLTYPE
+ typedef struct YYLTYPE
+ @{
+ int first_line;
+ int first_column;
+ int last_line;
+ int last_column;
+ char *filename;
+ @} YYLTYPE;
+@}
+@end group
+
+@group
+%code provides @{
+ void trace_token (enum yytokentype token, YYLTYPE loc);
+@}
+@end group
+
+@group
+%code @{
+ static void print_token_value (FILE *, int, YYSTYPE);
+ #define YYPRINT(F, N, L) print_token_value (F, N, L)
+@}
+@end group
+
+@dots{}
+@end example
+
+@noindent
+Bison will insert the @code{trace_token} prototype into both the
+parser header file and the parser implementation file after the
+definitions for @code{yytokentype}, @code{YYLTYPE}, and
+@code{YYSTYPE}.
+
+The above examples are careful to write directives in an order that
+reflects the layout of the generated parser implementation and header
+files: @code{%code top}, @code{%code requires}, @code{%code provides},
+and then @code{%code}. While your grammar files may generally be
+easier to read if you also follow this order, Bison does not require
+it. Instead, Bison lets you choose an organization that makes sense
+to you.
+
+You may declare any of these directives multiple times in the grammar file.
+In that case, Bison concatenates the contained code in declaration order.
+This is the only way in which the position of one of these directives within
+the grammar file affects its functionality.
+
+The result of the previous two properties is greater flexibility in how you may
+organize your grammar file.
+For example, you may organize semantic-type-related directives by semantic
+type:
+
+@example
+@group
+%code requires @{ #include "type1.h" @}
+%union @{ type1 field1; @}
+%destructor @{ type1_free ($$); @} <field1>
+%printer @{ type1_print ($$); @} <field1>
+@end group
+
+@group
+%code requires @{ #include "type2.h" @}
+%union @{ type2 field2; @}
+%destructor @{ type2_free ($$); @} <field2>
+%printer @{ type2_print ($$); @} <field2>
+@end group
+@end example
+
+@noindent
+You could even place each of the above directive groups in the rules section of
+the grammar file next to the set of rules that uses the associated semantic
+type.
+(In the rules section, you must terminate each of those directives with a
+semicolon.)
+And you don't have to worry that some directive (like a @code{%union}) in the
+definitions section is going to adversely affect their functionality in some
+counter-intuitive manner just because it comes first.
+Such an organization is not possible using @var{Prologue} sections.
+
+This section has been concerned with explaining the advantages of the four
+@var{Prologue} alternatives over the original Yacc @var{Prologue}.
+However, in most cases when using these directives, you shouldn't need to
+think about all the low-level ordering issues discussed here.
+Instead, you should simply use these directives to label each block of your
+code according to its purpose and let Bison handle the ordering.
+@code{%code} is the most generic label.
+Move code to @code{%code requires}, @code{%code provides}, or @code{%code top}
+as needed.
@node Bison Declarations
@subsection The Bison Declarations Section
@samp{%%} (which precedes the grammar rules) may never be omitted even
if it is the first thing in the file.
-@node Epilogue, , Grammar Rules, Grammar Outline
+@node Epilogue
@subsection The epilogue
@cindex additional C code section
@cindex epilogue
@cindex C code, section for additional
-The @var{epilogue} is copied verbatim to the end of the parser file, just as
-the @var{prologue} is copied to the beginning. This is the most convenient
-place to put anything that you want to have in the parser file but which need
-not come before the definition of @code{yyparse}. For example, the
-definitions of @code{yylex} and @code{yyerror} often go here.
-@xref{Interface, ,Parser C-Language Interface}.
+The @var{Epilogue} is copied verbatim to the end of the parser
+implementation file, just as the @var{Prologue} is copied to the
+beginning. This is the most convenient place to put anything that you
+want to have in the parser implementation file but which need not come
+before the definition of @code{yyparse}. For example, the definitions
+of @code{yylex} and @code{yyerror} often go here. Because C requires
+functions to be declared before being used, you often need to declare
+functions like @code{yylex} and @code{yyerror} in the Prologue, even
+if you define them in the Epilogue. @xref{Interface, ,Parser
+C-Language Interface}.
If the last section is empty, you may omit the @samp{%%} that separates it
from the grammar rules.
-The Bison parser itself contains many static variables whose names start
-with @samp{yy} and many macros whose names start with @samp{YY}. It is a
-good idea to avoid using any such names (except those documented in this
-manual) in the epilogue of the grammar file.
+The Bison parser itself contains many macros and identifiers whose names
+start with @samp{yy} or @samp{YY}, so it is a good idea to avoid using
+any such names (except those documented in this manual) in the epilogue
+of the grammar file.
@node Symbols
@section Symbols, Terminal and Nonterminal
class of syntactically equivalent tokens. You use the symbol in grammar
rules to mean that a token in that class is allowed. The symbol is
represented in the Bison parser by a numeric code, and the @code{yylex}
-function returns a token type code to indicate what kind of token has been
-read. You don't need to know what the code value is; you can use the
-symbol to stand for it.
-
-A @dfn{nonterminal symbol} stands for a class of syntactically equivalent
-groupings. The symbol name is used in writing grammar rules. By convention,
-it should be all lower case.
-
-Symbol names can contain letters, digits (not at the beginning),
-underscores and periods. Periods make sense only in nonterminals.
+function returns a token type code to indicate what kind of token has
+been read. You don't need to know what the code value is; you can use
+the symbol to stand for it.
+
+A @dfn{nonterminal symbol} stands for a class of syntactically
+equivalent groupings. The symbol name is used in writing grammar rules.
+By convention, it should be all lower case.
+
+Symbol names can contain letters, underscores, periods, and non-initial
+digits and dashes. Dashes in symbol names are a GNU extension, incompatible
+with POSIX Yacc. Periods and dashes make symbol names less convenient to
+use with named references, which require brackets around such names
+(@pxref{Named References}). Terminal symbols that contain periods or dashes
+make little sense: since they are not valid symbols (in most programming
+languages) they are not exported as token names.
There are three ways of writing terminal symbols in the grammar:
@itemize @bullet
@item
A @dfn{named token type} is written with an identifier, like an
-identifier in C. By convention, it should be all upper case. Each
+identifier in C@. By convention, it should be all upper case. Each
such name must be defined with a Bison declaration such as
@code{%token}. @xref{Token Decl, ,Token Type Names}.
All the usual escape sequences used in character literals in C can be
used in Bison as well, but you must not use the null character as a
-character literal because its ASCII code, zero, is the code @code{yylex}
-returns for end-of-input (@pxref{Calling Convention, ,Calling Convention
-for @code{yylex}}).
+character literal because its numeric code, zero, signifies
+end-of-input (@pxref{Calling Convention, ,Calling Convention
+for @code{yylex}}). Also, unlike standard C, trigraphs have no
+special meaning in Bison character literals, nor is backslash-newline
+allowed.
@item
@cindex string token
retrieve the token number for the literal string token from the
@code{yytname} table (@pxref{Calling Convention}).
-@strong{WARNING}: literal string tokens do not work in Yacc.
+@strong{Warning}: literal string tokens do not work in Yacc.
By convention, a literal string token is used only to represent a token
that consists of that particular string. Thus, you should use the token
read your program will be confused.
All the escape sequences used in string literals in C can be used in
-Bison as well. A literal string token must contain two or more
-characters; for a token containing just one character, use a character
-token (see above).
+Bison as well, except that you must not use a null character within a
+string literal. Also, unlike Standard C, trigraphs have no special
+meaning in Bison string literals, nor is backslash-newline allowed. A
+literal string token must contain two or more characters; for a token
+containing just one character, use a character token (see above).
@end itemize
How you choose to write a terminal symbol has no effect on its
grammatical meaning. That depends only on where it appears in rules and
on when the parser function returns that symbol.
-The value returned by @code{yylex} is always one of the terminal symbols
-(or 0 for end-of-input). Whichever way you write the token type in the
-grammar rules, you write it the same way in the definition of @code{yylex}.
-The numeric code for a character token type is simply the ASCII code for
-the character, so @code{yylex} can use the identical character constant to
-generate the requisite code. Each named token type becomes a C macro in
-the parser file, so @code{yylex} can use the name to stand for the code.
-(This is why periods don't make sense in terminal symbols.)
-@xref{Calling Convention, ,Calling Convention for @code{yylex}}.
+The value returned by @code{yylex} is always one of the terminal
+symbols, except that a zero or negative value signifies end-of-input.
+Whichever way you write the token type in the grammar rules, you write
+it the same way in the definition of @code{yylex}. The numeric code
+for a character token type is simply the positive numeric code of the
+character, so @code{yylex} can use the identical value to generate the
+requisite code, though you may need to convert it to @code{unsigned
+char} to avoid sign-extension on hosts where @code{char} is signed.
+Each named token type becomes a C macro in the parser implementation
+file, so @code{yylex} can use the name to stand for the code. (This
+is why periods don't make sense in terminal symbols.) @xref{Calling
+Convention, ,Calling Convention for @code{yylex}}.
If @code{yylex} is defined in a separate file, you need to arrange for the
token-type macro definitions to be available there. Use the @samp{-d}
into a separate header file @file{@var{name}.tab.h} which you can include
in the other source files that need it. @xref{Invocation, ,Invoking Bison}.
+If you want to write a grammar that is portable to any Standard C
+host, you must use only nonnull character tokens taken from the basic
+execution character set of Standard C@. This set consists of the ten
+digits, the 52 lower- and upper-case English letters, and the
+characters in the following C-language string:
+
+@example
+"\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_@{|@}~"
+@end example
+
+The @code{yylex} function and Bison must use a consistent character set
+and encoding for character tokens. For example, if you run Bison in an
+ASCII environment, but then compile and run the resulting
+program in an environment that uses an incompatible character set like
+EBCDIC, the resulting program may not work because the tables
+generated by Bison will assume ASCII numeric values for
+character tokens. It is standard practice for software distributions to
+contain C source files that were generated by Bison in an
+ASCII environment, so installers on platforms that are
+incompatible with ASCII must rebuild those files before
+compiling them.
+
The symbol @code{error} is a terminal symbol reserved for error recovery
(@pxref{Error Recovery}); you shouldn't use it for any other purpose.
-In particular, @code{yylex} should never return this value.
+In particular, @code{yylex} should never return this value. The default
+value of the error token is 256, unless you explicitly assigned 256 to
+one of your tokens with a @code{%token} declaration.
@node Rules
@section Syntax of Grammar Rules
@example
@group
-@var{result}: @var{components}@dots{}
- ;
+@var{result}: @var{components}@dots{};
@end group
@end example
@example
@group
-exp: exp '+' exp
- ;
+exp: exp '+' exp;
@end group
@end example
says that two groupings of type @code{exp}, with a @samp{+} token in between,
can be combined into a larger grouping of type @code{exp}.
-Whitespace in rules is significant only to separate symbols. You can add
-extra whitespace as you wish.
+White space in rules is significant only to separate symbols. You can add
+extra white space as you wish.
Scattered among the components can be @var{actions} that determine
the semantics of the rule. An action looks like this:
@end example
@noindent
+@cindex braced code
+This is an example of @dfn{braced code}, that is, C code surrounded by
+braces, much like a compound statement in C@. Braced code can contain
+any sequence of C tokens, so long as its braces are balanced. Bison
+does not check the braced code for correctness directly; it merely
+copies the code to the parser implementation file, where the C
+compiler can check it.
+
+Within braced code, the balanced-brace count is not affected by braces
+within comments, string literals, or character constants, but it is
+affected by the C digraphs @samp{<%} and @samp{%>} that represent
+braces. At the top level braced code must be terminated by @samp{@}}
+and not by a digraph. Bison does not look for trigraphs, so if braced
+code uses trigraphs you should ensure that they do not affect the
+nesting of braces or the boundaries of comments, string literals, or
+character constants.
+
Usually there is only one action and it follows the components.
@xref{Actions}.
Multiple rules for the same @var{result} can be written separately or can
be joined with the vertical-bar character @samp{|} as follows:
-@ifinfo
-@example
-@var{result}: @var{rule1-components}@dots{}
- | @var{rule2-components}@dots{}
- @dots{}
- ;
-@end example
-@end ifinfo
-@iftex
@example
@group
-@var{result}: @var{rule1-components}@dots{}
- | @var{rule2-components}@dots{}
- @dots{}
- ;
+@var{result}:
+ @var{rule1-components}@dots{}
+| @var{rule2-components}@dots{}
+@dots{}
+;
@end group
@end example
-@end iftex
@noindent
They are still considered distinct rules even when joined in this way.
@example
@group
-expseq: /* empty */
- | expseq1
- ;
+expseq:
+ /* empty */
+| expseq1
+;
@end group
@group
-expseq1: exp
- | expseq1 ',' exp
- ;
+expseq1:
+ exp
+| expseq1 ',' exp
+;
@end group
@end example
@section Recursive Rules
@cindex recursive rule
-A rule is called @dfn{recursive} when its @var{result} nonterminal appears
-also on its right hand side. Nearly all Bison grammars need to use
-recursion, because that is the only way to define a sequence of any number
-of a particular thing. Consider this recursive definition of a
+A rule is called @dfn{recursive} when its @var{result} nonterminal
+appears also on its right hand side. Nearly all Bison grammars need to
+use recursion, because that is the only way to define a sequence of any
+number of a particular thing. Consider this recursive definition of a
comma-separated sequence of one or more expressions:
@example
@group
-expseq1: exp
- | expseq1 ',' exp
- ;
+expseq1:
+ exp
+| expseq1 ',' exp
+;
@end group
@end example
@example
@group
-expseq1: exp
- | exp ',' expseq1
- ;
+expseq1:
+ exp
+| exp ',' expseq1
+;
@end group
@end example
@noindent
-Any kind of sequence can be defined using either left recursion or
-right recursion, but you should always use left recursion, because it
-can parse a sequence of any number of elements with bounded stack
-space. Right recursion uses up space on the Bison stack in proportion
-to the number of elements in the sequence, because all the elements
-must be shifted onto the stack before the rule can be applied even
-once. @xref{Algorithm, ,The Bison Parser Algorithm }, for
-further explanation of this.
+Any kind of sequence can be defined using either left recursion or right
+recursion, but you should always use left recursion, because it can
+parse a sequence of any number of elements with bounded stack space.
+Right recursion uses up space on the Bison stack in proportion to the
+number of elements in the sequence, because all the elements must be
+shifted onto the stack before the rule can be applied even once.
+@xref{Algorithm, ,The Bison Parser Algorithm}, for further explanation
+of this.
@cindex mutual recursion
@dfn{Indirect} or @dfn{mutual} recursion occurs when the result of the
@example
@group
-expr: primary
- | primary '+' primary
- ;
+expr:
+ primary
+| primary '+' primary
+;
@end group
@group
-primary: constant
- | '(' expr ')'
- ;
+primary:
+ constant
+| '(' expr ')'
+;
@end group
@end example
RPN and infix calculator examples (@pxref{RPN Calc, ,Reverse Polish
Notation Calculator}).
-Bison's default is to use type @code{int} for all semantic values. To
+Bison normally uses the type @code{int} for semantic values if your
+program uses the same data type for all language constructs. To
specify some other type, define @code{YYSTYPE} as a macro, like this:
@example
@end example
@noindent
+@code{YYSTYPE}'s replacement list should be a type name
+that does not contain parentheses or square brackets.
This macro definition must go in the prologue of the grammar file
(@pxref{Grammar Outline, ,Outline of a Bison Grammar}).
In most programs, you will need different data types for different kinds
of tokens and groupings. For example, a numeric constant may need type
-@code{int} or @code{long}, while a string constant needs type @code{char *},
-and an identifier might need a pointer to an entry in the symbol table.
+@code{int} or @code{long int}, while a string constant needs type
+@code{char *}, and an identifier might need a pointer to an entry in the
+symbol table.
To use more than one data type for semantic values in one parser, Bison
requires you to do two things:
@itemize @bullet
@item
-Specify the entire collection of possible data types, with the
-@code{%union} Bison declaration (@pxref{Union Decl, ,The Collection of Value Types}).
+Specify the entire collection of possible data types, either by using the
+@code{%union} Bison declaration (@pxref{Union Decl, ,The Collection of
+Value Types}), or by using a @code{typedef} or a @code{#define} to
+define @code{YYSTYPE} to be a union type whose member names are
+the type tags.
@item
Choose one of those types for each symbol (terminal or nonterminal) for
@cindex action
@vindex $$
@vindex $@var{n}
+@vindex $@var{name}
+@vindex $[@var{name}]
An action accompanies a syntactic rule and contains C code to be executed
each time an instance of that rule is recognized. The task of most actions
is to compute a semantic value for the grouping built by the rule from the
semantic values associated with tokens or smaller groupings.
-An action consists of C statements surrounded by braces, much like a
-compound statement in C. It can be placed at any position in the rule; it
-is executed at that position. Most rules have just one action at the end
-of the rule, following all the components. Actions in the middle of a rule
-are tricky and used only for special purposes (@pxref{Mid-Rule Actions, ,Actions in Mid-Rule}).
-
-The C code in an action can refer to the semantic values of the components
-matched by the rule with the construct @code{$@var{n}}, which stands for
-the value of the @var{n}th component. The semantic value for the grouping
-being constructed is @code{$$}. (Bison translates both of these constructs
-into array element references when it copies the actions into the parser
-file.)
+An action consists of braced code containing C statements, and can be
+placed at any position in the rule;
+it is executed at that position. Most rules have just one action at the
+end of the rule, following all the components. Actions in the middle of
+a rule are tricky and used only for special purposes (@pxref{Mid-Rule
+Actions, ,Actions in Mid-Rule}).
+
+The C code in an action can refer to the semantic values of the
+components matched by the rule with the construct @code{$@var{n}},
+which stands for the value of the @var{n}th component. The semantic
+value for the grouping being constructed is @code{$$}. In addition,
+the semantic values of symbols can be accessed with the named
+references construct @code{$@var{name}} or @code{$[@var{name}]}.
+Bison translates both of these constructs into expressions of the
+appropriate type when it copies the actions into the parser
+implementation file. @code{$$} (or @code{$@var{name}}, when it stands
+for the current grouping) is translated to a modifiable lvalue, so it
+can be assigned to.
Here is a typical example:
@example
@group
-exp: @dots{}
- | exp '+' exp
- @{ $$ = $1 + $3; @}
+exp:
+@dots{}
+| exp '+' exp @{ $$ = $1 + $3; @}
+@end group
+@end example
+
+Or, in terms of named references:
+
+@example
+@group
+exp[result]:
+@dots{}
+| exp[left] '+' exp[right] @{ $result = $left + $right; @}
@end group
@end example
@noindent
This rule constructs an @code{exp} from two smaller @code{exp} groupings
connected by a plus-sign token. In the action, @code{$1} and @code{$3}
+(@code{$left} and @code{$right})
refer to the semantic values of the two component @code{exp} groupings,
which are the first and third symbols on the right hand side of the rule.
-The sum is stored into @code{$$} so that it becomes the semantic value of
+The sum is stored into @code{$$} (@code{$result}) so that it becomes the
+semantic value of
the addition-expression just recognized by the rule. If there were a
useful semantic value associated with the @samp{+} token, it could be
-referred to as @code{$2}.@refill
+referred to as @code{$2}.
-@cindex default action
-If you don't specify an action for a rule, Bison supplies a default:
-@w{@code{$$ = $1}.} Thus, the value of the first symbol in the rule becomes
-the value of the whole rule. Of course, the default rule is valid only
-if the two data types match. There is no meaningful default action for
-an empty rule; every empty rule must have an explicit action unless the
-rule's value does not matter.
+@xref{Named References}, for more information about using the named
+references construct.
-@code{$@var{n}} with @var{n} zero or negative is allowed for reference
+Note that the vertical-bar character @samp{|} is really a rule
+separator, and actions are attached to a single rule. This is a
+difference with tools like Flex, for which @samp{|} stands for either
+``or'', or ``the same action as that of the next rule''. In the
+following example, the action is triggered only when @samp{b} is found:
+
+@example
+@group
+a-or-b: 'a'|'b' @{ a_or_b_found = 1; @};
+@end group
+@end example
+
+@cindex default action
+If you don't specify an action for a rule, Bison supplies a default:
+@w{@code{$$ = $1}.} Thus, the value of the first symbol in the rule
+becomes the value of the whole rule. Of course, the default action is
+valid only if the two data types match. There is no meaningful default
+action for an empty rule; every empty rule must have an explicit action
+unless the rule's value does not matter.
+
+@code{$@var{n}} with @var{n} zero or negative is allowed for reference
to tokens and groupings on the stack @emph{before} those that match the
current rule. This is a very risky practice, and to use it reliably
you must be certain of the context in which the rule is applied. Here
@example
@group
-foo: expr bar '+' expr @{ @dots{} @}
- | expr bar '-' expr @{ @dots{} @}
- ;
+foo:
+ expr bar '+' expr @{ @dots{} @}
+| expr bar '-' expr @{ @dots{} @}
+;
@end group
@group
-bar: /* empty */
- @{ previous_expr = $0; @}
- ;
+bar:
+ /* empty */ @{ previous_expr = $0; @}
+;
@end group
@end example
always refers to the @code{expr} which precedes @code{bar} in the
definition of @code{foo}.
+@vindex yylval
+It is also possible to access the semantic value of the lookahead token, if
+any, from a semantic action.
+This semantic value is stored in @code{yylval}.
+@xref{Action Features, ,Special Features for Use in Actions}.
+
@node Action Types
@subsection Data Types of Values in Actions
@cindex action data types
must declare a choice among these types for each terminal or nonterminal
symbol that can have a semantic value. Then each time you use @code{$$} or
@code{$@var{n}}, its data type is determined by which symbol it refers to
-in the rule. In this example,@refill
+in the rule. In this example,
@example
@group
-exp: @dots{}
- | exp '+' exp
- @{ $$ = $1 + $3; @}
+exp:
+ @dots{}
+| exp '+' exp @{ $$ = $1 + $3; @}
@end group
@end example
@code{$1} and @code{$3} refer to instances of @code{exp}, so they all
have the data type declared for the nonterminal symbol @code{exp}. If
@code{$2} were used, it would have the data type declared for the
-terminal symbol @code{'+'}, whatever that might be.@refill
+terminal symbol @code{'+'}, whatever that might be.
Alternatively, you can specify the data type when you refer to the value,
by inserting @samp{<@var{type}>} after the @samp{$} at the beginning of the
@example
@group
-stmt: LET '(' var ')'
- @{ $<context>$ = push_context ();
- declare_variable ($3); @}
- stmt @{ $$ = $6;
- pop_context ($<context>5); @}
+stmt:
+ LET '(' var ')'
+ @{ $<context>$ = push_context (); declare_variable ($3); @}
+ stmt
+ @{ $$ = $6; pop_context ($<context>5); @}
@end group
@end example
removes the temporary @code{let}-variable from the list so that it won't
appear to exist while the rest of the program is parsed.
+@findex %destructor
+@cindex discarded symbols, mid-rule actions
+@cindex error recovery, mid-rule actions
+In the above example, if the parser initiates error recovery (@pxref{Error
+Recovery}) while parsing the tokens in the embedded statement @code{stmt},
+it might discard the previous semantic context @code{$<context>5} without
+restoring it.
+Thus, @code{$<context>5} needs a destructor (@pxref{Destructor Decl, , Freeing
+Discarded Symbols}).
+However, Bison currently provides no means to declare a destructor specific to
+a particular mid-rule action's semantic value.
+
+One solution is to bury the mid-rule action inside a nonterminal symbol and to
+declare a destructor for that symbol:
+
+@example
+@group
+%type <context> let
+%destructor @{ pop_context ($$); @} let
+
+%%
+
+stmt:
+ let stmt
+ @{
+ $$ = $2;
+ pop_context ($1);
+ @};
+
+let:
+ LET '(' var ')'
+ @{
+ $$ = push_context ();
+ declare_variable ($3);
+ @};
+
+@end group
+@end example
+
+@noindent
+Note that the action is now at the end of its rule.
+Any mid-rule action can be converted to an end-of-rule action in this way, and
+this is what Bison actually does to implement mid-rule actions.
+
Taking action before a rule is completely recognized often leads to
conflicts since the parser must commit to a parse in order to execute the
action. For example, the following two rules, without mid-rule actions,
@example
@group
-compound: '@{' declarations statements '@}'
- | '@{' statements '@}'
- ;
+compound:
+ '@{' declarations statements '@}'
+| '@{' statements '@}'
+;
@end group
@end example
@example
@group
-compound: @{ prepare_for_local_variables (); @}
- '@{' declarations statements '@}'
+compound:
+ @{ prepare_for_local_variables (); @}
+ '@{' declarations statements '@}'
@end group
@group
- | '@{' statements '@}'
- ;
+| '@{' statements '@}'
+;
@end group
@end example
when it has read no farther than the open-brace. In other words, it
must commit to using one rule or the other, without sufficient
information to do it correctly. (The open-brace token is what is called
-the @dfn{look-ahead} token at this time, since the parser is still
-deciding what to do about it. @xref{Look-Ahead, ,Look-Ahead Tokens}.)
+the @dfn{lookahead} token at this time, since the parser is still
+deciding what to do about it. @xref{Lookahead, ,Lookahead Tokens}.)
You might think that you could correct the problem by putting identical
actions into the two rules, like this:
@example
@group
-compound: @{ prepare_for_local_variables (); @}
- '@{' declarations statements '@}'
- | @{ prepare_for_local_variables (); @}
- '@{' statements '@}'
- ;
+compound:
+ @{ prepare_for_local_variables (); @}
+ '@{' declarations statements '@}'
+| @{ prepare_for_local_variables (); @}
+ '@{' statements '@}'
+;
@end group
@end example
@example
@group
-compound: '@{' @{ prepare_for_local_variables (); @}
- declarations statements '@}'
- | '@{' statements '@}'
- ;
+compound:
+ '@{' @{ prepare_for_local_variables (); @}
+ declarations statements '@}'
+| '@{' statements '@}'
+;
@end group
@end example
@example
@group
-subroutine: /* empty */
- @{ prepare_for_local_variables (); @}
- ;
-
+subroutine:
+ /* empty */ @{ prepare_for_local_variables (); @}
+;
@end group
@group
-compound: subroutine
- '@{' declarations statements '@}'
- | subroutine
- '@{' statements '@}'
- ;
+compound:
+ subroutine '@{' declarations statements '@}'
+| subroutine '@{' statements '@}'
+;
@end group
@end example
@noindent
Now Bison can execute the action in the rule for @code{subroutine} without
-deciding which rule for @code{compound} it will eventually use. Note that
-the action is now at the end of its rule. Any mid-rule action can be
-converted to an end-of-rule action in this way, and this is what Bison
-actually does to implement mid-rule actions.
+deciding which rule for @code{compound} it will eventually use.
-@node Locations
+@node Tracking Locations
@section Tracking Locations
@cindex location
-@cindex textual position
-@cindex position, textual
+@cindex textual location
+@cindex location, textual
Though grammar rules and semantic actions are enough to write a fully
-functional parser, it can be useful to process some additionnal informations,
+functional parser, it can be useful to process some additional information,
especially symbol locations.
-@c (terminal or not) ?
-
-The way locations are handled is defined by providing a data type, and actions
-to take when rules are matched.
+The way locations are handled is defined by providing a data type, and
+actions to take when rules are matched.
@menu
* Location Type:: Specifying a data type for locations.
Defining a data type for locations is much simpler than for semantic values,
since all tokens and groupings always use the same type.
-The type of locations is specified by defining a macro called @code{YYLTYPE}.
+You can specify the type of locations by defining a macro called
+@code{YYLTYPE}, just as you can specify the semantic value type by
+defining a @code{YYSTYPE} macro (@pxref{Value Type}).
When @code{YYLTYPE} is not defined, Bison uses a default structure type with
four members:
@example
-struct
+typedef struct YYLTYPE
@{
int first_line;
int first_column;
int last_line;
int last_column;
-@}
+@} YYLTYPE;
@end example
+When @code{YYLTYPE} is not defined, at the beginning of the parsing, Bison
+initializes all these fields to 1 for @code{yylloc}. To initialize
+@code{yylloc} with a custom location type (or to chose a different
+initialization), use the @code{%initial-action} directive. @xref{Initial
+Action Decl, , Performing Actions before Parsing}.
+
@node Actions and Locations
@subsection Actions and Locations
@cindex location actions
@cindex actions, location
@vindex @@$
@vindex @@@var{n}
+@vindex @@@var{name}
+@vindex @@[@var{name}]
Actions are not only useful for defining language semantics, but also for
describing the behavior of the output parser with locations.
The most obvious way for building locations of syntactic groupings is very
-similar to the way semantic values are computed. In a given rule, several
+similar to the way semantic values are computed. In a given rule, several
constructs can be used to access the locations of the elements being matched.
The location of the @var{n}th component of the right hand side is
@code{@@@var{n}}, while the location of the left hand side grouping is
@code{@@$}.
+In addition, the named references construct @code{@@@var{name}} and
+@code{@@[@var{name}]} may also be used to address the symbol locations.
+@xref{Named References}, for more information about using the named
+references construct.
+
Here is a basic example using the default data type for locations:
@example
@group
-exp: @dots{}
- | exp '/' exp
- @{
- @@$.first_column = @@1.first_column;
- @@$.first_line = @@1.first_line;
- @@$.last_column = @@3.last_column;
- @@$.last_line = @@3.last_line;
- if ($3)
- $$ = $1 / $3;
- else
- @{
- $$ = 1;
- printf("Division by zero, l%d,c%d-l%d,c%d",
- @@3.first_line, @@3.first_column,
- @@3.last_line, @@3.last_column);
- @}
- @}
+exp:
+ @dots{}
+| exp '/' exp
+ @{
+ @@$.first_column = @@1.first_column;
+ @@$.first_line = @@1.first_line;
+ @@$.last_column = @@3.last_column;
+ @@$.last_line = @@3.last_line;
+ if ($3)
+ $$ = $1 / $3;
+ else
+ @{
+ $$ = 1;
+ fprintf (stderr,
+ "Division by zero, l%d,c%d-l%d,c%d",
+ @@3.first_line, @@3.first_column,
+ @@3.last_line, @@3.last_column);
+ @}
+ @}
@end group
@end example
As for semantic values, there is a default action for locations that is
-run each time a rule is matched. It sets the beginning of @code{@@$} to the
+run each time a rule is matched. It sets the beginning of @code{@@$} to the
beginning of the first symbol, and the end of @code{@@$} to the end of the
last symbol.
-With this default action, the location tracking can be fully automatic. The
+With this default action, the location tracking can be fully automatic. The
example above simply rewrites this way:
@example
@group
-exp: @dots{}
- | exp '/' exp
- @{
- if ($3)
- $$ = $1 / $3;
- else
- @{
- $$ = 1;
- printf("Division by zero, l%d,c%d-l%d,c%d",
- @@3.first_line, @@3.first_column,
- @@3.last_line, @@3.last_column);
- @}
- @}
+exp:
+ @dots{}
+| exp '/' exp
+ @{
+ if ($3)
+ $$ = $1 / $3;
+ else
+ @{
+ $$ = 1;
+ fprintf (stderr,
+ "Division by zero, l%d,c%d-l%d,c%d",
+ @@3.first_line, @@3.first_column,
+ @@3.last_line, @@3.last_column);
+ @}
+ @}
@end group
@end example
+@vindex yylloc
+It is also possible to access the location of the lookahead token, if any,
+from a semantic action.
+This location is stored in @code{yylloc}.
+@xref{Action Features, ,Special Features for Use in Actions}.
+
@node Location Default Action
@subsection Default Action for Locations
@vindex YYLLOC_DEFAULT
-
-Actually, actions are not the best place to compute locations. Since locations
-are much more general than semantic values, there is room in the output parser
-to redefine the default action to take for each rule. The
-@code{YYLLOC_DEFAULT} macro is called each time a rule is matched, before the
-associated action is run.
+@cindex GLR parsers and @code{YYLLOC_DEFAULT}
+
+Actually, actions are not the best place to compute locations. Since
+locations are much more general than semantic values, there is room in
+the output parser to redefine the default action to take for each
+rule. The @code{YYLLOC_DEFAULT} macro is invoked each time a rule is
+matched, before the associated action is run. It is also invoked
+while processing a syntax error, to compute the error's location.
+Before reporting an unresolvable syntactic ambiguity, a GLR
+parser invokes @code{YYLLOC_DEFAULT} recursively to compute the location
+of that ambiguity.
Most of the time, this macro is general enough to suppress location
dedicated code from semantic actions.
-The @code{YYLLOC_DEFAULT} macro takes three parameters. The first one is
-the location of the grouping (the result of the computation). The second one
-is an array holding locations of all right hand side elements of the rule
-being matched. The last one is the size of the right hand side rule.
+The @code{YYLLOC_DEFAULT} macro takes three parameters. The first one is
+the location of the grouping (the result of the computation). When a
+rule is matched, the second parameter identifies locations of
+all right hand side elements of the rule being matched, and the third
+parameter is the size of the rule's right hand side.
+When a GLR parser reports an ambiguity, which of multiple candidate
+right hand sides it passes to @code{YYLLOC_DEFAULT} is undefined.
+When processing a syntax error, the second parameter identifies locations
+of the symbols that were discarded during error processing, and the third
+parameter is the number of discarded symbols.
-By default, it is defined this way:
+By default, @code{YYLLOC_DEFAULT} is defined this way:
@example
@group
-#define YYLLOC_DEFAULT(Current, Rhs, N) \
- Current.last_line = Rhs[N].last_line; \
- Current.last_column = Rhs[N].last_column;
+# define YYLLOC_DEFAULT(Cur, Rhs, N) \
+do \
+ if (N) \
+ @{ \
+ (Cur).first_line = YYRHSLOC(Rhs, 1).first_line; \
+ (Cur).first_column = YYRHSLOC(Rhs, 1).first_column; \
+ (Cur).last_line = YYRHSLOC(Rhs, N).last_line; \
+ (Cur).last_column = YYRHSLOC(Rhs, N).last_column; \
+ @} \
+ else \
+ @{ \
+ (Cur).first_line = (Cur).last_line = \
+ YYRHSLOC(Rhs, 0).last_line; \
+ (Cur).first_column = (Cur).last_column = \
+ YYRHSLOC(Rhs, 0).last_column; \
+ @} \
+while (0)
@end group
@end example
+@noindent
+where @code{YYRHSLOC (rhs, k)} is the location of the @var{k}th symbol
+in @var{rhs} when @var{k} is positive, and the location of the symbol
+just before the reduction when @var{k} and @var{n} are both zero.
+
When defining @code{YYLLOC_DEFAULT}, you should consider that:
@itemize @bullet
@item
-All arguments are free of side-effects. However, only the first one (the
+All arguments are free of side-effects. However, only the first one (the
result) should be modified by @code{YYLLOC_DEFAULT}.
@item
-Before @code{YYLLOC_DEFAULT} is executed, the output parser sets @code{@@$}
-to @code{@@1}.
+For consistency with semantic actions, valid indexes within the
+right hand side range from 1 to @var{n}. When @var{n} is zero, only 0 is a
+valid index, and it refers to the symbol just before the reduction.
+During error processing @var{n} is always positive.
@item
-For consistency with semantic actions, valid indexes for the location array
-range from 1 to @var{n}.
+Your macro should parenthesize its arguments, if need be, since the
+actual arguments may not be surrounded by parentheses. Also, your
+macro should expand to something that can be used as a single
+statement when it is followed by a semicolon.
@end itemize
+@node Named References
+@section Named References
+@cindex named references
+
+As described in the preceding sections, the traditional way to refer to any
+semantic value or location is a @dfn{positional reference}, which takes the
+form @code{$@var{n}}, @code{$$}, @code{@@@var{n}}, and @code{@@$}. However,
+such a reference is not very descriptive. Moreover, if you later decide to
+insert or remove symbols in the right-hand side of a grammar rule, the need
+to renumber such references can be tedious and error-prone.
+
+To avoid these issues, you can also refer to a semantic value or location
+using a @dfn{named reference}. First of all, original symbol names may be
+used as named references. For example:
+
+@example
+@group
+invocation: op '(' args ')'
+ @{ $invocation = new_invocation ($op, $args, @@invocation); @}
+@end group
+@end example
+
+@noindent
+Positional and named references can be mixed arbitrarily. For example:
+
+@example
+@group
+invocation: op '(' args ')'
+ @{ $$ = new_invocation ($op, $args, @@$); @}
+@end group
+@end example
+
+@noindent
+However, sometimes regular symbol names are not sufficient due to
+ambiguities:
+
+@example
+@group
+exp: exp '/' exp
+ @{ $exp = $exp / $exp; @} // $exp is ambiguous.
+
+exp: exp '/' exp
+ @{ $$ = $1 / $exp; @} // One usage is ambiguous.
+
+exp: exp '/' exp
+ @{ $$ = $1 / $3; @} // No error.
+@end group
+@end example
+
+@noindent
+When ambiguity occurs, explicitly declared names may be used for values and
+locations. Explicit names are declared as a bracketed name after a symbol
+appearance in rule definitions. For example:
+@example
+@group
+exp[result]: exp[left] '/' exp[right]
+ @{ $result = $left / $right; @}
+@end group
+@end example
+
+@noindent
+In order to access a semantic value generated by a mid-rule action, an
+explicit name may also be declared by putting a bracketed name after the
+closing brace of the mid-rule action code:
+@example
+@group
+exp[res]: exp[x] '+' @{$left = $x;@}[left] exp[right]
+ @{ $res = $left + $right; @}
+@end group
+@end example
+
+@noindent
+
+In references, in order to specify names containing dots and dashes, an explicit
+bracketed syntax @code{$[name]} and @code{@@[name]} must be used:
+@example
+@group
+if-stmt: "if" '(' expr ')' "then" then.stmt ';'
+ @{ $[if-stmt] = new_if_stmt ($expr, $[then.stmt]); @}
+@end group
+@end example
+
+It often happens that named references are followed by a dot, dash or other
+C punctuation marks and operators. By default, Bison will read
+@samp{$name.suffix} as a reference to symbol value @code{$name} followed by
+@samp{.suffix}, i.e., an access to the @code{suffix} field of the semantic
+value. In order to force Bison to recognize @samp{name.suffix} in its
+entirety as the name of a semantic value, the bracketed syntax
+@samp{$[name.suffix]} must be used.
+
+The named references feature is experimental. More user feedback will help
+to stabilize it.
+
@node Declarations
@section Bison Declarations
@cindex declarations, Bison
declared if you need to specify which data type to use for the semantic
value (@pxref{Multiple Types, ,More Than One Value Type}).
-The first rule in the file also specifies the start symbol, by default.
-If you want some other symbol to be the start symbol, you must declare
-it explicitly (@pxref{Language and Grammar, ,Languages and Context-Free Grammars}).
+The first rule in the grammar file also specifies the start symbol, by
+default. If you want some other symbol to be the start symbol, you
+must declare it explicitly (@pxref{Language and Grammar, ,Languages
+and Context-Free Grammars}).
@menu
+* Require Decl:: Requiring a Bison version.
* Token Decl:: Declaring terminal symbols.
* Precedence Decl:: Declaring terminals with precedence and associativity.
* Union Decl:: Declaring the set of all semantic value types.
* Type Decl:: Declaring the choice of type for a nonterminal symbol.
-* Expect Decl:: Suppressing warnings about shift/reduce conflicts.
+* Initial Action Decl:: Code run before parsing starts.
+* Destructor Decl:: Declaring how symbols are freed.
+* Expect Decl:: Suppressing warnings about parsing conflicts.
* Start Decl:: Specifying the start symbol.
* Pure Decl:: Requesting a reentrant parser.
+* Push Decl:: Requesting a push parser.
* Decl Summary:: Table of all Bison declarations.
+* %define Summary:: Defining variables to adjust Bison's behavior.
+* %code Summary:: Inserting code into the parser source.
@end menu
+@node Require Decl
+@subsection Require a Version of Bison
+@cindex version requirement
+@cindex requiring a version of Bison
+@findex %require
+
+You may require the minimum version of Bison to process the grammar. If
+the requirement is not met, @command{bison} exits with an error (exit
+status 63).
+
+@example
+%require "@var{version}"
+@end example
+
@node Token Decl
@subsection Token Type Names
@cindex declaring token type names
the parser, so that the function @code{yylex} (if it is in this file)
can use the name @var{name} to stand for this token type's code.
-Alternatively, you can use @code{%left}, @code{%right}, or
+Alternatively, you can use @code{%left}, @code{%right},
+@code{%precedence}, or
@code{%nonassoc} instead of @code{%token}, if you wish to specify
associativity and precedence. @xref{Precedence Decl, ,Operator
Precedence}.
You can explicitly specify the numeric code for a token type by appending
-an integer value in the field immediately following the token name:
+a nonnegative decimal or hexadecimal integer value in the field immediately
+following the token name:
@example
%token NUM 300
+%token XNUM 0x12d // a GNU extension
@end example
@noindent
It is generally best, however, to let Bison choose the numeric codes for
all token types. Bison will automatically select codes that don't conflict
-with each other or with ASCII characters.
+with each other or with normal characters.
In the event that the stack type is a union, you must augment the
@code{%token} or other token declaration to include the data type
-alternative delimited by angle-brackets (@pxref{Multiple Types, ,More Than One Value Type}).
+alternative delimited by angle-brackets (@pxref{Multiple Types, ,More
+Than One Value Type}).
For example:
interchangeably in further declarations or the grammar rules. The
@code{yylex} function can use the token name or the literal string to
obtain the token type code number (@pxref{Calling Convention}).
+Syntax error messages passed to @code{yyerror} from the parser will reference
+the literal string instead of the token name.
+
+The token numbered as 0 corresponds to end of file; the following line
+allows for nicer error messages referring to ``end of file'' instead
+of ``$end'':
+
+@example
+%token END 0 "end of file"
+@end example
@node Precedence Decl
@subsection Operator Precedence
@cindex declaring operator precedence
@cindex operator precedence, declaring
-Use the @code{%left}, @code{%right} or @code{%nonassoc} declaration to
+Use the @code{%left}, @code{%right}, @code{%nonassoc}, or
+@code{%precedence} declaration to
declare a token and specify its precedence and associativity, all at
once. These are called @dfn{precedence declarations}.
-@xref{Precedence, ,Operator Precedence}, for general information on operator precedence.
+@xref{Precedence, ,Operator Precedence}, for general information on
+operator precedence.
-The syntax of a precedence declaration is the same as that of
+The syntax of a precedence declaration is nearly the same as that of
@code{%token}: either
@example
means that @samp{@var{x} @var{op} @var{y} @var{op} @var{z}} is
considered a syntax error.
+@code{%precedence} gives only precedence to the @var{symbols}, and
+defines no associativity at all. Use this to define precedence only,
+and leave any potential conflict due to associativity enabled.
+
@item
The precedence of an operator determines how it nests with other operators.
All the tokens declared in a single precedence declaration have equal
the one declared later has the higher precedence and is grouped first.
@end itemize
+For backward compatibility, there is a confusing difference between the
+argument lists of @code{%token} and precedence declarations.
+Only a @code{%token} can associate a literal string with a token type name.
+A precedence declaration always interprets a literal string as a reference to a
+separate token.
+For example:
+
+@example
+%left OR "<=" // Does not declare an alias.
+%left OR 134 "<=" 135 // Declares 134 for OR and 135 for "<=".
+@end example
+
@node Union Decl
@subsection The Collection of Value Types
@cindex declaring value types
@cindex value types, declaring
@findex %union
-The @code{%union} declaration specifies the entire collection of possible
-data types for semantic values. The keyword @code{%union} is followed by a
-pair of braces containing the same thing that goes inside a @code{union} in
-C.
+The @code{%union} declaration specifies the entire collection of
+possible data types for semantic values. The keyword @code{%union} is
+followed by braced code containing the same thing that goes inside a
+@code{union} in C@.
For example:
in the @code{%token} and @code{%type} declarations to pick one of the types
for a terminal or nonterminal symbol (@pxref{Type Decl, ,Nonterminal Symbols}).
-Note that, unlike making a @code{union} declaration in C, you do not write
+As an extension to POSIX, a tag is allowed after the
+@code{union}. For example:
+
+@example
+@group
+%union value @{
+ double val;
+ symrec *tptr;
+@}
+@end group
+@end example
+
+@noindent
+specifies the union tag @code{value}, so the corresponding C type is
+@code{union value}. If you do not specify a tag, it defaults to
+@code{YYSTYPE}.
+
+As another extension to POSIX, you may specify multiple
+@code{%union} declarations; their contents are concatenated. However,
+only the first @code{%union} declaration can specify a tag.
+
+Note that, unlike making a @code{union} declaration in C, you need not write
a semicolon after the closing brace.
+Instead of @code{%union}, you can define and use your own union type
+@code{YYSTYPE} if your grammar contains at least one
+@samp{<@var{type}>} tag. For example, you can put the following into
+a header file @file{parser.h}:
+
+@example
+@group
+union YYSTYPE @{
+ double val;
+ symrec *tptr;
+@};
+typedef union YYSTYPE YYSTYPE;
+@end group
+@end example
+
+@noindent
+and then your grammar can use the following
+instead of @code{%union}:
+
+@example
+@group
+%@{
+#include "parser.h"
+%@}
+%type <val> expr
+%token <tptr> ID
+@end group
+@end example
+
@node Type Decl
@subsection Nonterminal Symbols
@cindex declaring value types, nonterminals
@end example
@noindent
-Here @var{nonterminal} is the name of a nonterminal symbol, and @var{type}
-is the name given in the @code{%union} to the alternative that you want
-(@pxref{Union Decl, ,The Collection of Value Types}). You can give any number of nonterminal symbols in
-the same @code{%type} declaration, if they have the same value type. Use
-spaces to separate the symbol names.
+Here @var{nonterminal} is the name of a nonterminal symbol, and
+@var{type} is the name given in the @code{%union} to the alternative
+that you want (@pxref{Union Decl, ,The Collection of Value Types}). You
+can give any number of nonterminal symbols in the same @code{%type}
+declaration, if they have the same value type. Use spaces to separate
+the symbol names.
You can also declare the value type of a terminal symbol. To do this,
use the same @code{<@var{type}>} construction in a declaration for the
terminal symbol. All kinds of token declarations allow
@code{<@var{type}>}.
+@node Initial Action Decl
+@subsection Performing Actions before Parsing
+@findex %initial-action
+
+Sometimes your parser needs to perform some initializations before
+parsing. The @code{%initial-action} directive allows for such arbitrary
+code.
+
+@deffn {Directive} %initial-action @{ @var{code} @}
+@findex %initial-action
+Declare that the braced @var{code} must be invoked before parsing each time
+@code{yyparse} is called. The @var{code} may use @code{$$} and
+@code{@@$} --- initial value and location of the lookahead --- and the
+@code{%parse-param}.
+@end deffn
+
+For instance, if your locations use a file name, you may use
+
+@example
+%parse-param @{ char const *file_name @};
+%initial-action
+@{
+ @@$.initialize (file_name);
+@};
+@end example
+
+
+@node Destructor Decl
+@subsection Freeing Discarded Symbols
+@cindex freeing discarded symbols
+@findex %destructor
+@findex <*>
+@findex <>
+During error recovery (@pxref{Error Recovery}), symbols already pushed
+on the stack and tokens coming from the rest of the file are discarded
+until the parser falls on its feet. If the parser runs out of memory,
+or if it returns via @code{YYABORT} or @code{YYACCEPT}, all the
+symbols on the stack must be discarded. Even if the parser succeeds, it
+must discard the start symbol.
+
+When discarded symbols convey heap based information, this memory is
+lost. While this behavior can be tolerable for batch parsers, such as
+in traditional compilers, it is unacceptable for programs like shells or
+protocol implementations that may parse and execute indefinitely.
+
+The @code{%destructor} directive defines code that is called when a
+symbol is automatically discarded.
+
+@deffn {Directive} %destructor @{ @var{code} @} @var{symbols}
+@findex %destructor
+Invoke the braced @var{code} whenever the parser discards one of the
+@var{symbols}.
+Within @var{code}, @code{$$} designates the semantic value associated
+with the discarded symbol, and @code{@@$} designates its location.
+The additional parser parameters are also available (@pxref{Parser Function, ,
+The Parser Function @code{yyparse}}).
+
+When a symbol is listed among @var{symbols}, its @code{%destructor} is called a
+per-symbol @code{%destructor}.
+You may also define a per-type @code{%destructor} by listing a semantic type
+tag among @var{symbols}.
+In that case, the parser will invoke this @var{code} whenever it discards any
+grammar symbol that has that semantic type tag unless that symbol has its own
+per-symbol @code{%destructor}.
+
+Finally, you can define two different kinds of default @code{%destructor}s.
+(These default forms are experimental.
+More user feedback will help to determine whether they should become permanent
+features.)
+You can place each of @code{<*>} and @code{<>} in the @var{symbols} list of
+exactly one @code{%destructor} declaration in your grammar file.
+The parser will invoke the @var{code} associated with one of these whenever it
+discards any user-defined grammar symbol that has no per-symbol and no per-type
+@code{%destructor}.
+The parser uses the @var{code} for @code{<*>} in the case of such a grammar
+symbol for which you have formally declared a semantic type tag (@code{%type}
+counts as such a declaration, but @code{$<tag>$} does not).
+The parser uses the @var{code} for @code{<>} in the case of such a grammar
+symbol that has no declared semantic type tag.
+@end deffn
+
+@noindent
+For example:
+
+@example
+%union @{ char *string; @}
+%token <string> STRING1
+%token <string> STRING2
+%type <string> string1
+%type <string> string2
+%union @{ char character; @}
+%token <character> CHR
+%type <character> chr
+%token TAGLESS
+
+%destructor @{ @} <character>
+%destructor @{ free ($$); @} <*>
+%destructor @{ free ($$); printf ("%d", @@$.first_line); @} STRING1 string1
+%destructor @{ printf ("Discarding tagless symbol.\n"); @} <>
+@end example
+
+@noindent
+guarantees that, when the parser discards any user-defined symbol that has a
+semantic type tag other than @code{<character>}, it passes its semantic value
+to @code{free} by default.
+However, when the parser discards a @code{STRING1} or a @code{string1}, it also
+prints its line number to @code{stdout}.
+It performs only the second @code{%destructor} in this case, so it invokes
+@code{free} only once.
+Finally, the parser merely prints a message whenever it discards any symbol,
+such as @code{TAGLESS}, that has no semantic type tag.
+
+A Bison-generated parser invokes the default @code{%destructor}s only for
+user-defined as opposed to Bison-defined symbols.
+For example, the parser will not invoke either kind of default
+@code{%destructor} for the special Bison-defined symbols @code{$accept},
+@code{$undefined}, or @code{$end} (@pxref{Table of Symbols, ,Bison Symbols}),
+none of which you can reference in your grammar.
+It also will not invoke either for the @code{error} token (@pxref{Table of
+Symbols, ,error}), which is always defined by Bison regardless of whether you
+reference it in your grammar.
+However, it may invoke one of them for the end token (token 0) if you
+redefine it from @code{$end} to, for example, @code{END}:
+
+@example
+%token END 0
+@end example
+
+@cindex actions in mid-rule
+@cindex mid-rule actions
+Finally, Bison will never invoke a @code{%destructor} for an unreferenced
+mid-rule semantic value (@pxref{Mid-Rule Actions,,Actions in Mid-Rule}).
+That is, Bison does not consider a mid-rule to have a semantic value if you
+do not reference @code{$$} in the mid-rule's action or @code{$@var{n}}
+(where @var{n} is the right-hand side symbol position of the mid-rule) in
+any later action in that rule. However, if you do reference either, the
+Bison-generated parser will invoke the @code{<>} @code{%destructor} whenever
+it discards the mid-rule symbol.
+
+@ignore
+@noindent
+In the future, it may be possible to redefine the @code{error} token as a
+nonterminal that captures the discarded symbols.
+In that case, the parser will invoke the default destructor for it as well.
+@end ignore
+
+@sp 1
+
+@cindex discarded symbols
+@dfn{Discarded symbols} are the following:
+
+@itemize
+@item
+stacked symbols popped during the first phase of error recovery,
+@item
+incoming terminals during the second phase of error recovery,
+@item
+the current lookahead and the entire stack (except the current
+right-hand side symbols) when the parser returns immediately, and
+@item
+the start symbol, when the parser succeeds.
+@end itemize
+
+The parser can @dfn{return immediately} because of an explicit call to
+@code{YYABORT} or @code{YYACCEPT}, or failed error recovery, or memory
+exhaustion.
+
+Right-hand side symbols of a rule that explicitly triggers a syntax
+error via @code{YYERROR} are not discarded automatically. As a rule
+of thumb, destructors are invoked only when user actions cannot manage
+the memory.
+
@node Expect Decl
@subsection Suppressing Conflict Warnings
@cindex suppressing conflict warnings
@cindex warnings, preventing
@cindex conflicts, suppressing warnings of
@findex %expect
+@findex %expect-rr
Bison normally warns if there are any conflicts in the grammar
(@pxref{Shift/Reduce, ,Shift/Reduce Conflicts}), but most real grammars
%expect @var{n}
@end example
-Here @var{n} is a decimal integer. The declaration says there should be
-no warning if there are @var{n} shift/reduce conflicts and no
-reduce/reduce conflicts. An error, instead of the usual warning, is
-given if there are either more or fewer conflicts, or if there are any
-reduce/reduce conflicts.
+Here @var{n} is a decimal integer. The declaration says there should
+be @var{n} shift/reduce conflicts and no reduce/reduce conflicts.
+Bison reports an error if the number of shift/reduce conflicts differs
+from @var{n}, or if there are any reduce/reduce conflicts.
+
+For deterministic parsers, reduce/reduce conflicts are more
+serious, and should be eliminated entirely. Bison will always report
+reduce/reduce conflicts for these parsers. With GLR
+parsers, however, both kinds of conflicts are routine; otherwise,
+there would be no need to use GLR parsing. Therefore, it is
+also possible to specify an expected number of reduce/reduce conflicts
+in GLR parsers, using the declaration:
+
+@example
+%expect-rr @var{n}
+@end example
In general, using @code{%expect} involves these steps:
@item
Add an @code{%expect} declaration, copying the number @var{n} from the
-number which Bison printed.
+number which Bison printed. With GLR parsers, add an
+@code{%expect-rr} declaration as well.
@end itemize
-Now Bison will stop annoying you about the conflicts you have checked, but
-it will warn you again if changes in the grammar result in additional
-conflicts.
+Now Bison will report an error if you introduce an unexpected conflict,
+but will keep silent otherwise.
@node Start Decl
@subsection The Start-Symbol
@subsection A Pure (Reentrant) Parser
@cindex reentrant parser
@cindex pure parser
-@findex %pure_parser
+@findex %define api.pure
A @dfn{reentrant} program is one which does not alter in the course of
execution; in other words, it consists entirely of @dfn{pure} (read-only)
code. Reentrancy is important whenever asynchronous execution is possible;
-for example, a non-reentrant program may not be safe to call from a signal
-handler. In systems with multiple threads of control, a non-reentrant
+for example, a nonreentrant program may not be safe to call from a signal
+handler. In systems with multiple threads of control, a nonreentrant
program must be called only within interlocks.
Normally, Bison generates a parser which is not reentrant. This is
-suitable for most uses, and it permits compatibility with YACC. (The
-standard YACC interfaces are inherently nonreentrant, because they use
+suitable for most uses, and it permits compatibility with Yacc. (The
+standard Yacc interfaces are inherently nonreentrant, because they use
statically allocated variables for communication with @code{yylex},
including @code{yylval} and @code{yylloc}.)
Alternatively, you can generate a pure, reentrant parser. The Bison
-declaration @code{%pure_parser} says that you want the parser to be
+declaration @samp{%define api.pure} says that you want the parser to be
reentrant. It looks like this:
@example
-%pure_parser
+%define api.pure
@end example
The result is that the communication variables @code{yylval} and
@code{yylloc} become local variables in @code{yyparse}, and a different
calling convention is used for the lexical analyzer function
@code{yylex}. @xref{Pure Calling, ,Calling Conventions for Pure
-Parsers}, for the details of this. The variable @code{yynerrs} also
-becomes local in @code{yyparse} (@pxref{Error Reporting, ,The Error
+Parsers}, for the details of this. The variable @code{yynerrs}
+becomes local in @code{yyparse} in pull mode but it becomes a member
+of yypstate in push mode. (@pxref{Error Reporting, ,The Error
Reporting Function @code{yyerror}}). The convention for calling
@code{yyparse} itself is unchanged.
You can generate either a pure parser or a nonreentrant parser from any
valid grammar.
+@node Push Decl
+@subsection A Push Parser
+@cindex push parser
+@cindex push parser
+@findex %define api.push-pull
+
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+
+A pull parser is called once and it takes control until all its input
+is completely parsed. A push parser, on the other hand, is called
+each time a new token is made available.
+
+A push parser is typically useful when the parser is part of a
+main event loop in the client's application. This is typically
+a requirement of a GUI, when the main event loop needs to be triggered
+within a certain time period.
+
+Normally, Bison generates a pull parser.
+The following Bison declaration says that you want the parser to be a push
+parser (@pxref{%define Summary,,api.push-pull}):
+
+@example
+%define api.push-pull push
+@end example
+
+In almost all cases, you want to ensure that your push parser is also
+a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}). The only
+time you should create an impure push parser is to have backwards
+compatibility with the impure Yacc pull mode interface. Unless you know
+what you are doing, your declarations should look like this:
+
+@example
+%define api.pure
+%define api.push-pull push
+@end example
+
+There is a major notable functional difference between the pure push parser
+and the impure push parser. It is acceptable for a pure push parser to have
+many parser instances, of the same type of parser, in memory at the same time.
+An impure push parser should only use one parser at a time.
+
+When a push parser is selected, Bison will generate some new symbols in
+the generated parser. @code{yypstate} is a structure that the generated
+parser uses to store the parser's state. @code{yypstate_new} is the
+function that will create a new parser instance. @code{yypstate_delete}
+will free the resources associated with the corresponding parser instance.
+Finally, @code{yypush_parse} is the function that should be called whenever a
+token is available to provide the parser. A trivial example
+of using a pure push parser would look like this:
+
+@example
+int status;
+yypstate *ps = yypstate_new ();
+do @{
+ status = yypush_parse (ps, yylex (), NULL);
+@} while (status == YYPUSH_MORE);
+yypstate_delete (ps);
+@end example
+
+If the user decided to use an impure push parser, a few things about
+the generated parser will change. The @code{yychar} variable becomes
+a global variable instead of a variable in the @code{yypush_parse} function.
+For this reason, the signature of the @code{yypush_parse} function is
+changed to remove the token as a parameter. A nonreentrant push parser
+example would thus look like this:
+
+@example
+extern int yychar;
+int status;
+yypstate *ps = yypstate_new ();
+do @{
+ yychar = yylex ();
+ status = yypush_parse (ps);
+@} while (status == YYPUSH_MORE);
+yypstate_delete (ps);
+@end example
+
+That's it. Notice the next token is put into the global variable @code{yychar}
+for use by the next invocation of the @code{yypush_parse} function.
+
+Bison also supports both the push parser interface along with the pull parser
+interface in the same generated parser. In order to get this functionality,
+you should replace the @samp{%define api.push-pull push} declaration with the
+@samp{%define api.push-pull both} declaration. Doing this will create all of
+the symbols mentioned earlier along with the two extra symbols, @code{yyparse}
+and @code{yypull_parse}. @code{yyparse} can be used exactly as it normally
+would be used. However, the user should note that it is implemented in the
+generated parser by calling @code{yypull_parse}.
+This makes the @code{yyparse} function that is generated with the
+@samp{%define api.push-pull both} declaration slower than the normal
+@code{yyparse} function. If the user
+calls the @code{yypull_parse} function it will parse the rest of the input
+stream. It is possible to @code{yypush_parse} tokens to select a subgrammar
+and then @code{yypull_parse} the rest of the input stream. If you would like
+to switch back and forth between between parsing styles, you would have to
+write your own @code{yypull_parse} function that knows when to quit looking
+for input. An example of using the @code{yypull_parse} function would look
+like this:
+
+@example
+yypstate *ps = yypstate_new ();
+yypull_parse (ps); /* Will call the lexer */
+yypstate_delete (ps);
+@end example
+
+Adding the @samp{%define api.pure} declaration does exactly the same thing to
+the generated parser with @samp{%define api.push-pull both} as it did for
+@samp{%define api.push-pull push}.
+
@node Decl Summary
@subsection Bison Declaration Summary
@cindex Bison declaration summary
Here is a summary of the declarations used to define a grammar:
-@table @code
-@item %union
+@deffn {Directive} %union
Declare the collection of data types that semantic values may have
(@pxref{Union Decl, ,The Collection of Value Types}).
+@end deffn
-@item %token
+@deffn {Directive} %token
Declare a terminal symbol (token type name) with no precedence
or associativity specified (@pxref{Token Decl, ,Token Type Names}).
+@end deffn
-@item %right
+@deffn {Directive} %right
Declare a terminal symbol (token type name) that is right-associative
(@pxref{Precedence Decl, ,Operator Precedence}).
+@end deffn
-@item %left
+@deffn {Directive} %left
Declare a terminal symbol (token type name) that is left-associative
(@pxref{Precedence Decl, ,Operator Precedence}).
+@end deffn
-@item %nonassoc
+@deffn {Directive} %nonassoc
Declare a terminal symbol (token type name) that is nonassociative
-(using it in a way that would be associative is a syntax error)
(@pxref{Precedence Decl, ,Operator Precedence}).
+Using it in a way that would be associative is a syntax error.
+@end deffn
+
+@ifset defaultprec
+@deffn {Directive} %default-prec
+Assign a precedence to rules lacking an explicit @code{%prec} modifier
+(@pxref{Contextual Precedence, ,Context-Dependent Precedence}).
+@end deffn
+@end ifset
-@item %type
+@deffn {Directive} %type
Declare the type of semantic values for a nonterminal symbol
(@pxref{Type Decl, ,Nonterminal Symbols}).
+@end deffn
-@item %start
+@deffn {Directive} %start
Specify the grammar's start symbol (@pxref{Start Decl, ,The
Start-Symbol}).
+@end deffn
-@item %expect
+@deffn {Directive} %expect
Declare the expected number of shift-reduce conflicts
(@pxref{Expect Decl, ,Suppressing Conflict Warnings}).
-@end table
+@end deffn
+
@sp 1
@noindent
In order to change the behavior of @command{bison}, use the following
directives:
-@table @code
-@item %debug
-Output a definition of the macro @code{YYDEBUG} into the parser file, so
-that the debugging facilities are compiled. @xref{Debugging, ,Debugging
-Your Parser}.
-
-@item %defines
-Write an extra output file containing macro definitions for the token
-type names defined in the grammar and the semantic value type
-@code{YYSTYPE}, as well as a few @code{extern} variable declarations.
-
-If the parser output file is named @file{@var{name}.c} then this file
-is named @file{@var{name}.h}.@refill
-
-This output file is essential if you wish to put the definition of
-@code{yylex} in a separate source file, because @code{yylex} needs to
-be able to refer to token type codes and the variable
-@code{yylval}. @xref{Token Values, ,Semantic Values of Tokens}.@refill
-
-@item %file-prefix="@var{prefix}"
-Specify a prefix to use for all Bison output file names. The names are
-chosen as if the input file were named @file{@var{prefix}.y}.
-
-@c @item %header_extension
-@c Specify the extension of the parser header file generated when
-@c @code{%define} or @samp{-d} are used.
-@c
-@c For example, a grammar file named @file{foo.ypp} and containing a
-@c @code{%header_extension .hh} directive will produce a header file
-@c named @file{foo.tab.hh}
-
-@item %locations
+@deffn {Directive} %code @{@var{code}@}
+@deffnx {Directive} %code @var{qualifier} @{@var{code}@}
+@findex %code
+Insert @var{code} verbatim into the output parser source at the
+default location or at the location specified by @var{qualifier}.
+@xref{%code Summary}.
+@end deffn
+
+@deffn {Directive} %debug
+Instrument the output parser for traces. Obsoleted by @samp{%define
+parse.trace}.
+@xref{Tracing, ,Tracing Your Parser}.
+@end deffn
+
+@deffn {Directive} %define @var{variable}
+@deffnx {Directive} %define @var{variable} @var{value}
+@deffnx {Directive} %define @var{variable} "@var{value}"
+Define a variable to adjust Bison's behavior. @xref{%define Summary}.
+@end deffn
+
+@deffn {Directive} %defines
+Write a parser header file containing macro definitions for the token
+type names defined in the grammar as well as a few other declarations.
+If the parser implementation file is named @file{@var{name}.c} then
+the parser header file is named @file{@var{name}.h}.
+
+For C parsers, the parser header file declares @code{YYSTYPE} unless
+@code{YYSTYPE} is already defined as a macro or you have used a
+@code{<@var{type}>} tag without using @code{%union}. Therefore, if
+you are using a @code{%union} (@pxref{Multiple Types, ,More Than One
+Value Type}) with components that require other definitions, or if you
+have defined a @code{YYSTYPE} macro or type definition (@pxref{Value
+Type, ,Data Types of Semantic Values}), you need to arrange for these
+definitions to be propagated to all modules, e.g., by putting them in
+a prerequisite header that is included both by your parser and by any
+other module that needs @code{YYSTYPE}.
+
+Unless your parser is pure, the parser header file declares
+@code{yylval} as an external variable. @xref{Pure Decl, ,A Pure
+(Reentrant) Parser}.
+
+If you have also used locations, the parser header file declares
+@code{YYLTYPE} and @code{yylloc} using a protocol similar to that of the
+@code{YYSTYPE} macro and @code{yylval}. @xref{Tracking Locations}.
+
+This parser header file is normally essential if you wish to put the
+definition of @code{yylex} in a separate source file, because
+@code{yylex} typically needs to be able to refer to the
+above-mentioned declarations and to the token type codes. @xref{Token
+Values, ,Semantic Values of Tokens}.
+
+@findex %code requires
+@findex %code provides
+If you have declared @code{%code requires} or @code{%code provides}, the output
+header also contains their code.
+@xref{%code Summary}.
+@end deffn
+
+@deffn {Directive} %defines @var{defines-file}
+Same as above, but save in the file @var{defines-file}.
+@end deffn
+
+@deffn {Directive} %destructor
+Specify how the parser should reclaim the memory associated to
+discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
+
+@deffn {Directive} %file-prefix "@var{prefix}"
+Specify a prefix to use for all Bison output file names. The names
+are chosen as if the grammar file were named @file{@var{prefix}.y}.
+@end deffn
+
+@deffn {Directive} %language "@var{language}"
+Specify the programming language for the generated parser. Currently
+supported languages include C, C++, and Java.
+@var{language} is case-insensitive.
+
+This directive is experimental and its effect may be modified in future
+releases.
+@end deffn
+
+@deffn {Directive} %locations
Generate the code processing the locations (@pxref{Action Features,
,Special Features for Use in Actions}). This mode is enabled as soon as
the grammar uses the special @samp{@@@var{n}} tokens, but if your
grammar does not use it, using @samp{%locations} allows for more
-accurate parse error messages.
+accurate syntax error messages.
+@end deffn
-@item %name-prefix="@var{prefix}"
+@deffn {Directive} %name-prefix "@var{prefix}"
Rename the external symbols used in the parser so that they start with
@var{prefix} instead of @samp{yy}. The precise list of symbols renamed
+in C parsers
is @code{yyparse}, @code{yylex}, @code{yyerror}, @code{yynerrs},
-@code{yylval}, @code{yychar} and @code{yydebug}. For example, if you
-use @samp{%name-prefix="c_"}, the names become @code{c_parse},
-@code{c_lex}, and so on. @xref{Multiple Parsers, ,Multiple Parsers in
-the Same Program}.
-
-@item %no-parser
-Do not include any C code in the parser file; generate tables only. The
-parser file contains just @code{#define} directives and static variable
-declarations.
-
-This option also tells Bison to write the C code for the grammar actions
-into a file named @file{@var{filename}.act}, in the form of a
-brace-surrounded body fit for a @code{switch} statement.
+@code{yylval}, @code{yychar}, @code{yydebug}, and
+(if locations are used) @code{yylloc}. If you use a push parser,
+@code{yypush_parse}, @code{yypull_parse}, @code{yypstate},
+@code{yypstate_new} and @code{yypstate_delete} will
+also be renamed. For example, if you use @samp{%name-prefix "c_"}, the
+names become @code{c_parse}, @code{c_lex}, and so on.
+For C++ parsers, see the @samp{%define api.namespace} documentation in this
+section.
+@xref{Multiple Parsers, ,Multiple Parsers in the Same Program}.
+@end deffn
+
+@ifset defaultprec
+@deffn {Directive} %no-default-prec
+Do not assign a precedence to rules lacking an explicit @code{%prec}
+modifier (@pxref{Contextual Precedence, ,Context-Dependent
+Precedence}).
+@end deffn
+@end ifset
-@item %no-lines
+@deffn {Directive} %no-lines
Don't generate any @code{#line} preprocessor commands in the parser
-file. Ordinarily Bison writes these commands in the parser file so that
-the C compiler and debuggers will associate errors and object code with
-your source file (the grammar file). This directive causes them to
-associate errors with the parser file, treating it an independent source
-file in its own right.
-
-@item %output="@var{filename}"
-Specify the @var{filename} for the parser file.
-
-@item %pure-parser
-Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure
-(Reentrant) Parser}).
-
-@c @item %source_extension
-@c Specify the extension of the parser output file.
-@c
-@c For example, a grammar file named @file{foo.yy} and containing a
-@c @code{%source_extension .cpp} directive will produce a parser file
-@c named @file{foo.tab.cpp}
-
-@item %token_table
-Generate an array of token names in the parser file. The name of the
-array is @code{yytname}; @code{yytname[@var{i}]} is the name of the
-token whose internal Bison token code number is @var{i}. The first three
-elements of @code{yytname} are always @code{"$"}, @code{"error"}, and
-@code{"$illegal"}; after these come the symbols defined in the grammar
-file.
-
-For single-character literal tokens and literal string tokens, the name
-in the table includes the single-quote or double-quote characters: for
-example, @code{"'+'"} is a single-character literal and @code{"\"<=\""}
-is a literal string token. All the characters of the literal string
-token appear verbatim in the string found in the table; even
-double-quote characters are not escaped. For example, if the token
-consists of three characters @samp{*"*}, its string in @code{yytname}
-contains @samp{"*"*"}. (In C, that would be written as
-@code{"\"*\"*\""}).
-
-When you specify @code{%token_table}, Bison also generates macro
+implementation file. Ordinarily Bison writes these commands in the
+parser implementation file so that the C compiler and debuggers will
+associate errors and object code with your source file (the grammar
+file). This directive causes them to associate errors with the parser
+implementation file, treating it as an independent source file in its
+own right.
+@end deffn
+
+@deffn {Directive} %output "@var{file}"
+Specify @var{file} for the parser implementation file.
+@end deffn
+
+@deffn {Directive} %pure-parser
+Deprecated version of @samp{%define api.pure} (@pxref{%define
+Summary,,api.pure}), for which Bison is more careful to warn about
+unreasonable usage.
+@end deffn
+
+@deffn {Directive} %require "@var{version}"
+Require version @var{version} or higher of Bison. @xref{Require Decl, ,
+Require a Version of Bison}.
+@end deffn
+
+@deffn {Directive} %skeleton "@var{file}"
+Specify the skeleton to use.
+
+@c You probably don't need this option unless you are developing Bison.
+@c You should use @code{%language} if you want to specify the skeleton for a
+@c different language, because it is clearer and because it will always choose the
+@c correct skeleton for non-deterministic or push parsers.
+
+If @var{file} does not contain a @code{/}, @var{file} is the name of a skeleton
+file in the Bison installation directory.
+If it does, @var{file} is an absolute file name or a file name relative to the
+directory of the grammar file.
+This is similar to how most shells resolve commands.
+@end deffn
+
+@deffn {Directive} %token-table
+Generate an array of token names in the parser implementation file.
+The name of the array is @code{yytname}; @code{yytname[@var{i}]} is
+the name of the token whose internal Bison token code number is
+@var{i}. The first three elements of @code{yytname} correspond to the
+predefined tokens @code{"$end"}, @code{"error"}, and
+@code{"$undefined"}; after these come the symbols defined in the
+grammar file.
+
+The name in the table includes all the characters needed to represent
+the token in Bison. For single-character literals and literal
+strings, this includes the surrounding quoting characters and any
+escape sequences. For example, the Bison single-character literal
+@code{'+'} corresponds to a three-character name, represented in C as
+@code{"'+'"}; and the Bison two-character literal string @code{"\\/"}
+corresponds to a five-character name, represented in C as
+@code{"\"\\\\/\""}.
+
+When you specify @code{%token-table}, Bison also generates macro
definitions for macros @code{YYNTOKENS}, @code{YYNNTS}, and
@code{YYNRULES}, and @code{YYNSTATES}:
@item YYNSTATES
The number of parser states (@pxref{Parser States}).
@end table
+@end deffn
-@item %verbose
+@deffn {Directive} %verbose
Write an extra output file containing verbose descriptions of the
-parser states and what is done for each type of look-ahead token in
-that state.
-
-This file also describes all the conflicts, both those resolved by
-operator precedence and the unresolved ones.
+parser states and what is done for each type of lookahead token in
+that state. @xref{Understanding, , Understanding Your Parser}, for more
+information.
+@end deffn
-The file's name is made by removing @samp{.tab.c} or @samp{.c} from
-the parser output file name, and adding @samp{.output} instead.@refill
-
-Therefore, if the input file is @file{foo.y}, then the parser file is
-called @file{foo.tab.c} by default. As a consequence, the verbose
-output file is called @file{foo.output}.@refill
-
-@item %yacc
-@itemx %fixed-output-files
+@deffn {Directive} %yacc
Pretend the option @option{--yacc} was given, i.e., imitate Yacc,
including its naming conventions. @xref{Bison Options}, for more.
-@end table
+@end deffn
+@node %define Summary
+@subsection %define Summary
+There are many features of Bison's behavior that can be controlled by
+assigning the feature a single value. For historical reasons, some
+such features are assigned values by dedicated directives, such as
+@code{%start}, which assigns the start symbol. However, newer such
+features are associated with variables, which are assigned by the
+@code{%define} directive:
-@node Multiple Parsers
-@section Multiple Parsers in the Same Program
+@deffn {Directive} %define @var{variable}
+@deffnx {Directive} %define @var{variable} @var{value}
+@deffnx {Directive} %define @var{variable} "@var{value}"
+Define @var{variable} to @var{value}.
-Most programs that use Bison parse only one language and therefore contain
-only one Bison parser. But what if you want to parse more than one
-language with the same program? Then you need to avoid a name conflict
-between different definitions of @code{yyparse}, @code{yylval}, and so on.
+@var{value} must be placed in quotation marks if it contains any
+character other than a letter, underscore, period, or non-initial dash
+or digit. Omitting @code{"@var{value}"} entirely is always equivalent
+to specifying @code{""}.
-The easy way to do this is to use the option @samp{-p @var{prefix}}
-(@pxref{Invocation, ,Invoking Bison}). This renames the interface functions and
-variables of the Bison parser to start with @var{prefix} instead of
-@samp{yy}. You can use this to give each parser distinct names that do
-not conflict.
+It is an error if a @var{variable} is defined by @code{%define}
+multiple times, but see @ref{Bison Options,,-D
+@var{name}[=@var{value}]}.
+@end deffn
-The precise list of symbols renamed is @code{yyparse}, @code{yylex},
-@code{yyerror}, @code{yynerrs}, @code{yylval}, @code{yychar} and
-@code{yydebug}. For example, if you use @samp{-p c}, the names become
-@code{cparse}, @code{clex}, and so on.
+The rest of this section summarizes variables and values that
+@code{%define} accepts.
-@strong{All the other variables and macros associated with Bison are not
-renamed.} These others are not global; there is no conflict if the same
-name is used in different parsers. For example, @code{YYSTYPE} is not
-renamed, but defining this in different ways in different parsers causes
-no trouble (@pxref{Value Type, ,Data Types of Semantic Values}).
+Some @var{variable}s take Boolean values. In this case, Bison will
+complain if the variable definition does not meet one of the following
+four conditions:
-The @samp{-p} option works by adding macro definitions to the beginning
-of the parser source file, defining @code{yyparse} as
-@code{@var{prefix}parse}, and so on. This effectively substitutes one
-name for the other in the entire parser file.
+@enumerate
+@item @code{@var{value}} is @code{true}
+
+@item @code{@var{value}} is omitted (or @code{""} is specified).
+This is equivalent to @code{true}.
+
+@item @code{@var{value}} is @code{false}.
+
+@item @var{variable} is never defined.
+In this case, Bison selects a default value.
+@end enumerate
+
+What @var{variable}s are accepted, as well as their meanings and default
+values, depend on the selected target language and/or the parser
+skeleton (@pxref{Decl Summary,,%language}, @pxref{Decl
+Summary,,%skeleton}).
+Unaccepted @var{variable}s produce an error.
+Some of the accepted @var{variable}s are:
+
+@table @code
+@c ================================================== api.namespace
+@item api.namespace
+@findex %define api.namespace
+@itemize
+@item Languages(s): C++
+
+@item Purpose: Specify the namespace for the parser class.
+For example, if you specify:
+
+@example
+%define api.namespace "foo::bar"
+@end example
+
+Bison uses @code{foo::bar} verbatim in references such as:
+
+@example
+foo::bar::parser::semantic_type
+@end example
+
+However, to open a namespace, Bison removes any leading @code{::} and then
+splits on any remaining occurrences:
+
+@example
+namespace foo @{ namespace bar @{
+ class position;
+ class location;
+@} @}
+@end example
+
+@item Accepted Values:
+Any absolute or relative C++ namespace reference without a trailing
+@code{"::"}. For example, @code{"foo"} or @code{"::foo::bar"}.
+
+@item Default Value:
+The value specified by @code{%name-prefix}, which defaults to @code{yy}.
+This usage of @code{%name-prefix} is for backward compatibility and can
+be confusing since @code{%name-prefix} also specifies the textual prefix
+for the lexical analyzer function. Thus, if you specify
+@code{%name-prefix}, it is best to also specify @samp{%define
+api.namespace} so that @code{%name-prefix} @emph{only} affects the
+lexical analyzer function. For example, if you specify:
+
+@example
+%define api.namespace "foo"
+%name-prefix "bar::"
+@end example
+
+The parser namespace is @code{foo} and @code{yylex} is referenced as
+@code{bar::lex}.
+@end itemize
+@c namespace
+
+
+
+@c ================================================== api.pure
+@item api.pure
+@findex %define api.pure
+
+@itemize @bullet
+@item Language(s): C
+
+@item Purpose: Request a pure (reentrant) parser program.
+@xref{Pure Decl, ,A Pure (Reentrant) Parser}.
+
+@item Accepted Values: Boolean
+
+@item Default Value: @code{false}
+@end itemize
+@c api.pure
+
+
+
+@c ================================================== api.push-pull
+@item api.push-pull
+@findex %define api.push-pull
+
+@itemize @bullet
+@item Language(s): C (deterministic parsers only)
+
+@item Purpose: Request a pull parser, a push parser, or both.
+@xref{Push Decl, ,A Push Parser}.
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+
+@item Accepted Values: @code{pull}, @code{push}, @code{both}
+
+@item Default Value: @code{pull}
+@end itemize
+@c api.push-pull
+
+
+
+@c ================================================== api.tokens.prefix
+@item api.tokens.prefix
+@findex %define api.tokens.prefix
+
+@itemize
+@item Languages(s): all
+
+@item Purpose:
+Add a prefix to the token names when generating their definition in the
+target language. For instance
+
+@example
+%token FILE for ERROR
+%define api.tokens.prefix "TOK_"
+%%
+start: FILE for ERROR;
+@end example
+
+@noindent
+generates the definition of the symbols @code{TOK_FILE}, @code{TOK_for},
+and @code{TOK_ERROR} in the generated source files. In particular, the
+scanner must use these prefixed token names, while the grammar itself
+may still use the short names (as in the sample rule given above). The
+generated informational files (@file{*.output}, @file{*.xml},
+@file{*.dot}) are not modified by this prefix. See @ref{Calc++ Parser}
+and @ref{Calc++ Scanner}, for a complete example.
+
+@item Accepted Values:
+Any string. Should be a valid identifier prefix in the target language,
+in other words, it should typically be an identifier itself (sequence of
+letters, underscores, and ---not at the beginning--- digits).
+
+@item Default Value:
+empty
+@end itemize
+@c api.tokens.prefix
+
+
+@c ================================================== lex_symbol
+@item lex_symbol
+@findex %define lex_symbol
+
+@itemize @bullet
+@item Language(s):
+C++
+
+@item Purpose:
+When variant-based semantic values are enabled (@pxref{C++ Variants}),
+request that symbols be handled as a whole (type, value, and possibly
+location) in the scanner. @xref{Complete Symbols}, for details.
+
+@item Accepted Values:
+Boolean.
+
+@item Default Value:
+@code{false}
+@end itemize
+@c lex_symbol
+
+
+@c ================================================== lr.default-reductions
+
+@item lr.default-reductions
+@findex %define lr.default-reductions
+
+@itemize @bullet
+@item Language(s): all
+
+@item Purpose: Specify the kind of states that are permitted to
+contain default reductions. @xref{Default Reductions}. (The ability to
+specify where default reductions should be used is experimental. More user
+feedback will help to stabilize it.)
+
+@item Accepted Values: @code{most}, @code{consistent}, @code{accepting}
+@item Default Value:
+@itemize
+@item @code{accepting} if @code{lr.type} is @code{canonical-lr}.
+@item @code{most} otherwise.
+@end itemize
+@end itemize
+
+@c ============================================ lr.keep-unreachable-states
+
+@item lr.keep-unreachable-states
+@findex %define lr.keep-unreachable-states
+
+@itemize @bullet
+@item Language(s): all
+@item Purpose: Request that Bison allow unreachable parser states to
+remain in the parser tables. @xref{Unreachable States}.
+@item Accepted Values: Boolean
+@item Default Value: @code{false}
+@end itemize
+@c lr.keep-unreachable-states
+
+@c ================================================== lr.type
+
+@item lr.type
+@findex %define lr.type
+
+@itemize @bullet
+@item Language(s): all
+
+@item Purpose: Specify the type of parser tables within the
+LR(1) family. @xref{LR Table Construction}. (This feature is experimental.
+More user feedback will help to stabilize it.)
+
+@item Accepted Values: @code{lalr}, @code{ielr}, @code{canonical-lr}
+
+@item Default Value: @code{lalr}
+@end itemize
+
+
+@c ================================================== namespace
+@item namespace
+@findex %define namespace
+Obsoleted by @code{api.namespace}
+@c namespace
+
+
+@c ================================================== parse.assert
+@item parse.assert
+@findex %define parse.assert
+
+@itemize
+@item Languages(s): C++
+
+@item Purpose: Issue runtime assertions to catch invalid uses.
+In C++, when variants are used (@pxref{C++ Variants}), symbols must be
+constructed and
+destroyed properly. This option checks these constraints.
+
+@item Accepted Values: Boolean
+
+@item Default Value: @code{false}
+@end itemize
+@c parse.assert
+
+
+@c ================================================== parse.error
+@item parse.error
+@findex %define parse.error
+@itemize
+@item Languages(s):
+all
+@item Purpose:
+Control the kind of error messages passed to the error reporting
+function. @xref{Error Reporting, ,The Error Reporting Function
+@code{yyerror}}.
+@item Accepted Values:
+@itemize
+@item @code{simple}
+Error messages passed to @code{yyerror} are simply @w{@code{"syntax
+error"}}.
+@item @code{verbose}
+Error messages report the unexpected token, and possibly the expected ones.
+However, this report can often be incorrect when LAC is not enabled
+(@pxref{LAC}).
+@end itemize
+
+@item Default Value:
+@code{simple}
+@end itemize
+@c parse.error
+
+
+@c ================================================== parse.lac
+@item parse.lac
+@findex %define parse.lac
+
+@itemize
+@item Languages(s): C (deterministic parsers only)
+
+@item Purpose: Enable LAC (lookahead correction) to improve
+syntax error handling. @xref{LAC}.
+@item Accepted Values: @code{none}, @code{full}
+@item Default Value: @code{none}
+@end itemize
+@c parse.lac
+
+@c ================================================== parse.trace
+@item parse.trace
+@findex %define parse.trace
+
+@itemize
+@item Languages(s): C, C++
+
+@item Purpose: Require parser instrumentation for tracing.
+In C/C++, define the macro @code{YYDEBUG} to 1 in the parser implementation
+file if it is not already defined, so that the debugging facilities are
+compiled. @xref{Tracing, ,Tracing Your Parser}.
+
+@item Accepted Values: Boolean
+
+@item Default Value: @code{false}
+@end itemize
+@c parse.trace
+
+@c ================================================== variant
+@item variant
+@findex %define variant
+
+@itemize @bullet
+@item Language(s):
+C++
+
+@item Purpose:
+Request variant-based semantic values.
+@xref{C++ Variants}.
+
+@item Accepted Values:
+Boolean.
+
+@item Default Value:
+@code{false}
+@end itemize
+@c variant
+@end table
+
+
+@node %code Summary
+@subsection %code Summary
+@findex %code
+@cindex Prologue
+
+The @code{%code} directive inserts code verbatim into the output
+parser source at any of a predefined set of locations. It thus serves
+as a flexible and user-friendly alternative to the traditional Yacc
+prologue, @code{%@{@var{code}%@}}. This section summarizes the
+functionality of @code{%code} for the various target languages
+supported by Bison. For a detailed discussion of how to use
+@code{%code} in place of @code{%@{@var{code}%@}} for C/C++ and why it
+is advantageous to do so, @pxref{Prologue Alternatives}.
+
+@deffn {Directive} %code @{@var{code}@}
+This is the unqualified form of the @code{%code} directive. It
+inserts @var{code} verbatim at a language-dependent default location
+in the parser implementation.
+
+For C/C++, the default location is the parser implementation file
+after the usual contents of the parser header file. Thus, the
+unqualified form replaces @code{%@{@var{code}%@}} for most purposes.
+
+For Java, the default location is inside the parser class.
+@end deffn
+
+@deffn {Directive} %code @var{qualifier} @{@var{code}@}
+This is the qualified form of the @code{%code} directive.
+@var{qualifier} identifies the purpose of @var{code} and thus the
+location(s) where Bison should insert it. That is, if you need to
+specify location-sensitive @var{code} that does not belong at the
+default location selected by the unqualified @code{%code} form, use
+this form instead.
+@end deffn
+
+For any particular qualifier or for the unqualified form, if there are
+multiple occurrences of the @code{%code} directive, Bison concatenates
+the specified code in the order in which it appears in the grammar
+file.
+
+Not all qualifiers are accepted for all target languages. Unaccepted
+qualifiers produce an error. Some of the accepted qualifiers are:
+
+@table @code
+@item requires
+@findex %code requires
+
+@itemize @bullet
+@item Language(s): C, C++
+
+@item Purpose: This is the best place to write dependency code required for
+@code{YYSTYPE} and @code{YYLTYPE}.
+In other words, it's the best place to define types referenced in @code{%union}
+directives, and it's the best place to override Bison's default @code{YYSTYPE}
+and @code{YYLTYPE} definitions.
+
+@item Location(s): The parser header file and the parser implementation file
+before the Bison-generated @code{YYSTYPE} and @code{YYLTYPE}
+definitions.
+@end itemize
+
+@item provides
+@findex %code provides
+
+@itemize @bullet
+@item Language(s): C, C++
+
+@item Purpose: This is the best place to write additional definitions and
+declarations that should be provided to other modules.
+
+@item Location(s): The parser header file and the parser implementation
+file after the Bison-generated @code{YYSTYPE}, @code{YYLTYPE}, and
+token definitions.
+@end itemize
+
+@item top
+@findex %code top
+
+@itemize @bullet
+@item Language(s): C, C++
+
+@item Purpose: The unqualified @code{%code} or @code{%code requires}
+should usually be more appropriate than @code{%code top}. However,
+occasionally it is necessary to insert code much nearer the top of the
+parser implementation file. For example:
+
+@example
+%code top @{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+@}
+@end example
+
+@item Location(s): Near the top of the parser implementation file.
+@end itemize
+
+@item imports
+@findex %code imports
+
+@itemize @bullet
+@item Language(s): Java
+
+@item Purpose: This is the best place to write Java import directives.
+
+@item Location(s): The parser Java file after any Java package directive and
+before any class definitions.
+@end itemize
+@end table
+
+Though we say the insertion locations are language-dependent, they are
+technically skeleton-dependent. Writers of non-standard skeletons
+however should choose their locations consistently with the behavior
+of the standard Bison skeletons.
+
+
+@node Multiple Parsers
+@section Multiple Parsers in the Same Program
+
+Most programs that use Bison parse only one language and therefore contain
+only one Bison parser. But what if you want to parse more than one
+language with the same program? Then you need to avoid a name conflict
+between different definitions of @code{yyparse}, @code{yylval}, and so on.
+
+The easy way to do this is to use the option @samp{-p @var{prefix}}
+(@pxref{Invocation, ,Invoking Bison}). This renames the interface
+functions and variables of the Bison parser to start with @var{prefix}
+instead of @samp{yy}. You can use this to give each parser distinct
+names that do not conflict.
+
+The precise list of symbols renamed is @code{yyparse}, @code{yylex},
+@code{yyerror}, @code{yynerrs}, @code{yylval}, @code{yylloc},
+@code{yychar} and @code{yydebug}. If you use a push parser,
+@code{yypush_parse}, @code{yypull_parse}, @code{yypstate},
+@code{yypstate_new} and @code{yypstate_delete} will also be renamed.
+For example, if you use @samp{-p c}, the names become @code{cparse},
+@code{clex}, and so on.
+
+@strong{All the other variables and macros associated with Bison are not
+renamed.} These others are not global; there is no conflict if the same
+name is used in different parsers. For example, @code{YYSTYPE} is not
+renamed, but defining this in different ways in different parsers causes
+no trouble (@pxref{Value Type, ,Data Types of Semantic Values}).
+
+The @samp{-p} option works by adding macro definitions to the
+beginning of the parser implementation file, defining @code{yyparse}
+as @code{@var{prefix}parse}, and so on. This effectively substitutes
+one name for the other in the entire parser implementation file.
@node Interface
@chapter Parser C-Language Interface
in the grammar file, you are likely to run into trouble.
@menu
-* Parser Function:: How to call @code{yyparse} and what it returns.
-* Lexical:: You must supply a function @code{yylex}
- which reads tokens.
-* Error Reporting:: You must supply a function @code{yyerror}.
-* Action Features:: Special features for use in actions.
+* Parser Function:: How to call @code{yyparse} and what it returns.
+* Push Parser Function:: How to call @code{yypush_parse} and what it returns.
+* Pull Parser Function:: How to call @code{yypull_parse} and what it returns.
+* Parser Create Function:: How to call @code{yypstate_new} and what it returns.
+* Parser Delete Function:: How to call @code{yypstate_delete} and what it returns.
+* Lexical:: You must supply a function @code{yylex}
+ which reads tokens.
+* Error Reporting:: You must supply a function @code{yyerror}.
+* Action Features:: Special features for use in actions.
+* Internationalization:: How to let the parser speak in the user's
+ native language.
@end menu
@node Parser Function
write an action which directs @code{yyparse} to return immediately
without reading further.
+
+@deftypefun int yyparse (void)
The value returned by @code{yyparse} is 0 if parsing was successful (return
is due to end-of-input).
-The value is 1 if parsing failed (return is due to a syntax error).
+The value is 1 if parsing failed because of invalid input, i.e., input
+that contains a syntax error or that causes @code{YYABORT} to be
+invoked.
+
+The value is 2 if parsing failed due to memory exhaustion.
+@end deftypefun
In an action, you can cause immediate return from @code{yyparse} by using
these macros:
-@table @code
-@item YYACCEPT
+@defmac YYACCEPT
@findex YYACCEPT
Return immediately with value 0 (to report success).
+@end defmac
-@item YYABORT
+@defmac YYABORT
@findex YYABORT
Return immediately with value 1 (to report failure).
-@end table
+@end defmac
+
+If you use a reentrant parser, you can optionally pass additional
+parameter information to it in a reentrant way. To do so, use the
+declaration @code{%parse-param}:
+
+@deffn {Directive} %parse-param @{@var{argument-declaration}@} @dots{}
+@findex %parse-param
+Declare that one or more
+@var{argument-declaration} are additional @code{yyparse} arguments.
+The @var{argument-declaration} is used when declaring
+functions or prototypes. The last identifier in
+@var{argument-declaration} must be the argument name.
+@end deffn
+
+Here's an example. Write this in the parser:
+
+@example
+%parse-param @{int *nastiness@} @{int *randomness@}
+@end example
+
+@noindent
+Then call the parser like this:
+
+@example
+@{
+ int nastiness, randomness;
+ @dots{} /* @r{Store proper data in @code{nastiness} and @code{randomness}.} */
+ value = yyparse (&nastiness, &randomness);
+ @dots{}
+@}
+@end example
+
+@noindent
+In the grammar actions, use expressions like this to refer to the data:
+
+@example
+exp: @dots{} @{ @dots{}; *randomness += 1; @dots{} @}
+@end example
+
+@node Push Parser Function
+@section The Push Parser Function @code{yypush_parse}
+@findex yypush_parse
+
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+
+You call the function @code{yypush_parse} to parse a single token. This
+function is available if either the @samp{%define api.push-pull push} or
+@samp{%define api.push-pull both} declaration is used.
+@xref{Push Decl, ,A Push Parser}.
+
+@deftypefun int yypush_parse (yypstate *yyps)
+The value returned by @code{yypush_parse} is the same as for yyparse with the
+following exception. @code{yypush_parse} will return YYPUSH_MORE if more input
+is required to finish parsing the grammar.
+@end deftypefun
+
+@node Pull Parser Function
+@section The Pull Parser Function @code{yypull_parse}
+@findex yypull_parse
+
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+
+You call the function @code{yypull_parse} to parse the rest of the input
+stream. This function is available if the @samp{%define api.push-pull both}
+declaration is used.
+@xref{Push Decl, ,A Push Parser}.
+
+@deftypefun int yypull_parse (yypstate *yyps)
+The value returned by @code{yypull_parse} is the same as for @code{yyparse}.
+@end deftypefun
+
+@node Parser Create Function
+@section The Parser Create Function @code{yystate_new}
+@findex yypstate_new
+
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+
+You call the function @code{yypstate_new} to create a new parser instance.
+This function is available if either the @samp{%define api.push-pull push} or
+@samp{%define api.push-pull both} declaration is used.
+@xref{Push Decl, ,A Push Parser}.
+
+@deftypefun yypstate *yypstate_new (void)
+The function will return a valid parser instance if there was memory available
+or 0 if no memory was available.
+In impure mode, it will also return 0 if a parser instance is currently
+allocated.
+@end deftypefun
+
+@node Parser Delete Function
+@section The Parser Delete Function @code{yystate_delete}
+@findex yypstate_delete
+
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+
+You call the function @code{yypstate_delete} to delete a parser instance.
+function is available if either the @samp{%define api.push-pull push} or
+@samp{%define api.push-pull both} declaration is used.
+@xref{Push Decl, ,A Push Parser}.
+
+@deftypefun void yypstate_delete (yypstate *yyps)
+This function will reclaim the memory associated with a parser instance.
+After this call, you should no longer attempt to use the parser instance.
+@end deftypefun
@node Lexical
@section The Lexical Analyzer Function @code{yylex}
this function automatically; you must write it so that @code{yyparse} can
call it. The function is sometimes referred to as a lexical scanner.
-In simple programs, @code{yylex} is often defined at the end of the Bison
-grammar file. If @code{yylex} is defined in a separate source file, you
-need to arrange for the token-type macro definitions to be available there.
-To do this, use the @samp{-d} option when you run Bison, so that it will
-write these macro definitions into a separate header file
-@file{@var{name}.tab.h} which you can include in the other source files
-that need it. @xref{Invocation, ,Invoking Bison}.@refill
+In simple programs, @code{yylex} is often defined at the end of the
+Bison grammar file. If @code{yylex} is defined in a separate source
+file, you need to arrange for the token-type macro definitions to be
+available there. To do this, use the @samp{-d} option when you run
+Bison, so that it will write these macro definitions into the separate
+parser header file, @file{@var{name}.tab.h}, which you can include in
+the other source files that need it. @xref{Invocation, ,Invoking
+Bison}.
@menu
* Calling Convention:: How @code{yyparse} calls @code{yylex}.
-* Token Values:: How @code{yylex} must return the semantic value
- of the token it has read.
-* Token Positions:: How @code{yylex} must return the text position
- (line number, etc.) of the token, if the
- actions want that.
-* Pure Calling:: How the calling convention differs
- in a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}).
+* Token Values:: How @code{yylex} must return the semantic value
+ of the token it has read.
+* Token Locations:: How @code{yylex} must return the text location
+ (line number, etc.) of the token, if the
+ actions want that.
+* Pure Calling:: How the calling convention differs in a pure parser
+ (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}).
@end menu
@node Calling Convention
@subsection Calling Convention for @code{yylex}
-The value that @code{yylex} returns must be the numeric code for the type
-of token it has just found, or 0 for end-of-input.
+The value that @code{yylex} returns must be the positive numeric code
+for the type of token it has just found; a zero or negative value
+signifies end-of-input.
When a token is referred to in the grammar rules by a name, that name
-in the parser file becomes a C macro whose definition is the proper
-numeric code for that token type. So @code{yylex} can use the name
-to indicate that type. @xref{Symbols}.
+in the parser implementation file becomes a C macro whose definition
+is the proper numeric code for that token type. So @code{yylex} can
+use the name to indicate that type. @xref{Symbols}.
When a token is referred to in the grammar rules by a character literal,
the numeric code for that character is also the code for the token type.
-So @code{yylex} can simply return that character code. The null character
-must not be used this way, because its code is zero and that is what
+So @code{yylex} can simply return that character code, possibly converted
+to @code{unsigned char} to avoid sign-extension. The null character
+must not be used this way, because its code is zero and that
signifies end-of-input.
Here is an example showing these things:
yylex (void)
@{
@dots{}
- if (c == EOF) /* Detect end of file. */
+ if (c == EOF) /* Detect end-of-input. */
return 0;
@dots{}
if (c == '+' || c == '-')
- return c; /* Assume token type for `+' is '+'. */
+ return c; /* Assume token type for `+' is '+'. */
@dots{}
- return INT; /* Return the type of the token. */
+ return INT; /* Return the type of the token. */
@dots{}
@}
@end example
table. The index of the token in the table is the token type's code.
The name of a multicharacter token is recorded in @code{yytname} with a
double-quote, the token's characters, and another double-quote. The
-token's characters are not escaped in any way; they appear verbatim in
-the contents of the string in the table.
+token's characters are escaped as necessary to be suitable as input
+to Bison.
-Here's code for looking up a token in @code{yytname}, assuming that the
-characters of the token are stored in @code{token_buffer}.
+Here's code for looking up a multicharacter token in @code{yytname},
+assuming that the characters of the token are stored in
+@code{token_buffer}, and assuming that the token does not contain any
+characters like @samp{"} that require escaping.
-@smallexample
+@example
for (i = 0; i < YYNTOKENS; i++)
@{
if (yytname[i] != 0
&& yytname[i][0] == '"'
- && strncmp (yytname[i] + 1, token_buffer,
- strlen (token_buffer))
+ && ! strncmp (yytname[i] + 1, token_buffer,
+ strlen (token_buffer))
&& yytname[i][strlen (token_buffer) + 1] == '"'
&& yytname[i][strlen (token_buffer) + 2] == 0)
break;
@}
-@end smallexample
+@end example
The @code{yytname} table is generated only if you use the
-@code{%token_table} declaration. @xref{Decl Summary}.
+@code{%token-table} declaration. @xref{Decl Summary}.
@end itemize
@node Token Values
@subsection Semantic Values of Tokens
@vindex yylval
-In an ordinary (non-reentrant) parser, the semantic value of the token must
+In an ordinary (nonreentrant) parser, the semantic value of the token must
be stored into the global variable @code{yylval}. When you are using
just one data type for semantic values, @code{yylval} has that type.
Thus, if the type is @code{int} (the default), you might write this in
@example
@group
@dots{}
- yylval = value; /* Put value onto Bison stack. */
- return INT; /* Return the type of the token. */
+ yylval = value; /* Put value onto Bison stack. */
+ return INT; /* Return the type of the token. */
@dots{}
@end group
@end example
When you are using multiple data types, @code{yylval}'s type is a union
-made from the @code{%union} declaration (@pxref{Union Decl, ,The Collection of Value Types}). So when
-you store a token's value, you must use the proper member of the union.
-If the @code{%union} declaration looks like this:
+made from the @code{%union} declaration (@pxref{Union Decl, ,The
+Collection of Value Types}). So when you store a token's value, you
+must use the proper member of the union. If the @code{%union}
+declaration looks like this:
@example
@group
@example
@group
@dots{}
- yylval.intval = value; /* Put value onto Bison stack. */
- return INT; /* Return the type of the token. */
+ yylval.intval = value; /* Put value onto Bison stack. */
+ return INT; /* Return the type of the token. */
@dots{}
@end group
@end example
-@node Token Positions
-@subsection Textual Positions of Tokens
+@node Token Locations
+@subsection Textual Locations of Tokens
@vindex yylloc
-If you are using the @samp{@@@var{n}}-feature (@pxref{Locations, ,
-Tracking Locations}) in actions to keep track of the
-textual locations of tokens and groupings, then you must provide this
-information in @code{yylex}. The function @code{yyparse} expects to
-find the textual location of a token just parsed in the global variable
-@code{yylloc}. So @code{yylex} must store the proper data in that
-variable.
+If you are using the @samp{@@@var{n}}-feature (@pxref{Tracking Locations})
+in actions to keep track of the textual locations of tokens and groupings,
+then you must provide this information in @code{yylex}. The function
+@code{yyparse} expects to find the textual location of a token just parsed
+in the global variable @code{yylloc}. So @code{yylex} must store the proper
+data in that variable.
By default, the value of @code{yylloc} is a structure and you need only
initialize the members that are going to be used by the actions. The
@node Pure Calling
@subsection Calling Conventions for Pure Parsers
-When you use the Bison declaration @code{%pure_parser} to request a
+When you use the Bison declaration @samp{%define api.pure} to request a
pure, reentrant parser, the global communication variables @code{yylval}
and @code{yylloc} cannot be used. (@xref{Pure Decl, ,A Pure (Reentrant)
Parser}.) In such parsers the two global variables are replaced by
@end example
If the grammar file does not use the @samp{@@} constructs to refer to
-textual positions, then the type @code{YYLTYPE} will not be defined. In
+textual locations, then the type @code{YYLTYPE} will not be defined. In
this case, omit the second argument; @code{yylex} will be called with
only one argument.
-@vindex YYPARSE_PARAM
-If you use a reentrant parser, you can optionally pass additional
-parameter information to it in a reentrant way. To do so, define the
-macro @code{YYPARSE_PARAM} as a variable name. This modifies the
-@code{yyparse} function to accept one argument, of type @code{void *},
-with that name.
-
-When you call @code{yyparse}, pass the address of an object, casting the
-address to @code{void *}. The grammar actions can refer to the contents
-of the object by casting the pointer value back to its proper type and
-then dereferencing it. Here's an example. Write this in the parser:
+If you wish to pass additional arguments to @code{yylex}, use
+@code{%lex-param} just like @code{%parse-param} (@pxref{Parser
+Function}). To pass additional arguments to both @code{yylex} and
+@code{yyparse}, use @code{%param}.
+
+@deffn {Directive} %lex-param @{@var{argument-declaration}@} @dots{}
+@findex %lex-param
+Specify that @var{argument-declaration} are additional @code{yylex} argument
+declarations. You may pass one or more such declarations, which is
+equivalent to repeating @code{%lex-param}.
+@end deffn
+
+@deffn {Directive} %param @{@var{argument-declaration}@} @dots{}
+@findex %param
+Specify that @var{argument-declaration} are additional
+@code{yylex}/@code{yyparse} argument declaration. This is equivalent to
+@samp{%lex-param @{@var{argument-declaration}@} @dots{} %parse-param
+@{@var{argument-declaration}@} @dots{}}. You may pass one or more
+declarations, which is equivalent to repeating @code{%param}.
+@end deffn
+
+For instance:
@example
-%@{
-struct parser_control
-@{
- int nastiness;
- int randomness;
-@};
-
-#define YYPARSE_PARAM parm
-%@}
+%lex-param @{scanner_mode *mode@}
+%parse-param @{parser_mode *mode@}
+%param @{environment_type *env@}
@end example
@noindent
-Then call the parser like this:
+results in the following signature:
@example
-struct parser_control
-@{
- int nastiness;
- int randomness;
-@};
-
-@dots{}
-
-@{
- struct parser_control foo;
- @dots{} /* @r{Store proper data in @code{foo}.} */
- value = yyparse ((void *) &foo);
- @dots{}
-@}
+int yylex (scanner_mode *mode, environment_type *env);
+int yyparse (parser_mode *mode, environment_type *env);
@end example
-@noindent
-In the grammar actions, use expressions like this to refer to the data:
+If @samp{%define api.pure} is added:
@example
-((struct parser_control *) parm)->randomness
+int yylex (YYSTYPE *lvalp, scanner_mode *mode, environment_type *env);
+int yyparse (parser_mode *mode, environment_type *env);
@end example
-@vindex YYLEX_PARAM
-If you wish to pass the additional parameter data to @code{yylex},
-define the macro @code{YYLEX_PARAM} just like @code{YYPARSE_PARAM}, as
-shown here:
+@noindent
+and finally, if both @samp{%define api.pure} and @code{%locations} are used:
@example
-%@{
-struct parser_control
-@{
- int nastiness;
- int randomness;
-@};
-
-#define YYPARSE_PARAM parm
-#define YYLEX_PARAM parm
-%@}
+int yylex (YYSTYPE *lvalp, YYLTYPE *llocp,
+ scanner_mode *mode, environment_type *env);
+int yyparse (parser_mode *mode, environment_type *env);
@end example
-You should then define @code{yylex} to accept one additional
-argument---the value of @code{parm}. (This makes either two or three
-arguments in total, depending on whether an argument of type
-@code{YYLTYPE} is passed.) You can declare the argument as a pointer to
-the proper object type, or you can declare it as @code{void *} and
-access the contents as shown above.
-
-You can use @samp{%pure_parser} to request a reentrant parser without
-also using @code{YYPARSE_PARAM}. Then you should call @code{yyparse}
-with no arguments, as usual.
-
@node Error Reporting
@section The Error Reporting Function @code{yyerror}
@cindex error reporting function
@cindex parse error
@cindex syntax error
-The Bison parser detects a @dfn{parse error} or @dfn{syntax error}
+The Bison parser detects a @dfn{syntax error} (or @dfn{parse error})
whenever it reads a token which cannot satisfy any syntax rule. An
action in the grammar can also explicitly proclaim an error, using the
macro @code{YYERROR} (@pxref{Action Features, ,Special Features for Use
The Bison parser expects to report the error by calling an error
reporting function named @code{yyerror}, which you must supply. It is
called by @code{yyparse} whenever a syntax error is found, and it
-receives one argument. For a parse error, the string is normally
-@w{@code{"parse error"}}.
-
-@findex YYERROR_VERBOSE
-If you define the macro @code{YYERROR_VERBOSE} in the Bison declarations
-section (@pxref{Bison Declarations, ,The Bison Declarations Section}),
-then Bison provides a more verbose and specific error message string
-instead of just plain @w{@code{"parse error"}}. It doesn't matter what
-definition you use for @code{YYERROR_VERBOSE}, just whether you define
-it.
-
-The parser can detect one other kind of error: stack overflow. This
-happens when the input contains constructions that are very deeply
+receives one argument. For a syntax error, the string is normally
+@w{@code{"syntax error"}}.
+
+@findex %define parse.error
+If you invoke @samp{%define parse.error verbose} in the Bison declarations
+section (@pxref{Bison Declarations, ,The Bison Declarations Section}), then
+Bison provides a more verbose and specific error message string instead of
+just plain @w{@code{"syntax error"}}. However, that message sometimes
+contains incorrect information if LAC is not enabled (@pxref{LAC}).
+
+The parser can detect one other kind of error: memory exhaustion. This
+can happen when the input contains constructions that are very deeply
nested. It isn't likely you will encounter this, since the Bison
-parser extends its stack automatically up to a very large limit. But
-if overflow happens, @code{yyparse} calls @code{yyerror} in the usual
-fashion, except that the argument string is @w{@code{"parser stack
-overflow"}}.
+parser normally extends its stack automatically up to a very large limit. But
+if memory is exhausted, @code{yyparse} calls @code{yyerror} in the usual
+fashion, except that the argument string is @w{@code{"memory exhausted"}}.
+
+In some cases diagnostics like @w{@code{"syntax error"}} are
+translated automatically from English to some other language before
+they are passed to @code{yyerror}. @xref{Internationalization}.
The following definition suffices in simple programs:
@example
@group
void
-yyerror (char *s)
+yyerror (char const *s)
@{
@end group
@group
(@pxref{Error Recovery}). If recovery is impossible, @code{yyparse} will
immediately return 1.
-@vindex yynerrs
-The variable @code{yynerrs} contains the number of syntax errors
-encountered so far. Normally this variable is global; but if you
-request a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}) then it is a local variable
-which only the actions can access.
+Obviously, in location tracking pure parsers, @code{yyerror} should have
+an access to the current location.
+This is indeed the case for the GLR
+parsers, but not for the Yacc parser, for historical reasons. I.e., if
+@samp{%locations %define api.pure} is passed then the prototypes for
+@code{yyerror} are:
-@node Action Features
-@section Special Features for Use in Actions
-@cindex summary, action features
-@cindex action features summary
+@example
+void yyerror (char const *msg); /* Yacc parsers. */
+void yyerror (YYLTYPE *locp, char const *msg); /* GLR parsers. */
+@end example
+
+If @samp{%parse-param @{int *nastiness@}} is used, then:
+
+@example
+void yyerror (int *nastiness, char const *msg); /* Yacc parsers. */
+void yyerror (int *nastiness, char const *msg); /* GLR parsers. */
+@end example
+
+Finally, GLR and Yacc parsers share the same @code{yyerror} calling
+convention for absolutely pure parsers, i.e., when the calling
+convention of @code{yylex} @emph{and} the calling convention of
+@samp{%define api.pure} are pure.
+I.e.:
+
+@example
+/* Location tracking. */
+%locations
+/* Pure yylex. */
+%define api.pure
+%lex-param @{int *nastiness@}
+/* Pure yyparse. */
+%parse-param @{int *nastiness@}
+%parse-param @{int *randomness@}
+@end example
+
+@noindent
+results in the following signatures for all the parser kinds:
+
+@example
+int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
+int yyparse (int *nastiness, int *randomness);
+void yyerror (YYLTYPE *locp,
+ int *nastiness, int *randomness,
+ char const *msg);
+@end example
+
+@noindent
+The prototypes are only indications of how the code produced by Bison
+uses @code{yyerror}. Bison-generated code always ignores the returned
+value, so @code{yyerror} can return any type, including @code{void}.
+Also, @code{yyerror} can be a variadic function; that is why the
+message is always passed last.
+
+Traditionally @code{yyerror} returns an @code{int} that is always
+ignored, but this is purely for historical reasons, and @code{void} is
+preferable since it more accurately describes the return type for
+@code{yyerror}.
+
+@vindex yynerrs
+The variable @code{yynerrs} contains the number of syntax errors
+reported so far. Normally this variable is global; but if you
+request a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser})
+then it is a local variable which only the actions can access.
+
+@node Action Features
+@section Special Features for Use in Actions
+@cindex summary, action features
+@cindex action features summary
Here is a table of Bison constructs, variables and macros that
are useful in actions.
-@table @samp
-@item $$
+@deffn {Variable} $$
Acts like a variable that contains the semantic value for the
grouping made by the current rule. @xref{Actions}.
+@end deffn
-@item $@var{n}
+@deffn {Variable} $@var{n}
Acts like a variable that contains the semantic value for the
@var{n}th component of the current rule. @xref{Actions}.
+@end deffn
-@item $<@var{typealt}>$
+@deffn {Variable} $<@var{typealt}>$
Like @code{$$} but specifies alternative @var{typealt} in the union
-specified by the @code{%union} declaration. @xref{Action Types, ,Data Types of Values in Actions}.
+specified by the @code{%union} declaration. @xref{Action Types, ,Data
+Types of Values in Actions}.
+@end deffn
-@item $<@var{typealt}>@var{n}
+@deffn {Variable} $<@var{typealt}>@var{n}
Like @code{$@var{n}} but specifies alternative @var{typealt} in the
union specified by the @code{%union} declaration.
-@xref{Action Types, ,Data Types of Values in Actions}.@refill
+@xref{Action Types, ,Data Types of Values in Actions}.
+@end deffn
-@item YYABORT;
+@deffn {Macro} YYABORT;
Return immediately from @code{yyparse}, indicating failure.
@xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@end deffn
-@item YYACCEPT;
+@deffn {Macro} YYACCEPT;
Return immediately from @code{yyparse}, indicating success.
@xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@end deffn
-@item YYBACKUP (@var{token}, @var{value});
+@deffn {Macro} YYBACKUP (@var{token}, @var{value});
@findex YYBACKUP
Unshift a token. This macro is allowed only for rules that reduce
-a single value, and only when there is no look-ahead token.
-It installs a look-ahead token with token type @var{token} and
+a single value, and only when there is no lookahead token.
+It is also disallowed in GLR parsers.
+It installs a lookahead token with token type @var{token} and
semantic value @var{value}; then it discards the value that was
going to be reduced by this rule.
If the macro is used when it is not valid, such as when there is
-a look-ahead token already, then it reports a syntax error with
+a lookahead token already, then it reports a syntax error with
a message @samp{cannot back up} and performs ordinary error
recovery.
In either case, the rest of the action is not executed.
+@end deffn
-@item YYEMPTY
+@deffn {Macro} YYEMPTY
@vindex YYEMPTY
-Value stored in @code{yychar} when there is no look-ahead token.
+Value stored in @code{yychar} when there is no lookahead token.
+@end deffn
-@item YYERROR;
+@deffn {Macro} YYEOF
+@vindex YYEOF
+Value stored in @code{yychar} when the lookahead is the end of the input
+stream.
+@end deffn
+
+@deffn {Macro} YYERROR;
@findex YYERROR
Cause an immediate syntax error. This statement initiates error
recovery just as if the parser itself had detected an error; however, it
does not call @code{yyerror}, and does not print any message. If you
want to print an error message, call @code{yyerror} explicitly before
the @samp{YYERROR;} statement. @xref{Error Recovery}.
+@end deffn
-@item YYRECOVERING
-This macro stands for an expression that has the value 1 when the parser
-is recovering from a syntax error, and 0 the rest of the time.
+@deffn {Macro} YYRECOVERING
+@findex YYRECOVERING
+The expression @code{YYRECOVERING ()} yields 1 when the parser
+is recovering from a syntax error, and 0 otherwise.
@xref{Error Recovery}.
+@end deffn
+
+@deffn {Variable} yychar
+Variable containing either the lookahead token, or @code{YYEOF} when the
+lookahead is the end of the input stream, or @code{YYEMPTY} when no lookahead
+has been performed so the next token is not yet known.
+Do not modify @code{yychar} in a deferred semantic action (@pxref{GLR Semantic
+Actions}).
+@xref{Lookahead, ,Lookahead Tokens}.
+@end deffn
+
+@deffn {Macro} yyclearin;
+Discard the current lookahead token. This is useful primarily in
+error rules.
+Do not invoke @code{yyclearin} in a deferred semantic action (@pxref{GLR
+Semantic Actions}).
+@xref{Error Recovery}.
+@end deffn
-@item yychar
-Variable containing the current look-ahead token. (In a pure parser,
-this is actually a local variable within @code{yyparse}.) When there is
-no look-ahead token, the value @code{YYEMPTY} is stored in the variable.
-@xref{Look-Ahead, ,Look-Ahead Tokens}.
-
-@item yyclearin;
-Discard the current look-ahead token. This is useful primarily in
-error rules. @xref{Error Recovery}.
-
-@item yyerrok;
+@deffn {Macro} yyerrok;
Resume generating error messages immediately for subsequent syntax
errors. This is useful primarily in error rules.
@xref{Error Recovery}.
-
-@item @@$
+@end deffn
+
+@deffn {Variable} yylloc
+Variable containing the lookahead token location when @code{yychar} is not set
+to @code{YYEMPTY} or @code{YYEOF}.
+Do not modify @code{yylloc} in a deferred semantic action (@pxref{GLR Semantic
+Actions}).
+@xref{Actions and Locations, ,Actions and Locations}.
+@end deffn
+
+@deffn {Variable} yylval
+Variable containing the lookahead token semantic value when @code{yychar} is
+not set to @code{YYEMPTY} or @code{YYEOF}.
+Do not modify @code{yylval} in a deferred semantic action (@pxref{GLR Semantic
+Actions}).
+@xref{Actions, ,Actions}.
+@end deffn
+
+@deffn {Value} @@$
@findex @@$
-Acts like a structure variable containing information on the textual position
-of the grouping made by the current rule. @xref{Locations, ,
-Tracking Locations}.
+Acts like a structure variable containing information on the textual
+location of the grouping made by the current rule. @xref{Tracking
+Locations}.
@c Check if those paragraphs are still useful or not.
@c those members.
@c The use of this feature makes the parser noticeably slower.
+@end deffn
-@item @@@var{n}
+@deffn {Value} @@@var{n}
@findex @@@var{n}
-Acts like a structure variable containing information on the textual position
-of the @var{n}th component of the current rule. @xref{Locations, ,
-Tracking Locations}.
+Acts like a structure variable containing information on the textual
+location of the @var{n}th component of the current rule. @xref{Tracking
+Locations}.
+@end deffn
+
+@node Internationalization
+@section Parser Internationalization
+@cindex internationalization
+@cindex i18n
+@cindex NLS
+@cindex gettext
+@cindex bison-po
+
+A Bison-generated parser can print diagnostics, including error and
+tracing messages. By default, they appear in English. However, Bison
+also supports outputting diagnostics in the user's native language. To
+make this work, the user should set the usual environment variables.
+@xref{Users, , The User's View, gettext, GNU @code{gettext} utilities}.
+For example, the shell command @samp{export LC_ALL=fr_CA.UTF-8} might
+set the user's locale to French Canadian using the UTF-8
+encoding. The exact set of available locales depends on the user's
+installation.
+
+The maintainer of a package that uses a Bison-generated parser enables
+the internationalization of the parser's output through the following
+steps. Here we assume a package that uses GNU Autoconf and
+GNU Automake.
+
+@enumerate
+@item
+@cindex bison-i18n.m4
+Into the directory containing the GNU Autoconf macros used
+by the package---often called @file{m4}---copy the
+@file{bison-i18n.m4} file installed by Bison under
+@samp{share/aclocal/bison-i18n.m4} in Bison's installation directory.
+For example:
+
+@example
+cp /usr/local/share/aclocal/bison-i18n.m4 m4/bison-i18n.m4
+@end example
+
+@item
+@findex BISON_I18N
+@vindex BISON_LOCALEDIR
+@vindex YYENABLE_NLS
+In the top-level @file{configure.ac}, after the @code{AM_GNU_GETTEXT}
+invocation, add an invocation of @code{BISON_I18N}. This macro is
+defined in the file @file{bison-i18n.m4} that you copied earlier. It
+causes @samp{configure} to find the value of the
+@code{BISON_LOCALEDIR} variable, and it defines the source-language
+symbol @code{YYENABLE_NLS} to enable translations in the
+Bison-generated parser.
+
+@item
+In the @code{main} function of your program, designate the directory
+containing Bison's runtime message catalog, through a call to
+@samp{bindtextdomain} with domain name @samp{bison-runtime}.
+For example:
+
+@example
+bindtextdomain ("bison-runtime", BISON_LOCALEDIR);
+@end example
+
+Typically this appears after any other call @code{bindtextdomain
+(PACKAGE, LOCALEDIR)} that your package already has. Here we rely on
+@samp{BISON_LOCALEDIR} to be defined as a string through the
+@file{Makefile}.
+
+@item
+In the @file{Makefile.am} that controls the compilation of the @code{main}
+function, make @samp{BISON_LOCALEDIR} available as a C preprocessor macro,
+either in @samp{DEFS} or in @samp{AM_CPPFLAGS}. For example:
+
+@example
+DEFS = @@DEFS@@ -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
+@end example
+
+or:
+
+@example
+AM_CPPFLAGS = -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
+@end example
+
+@item
+Finally, invoke the command @command{autoreconf} to generate the build
+infrastructure.
+@end enumerate
-@end table
@node Algorithm
@chapter The Bison Parser Algorithm
This kind of parser is known in the literature as a bottom-up parser.
@menu
-* Look-Ahead:: Parser looks one token ahead when deciding what to do.
+* Lookahead:: Parser looks one token ahead when deciding what to do.
* Shift/Reduce:: Conflicts: when either shifting or reduction is valid.
* Precedence:: Operator precedence works by resolving conflicts.
* Contextual Precedence:: When an operator's precedence depends on context.
* Parser States:: The parser is a finite-state-machine with stack.
* Reduce/Reduce:: When two rules are applicable in the same situation.
-* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
-* Stack Overflow:: What happens when stack gets full. How to avoid it.
+* Mysterious Conflicts:: Conflicts that look unjustified.
+* Tuning LR:: How to tune fundamental aspects of LR-based parsing.
+* Generalized LR Parsing:: Parsing arbitrary context-free grammars.
+* Memory Management:: What happens when memory is exhausted. How to avoid it.
@end menu
-@node Look-Ahead
-@section Look-Ahead Tokens
-@cindex look-ahead token
+@node Lookahead
+@section Lookahead Tokens
+@cindex lookahead token
The Bison parser does @emph{not} always reduce immediately as soon as the
last @var{n} tokens and groupings match a rule. This is because such a
token in order to decide what to do.
When a token is read, it is not immediately shifted; first it becomes the
-@dfn{look-ahead token}, which is not on the stack. Now the parser can
+@dfn{lookahead token}, which is not on the stack. Now the parser can
perform one or more reductions of tokens and groupings on the stack, while
-the look-ahead token remains off to the side. When no more reductions
-should take place, the look-ahead token is shifted onto the stack. This
+the lookahead token remains off to the side. When no more reductions
+should take place, the lookahead token is shifted onto the stack. This
does not mean that all possible reductions have been done; depending on the
-token type of the look-ahead token, some rules may choose to delay their
+token type of the lookahead token, some rules may choose to delay their
application.
-Here is a simple case where look-ahead is needed. These three rules define
+Here is a simple case where lookahead is needed. These three rules define
expressions which contain binary addition operators and postfix unary
factorial operators (@samp{!}), and allow parentheses for grouping.
@example
@group
-expr: term '+' expr
- | term
- ;
+expr:
+ term '+' expr
+| term
+;
@end group
@group
-term: '(' expr ')'
- | term '!'
- | NUMBER
- ;
+term:
+ '(' expr ')'
+| term '!'
+| NUMBER
+;
@end group
@end example
'!'}. No rule allows that sequence.
@vindex yychar
-The current look-ahead token is stored in the variable @code{yychar}.
+@vindex yylval
+@vindex yylloc
+The lookahead token is stored in the variable @code{yychar}.
+Its semantic value and location, if any, are stored in the variables
+@code{yylval} and @code{yylloc}.
@xref{Action Features, ,Special Features for Use in Actions}.
@node Shift/Reduce
@example
@group
if_stmt:
- IF expr THEN stmt
- | IF expr THEN stmt ELSE stmt
- ;
+ IF expr THEN stmt
+| IF expr THEN stmt ELSE stmt
+;
@end group
@end example
Here we assume that @code{IF}, @code{THEN} and @code{ELSE} are
terminal symbols for specific keyword tokens.
-When the @code{ELSE} token is read and becomes the look-ahead token, the
+When the @code{ELSE} token is read and becomes the lookahead token, the
contents of the stack (assuming the input is valid) are just right for
reduction by the first rule. But it is also legitimate to shift the
@code{ELSE}, because that would lead to eventual reduction by the second
Algol 60 and is called the ``dangling @code{else}'' ambiguity.
To avoid warnings from Bison about predictable, legitimate shift/reduce
-conflicts, use the @code{%expect @var{n}} declaration. There will be no
-warning as long as the number of shift/reduce conflicts is exactly @var{n}.
+conflicts, use the @code{%expect @var{n}} declaration.
+There will be no warning as long as the number of shift/reduce conflicts
+is exactly @var{n}, and Bison will report an error if there is a
+different number.
@xref{Expect Decl, ,Suppressing Conflict Warnings}.
The definition of @code{if_stmt} above is solely to blame for the
conflict, but the conflict does not actually appear without additional
-rules. Here is a complete Bison input file that actually manifests the
-conflict:
+rules. Here is a complete Bison grammar file that actually manifests
+the conflict:
@example
@group
%%
@end group
@group
-stmt: expr
- | if_stmt
- ;
+stmt:
+ expr
+| if_stmt
+;
@end group
@group
if_stmt:
- IF expr THEN stmt
- | IF expr THEN stmt ELSE stmt
- ;
+ IF expr THEN stmt
+| IF expr THEN stmt ELSE stmt
+;
@end group
-expr: variable
- ;
+expr:
+ variable
+;
@end example
@node Precedence
@menu
* Why Precedence:: An example showing why precedence is needed.
-* Using Precedence:: How to specify precedence in Bison grammars.
+* Using Precedence:: How to specify precedence and associativity.
+* Precedence Only:: How to specify precedence only.
* Precedence Examples:: How these features are used in the previous example.
* How Precedence:: How they work.
@end menu
@example
@group
-expr: expr '-' expr
- | expr '*' expr
- | expr '<' expr
- | '(' expr ')'
- @dots{}
- ;
+expr:
+ expr '-' expr
+| expr '*' expr
+| expr '<' expr
+| '(' expr ')'
+@dots{}
+;
@end group
@end example
The latter alternative, @dfn{right association}, is desirable for
assignment operators. The choice of left or right association is a
matter of whether the parser chooses to shift or reduce when the stack
-contains @w{@samp{1 - 2}} and the look-ahead token is @samp{-}: shifting
+contains @w{@samp{1 - 2}} and the lookahead token is @samp{-}: shifting
makes right-associativity.
@node Using Precedence
@subsection Specifying Operator Precedence
@findex %left
-@findex %right
@findex %nonassoc
+@findex %precedence
+@findex %right
Bison allows you to specify these choices with the operator precedence
declarations @code{%left} and @code{%right}. Each such declaration
them right-associative. A third alternative is @code{%nonassoc}, which
declares that it is a syntax error to find the same operator twice ``in a
row''.
+The last alternative, @code{%precedence}, allows to define only
+precedence and no associativity at all. As a result, any
+associativity-related conflict that remains will be reported as an
+compile-time error. The directive @code{%nonassoc} creates run-time
+error: using the operator in a associative way is a syntax error. The
+directive @code{%precedence} creates compile-time errors: an operator
+@emph{can} be involved in an associativity-related conflict, contrary to
+what expected the grammar author.
The relative precedence of different operators is controlled by the
-order in which they are declared. The first @code{%left} or
-@code{%right} declaration in the file declares the operators whose
+order in which they are declared. The first precedence/associativity
+declaration in the file declares the operators whose
precedence is lowest, the next such declaration declares the operators
whose precedence is a little higher, and so on.
+@node Precedence Only
+@subsection Specifying Precedence Only
+@findex %precedence
+
+Since POSIX Yacc defines only @code{%left}, @code{%right}, and
+@code{%nonassoc}, which all defines precedence and associativity, little
+attention is paid to the fact that precedence cannot be defined without
+defining associativity. Yet, sometimes, when trying to solve a
+conflict, precedence suffices. In such a case, using @code{%left},
+@code{%right}, or @code{%nonassoc} might hide future (associativity
+related) conflicts that would remain hidden.
+
+The dangling @code{else} ambiguity (@pxref{Shift/Reduce, , Shift/Reduce
+Conflicts}) can be solved explicitly. This shift/reduce conflicts occurs
+in the following situation, where the period denotes the current parsing
+state:
+
+@example
+if @var{e1} then if @var{e2} then @var{s1} . else @var{s2}
+@end example
+
+The conflict involves the reduction of the rule @samp{IF expr THEN
+stmt}, which precedence is by default that of its last token
+(@code{THEN}), and the shifting of the token @code{ELSE}. The usual
+disambiguation (attach the @code{else} to the closest @code{if}),
+shifting must be preferred, i.e., the precedence of @code{ELSE} must be
+higher than that of @code{THEN}. But neither is expected to be involved
+in an associativity related conflict, which can be specified as follows.
+
+@example
+%precedence THEN
+%precedence ELSE
+@end example
+
+The unary-minus is another typical example where associativity is
+usually over-specified, see @ref{Infix Calc, , Infix Notation
+Calculator: @code{calc}}. The @code{%left} directive is traditionally
+used to declare the precedence of @code{NEG}, which is more than needed
+since it also defines its associativity. While this is harmless in the
+traditional example, who knows how @code{NEG} might be used in future
+evolutions of the grammar@dots{}
+
@node Precedence Examples
@subsection Precedence Examples
The first effect of the precedence declarations is to assign precedence
levels to the terminal symbols declared. The second effect is to assign
-precedence levels to certain rules: each rule gets its precedence from the
-last terminal symbol mentioned in the components. (You can also specify
-explicitly the precedence of a rule. @xref{Contextual Precedence, ,Context-Dependent Precedence}.)
-
-Finally, the resolution of conflicts works by comparing the
-precedence of the rule being considered with that of the
-look-ahead token. If the token's precedence is higher, the
-choice is to shift. If the rule's precedence is higher, the
-choice is to reduce. If they have equal precedence, the choice
-is made based on the associativity of that precedence level. The
-verbose output file made by @samp{-v} (@pxref{Invocation, ,Invoking Bison}) says
-how each conflict was resolved.
+precedence levels to certain rules: each rule gets its precedence from
+the last terminal symbol mentioned in the components. (You can also
+specify explicitly the precedence of a rule. @xref{Contextual
+Precedence, ,Context-Dependent Precedence}.)
+
+Finally, the resolution of conflicts works by comparing the precedence
+of the rule being considered with that of the lookahead token. If the
+token's precedence is higher, the choice is to shift. If the rule's
+precedence is higher, the choice is to reduce. If they have equal
+precedence, the choice is made based on the associativity of that
+precedence level. The verbose output file made by @samp{-v}
+(@pxref{Invocation, ,Invoking Bison}) says how each conflict was
+resolved.
Not all rules and not all tokens have precedence. If either the rule or
-the look-ahead token has no precedence, then the default is to shift.
+the lookahead token has no precedence, then the default is to shift.
@node Contextual Precedence
@section Context-Dependent Precedence
sign typically has a very high precedence as a unary operator, and a
somewhat lower precedence (lower than multiplication) as a binary operator.
-The Bison precedence declarations, @code{%left}, @code{%right} and
-@code{%nonassoc}, can only be used once for a given token; so a token has
+The Bison precedence declarations
+can only be used once for a given token; so a token has
only one precedence declared in this way. For context-dependent
precedence, you need to use an additional mechanism: the @code{%prec}
-modifier for rules.@refill
+modifier for rules.
The @code{%prec} modifier declares the precedence of a particular rule by
specifying a terminal symbol whose precedence should be used for that rule.
@example
@group
-exp: @dots{}
- | exp '-' exp
- @dots{}
- | '-' exp %prec UMINUS
+exp:
+ @dots{}
+| exp '-' exp
+ @dots{}
+| '-' exp %prec UMINUS
@end group
@end example
+@ifset defaultprec
+If you forget to append @code{%prec UMINUS} to the rule for unary
+minus, Bison silently assumes that minus has its usual precedence.
+This kind of problem can be tricky to debug, since one typically
+discovers the mistake only by testing the code.
+
+The @code{%no-default-prec;} declaration makes it easier to discover
+this kind of problem systematically. It causes rules that lack a
+@code{%prec} modifier to have no precedence, even if the last terminal
+symbol mentioned in their components has a declared precedence.
+
+If @code{%no-default-prec;} is in effect, you must specify @code{%prec}
+for all rules that participate in precedence conflict resolution.
+Then you will see any shift/reduce conflict until you tell Bison how
+to resolve it, either by changing your grammar or by adding an
+explicit precedence. This will probably add declarations to the
+grammar, but it helps to protect against incorrect rule precedences.
+
+The effect of @code{%no-default-prec;} can be reversed by giving
+@code{%default-prec;}, which is the default.
+@end ifset
+
@node Parser States
@section Parser States
@cindex finite-state machine
near the top of the stack. The current state collects all the information
about previous input which is relevant to deciding what to do next.
-Each time a look-ahead token is read, the current parser state together
-with the type of look-ahead token are looked up in a table. This table
-entry can say, ``Shift the look-ahead token.'' In this case, it also
+Each time a lookahead token is read, the current parser state together
+with the type of lookahead token are looked up in a table. This table
+entry can say, ``Shift the lookahead token.'' In this case, it also
specifies the new parser state, which is pushed onto the top of the
parser stack. Or it can say, ``Reduce using rule number @var{n}.''
This means that a certain number of tokens or groupings are taken off
that number of states are popped from the stack, and one new state is
pushed.
-There is one other alternative: the table can say that the look-ahead token
+There is one other alternative: the table can say that the lookahead token
is erroneous in the current state. This causes error processing to begin
(@pxref{Error Recovery}).
of zero or more @code{word} groupings.
@example
-sequence: /* empty */
- @{ printf ("empty sequence\n"); @}
- | maybeword
- | sequence word
- @{ printf ("added word %s\n", $2); @}
- ;
+@group
+sequence:
+ /* empty */ @{ printf ("empty sequence\n"); @}
+| maybeword
+| sequence word @{ printf ("added word %s\n", $2); @}
+;
+@end group
-maybeword: /* empty */
- @{ printf ("empty maybeword\n"); @}
- | word
- @{ printf ("single word %s\n", $1); @}
- ;
+@group
+maybeword:
+ /* empty */ @{ printf ("empty maybeword\n"); @}
+| word @{ printf ("single word %s\n", $1); @}
+;
+@end group
@end example
@noindent
proper way to define @code{sequence}:
@example
-sequence: /* empty */
- @{ printf ("empty sequence\n"); @}
- | sequence word
- @{ printf ("added word %s\n", $2); @}
- ;
+sequence:
+ /* empty */ @{ printf ("empty sequence\n"); @}
+| sequence word @{ printf ("added word %s\n", $2); @}
+;
@end example
Here is another common error that yields a reduce/reduce conflict:
@example
-sequence: /* empty */
- | sequence words
- | sequence redirects
- ;
+sequence:
+ /* empty */
+| sequence words
+| sequence redirects
+;
-words: /* empty */
- | words word
- ;
+words:
+ /* empty */
+| words word
+;
-redirects:/* empty */
- | redirects redirect
- ;
+redirects:
+ /* empty */
+| redirects redirect
+;
@end example
@noindent
of sequence:
@example
-sequence: /* empty */
- | sequence word
- | sequence redirect
- ;
+sequence:
+ /* empty */
+| sequence word
+| sequence redirect
+;
@end example
Second, to prevent either a @code{words} or a @code{redirects}
from being empty:
@example
-sequence: /* empty */
- | sequence words
- | sequence redirects
- ;
+@group
+sequence:
+ /* empty */
+| sequence words
+| sequence redirects
+;
+@end group
-words: word
- | words word
- ;
+@group
+words:
+ word
+| words word
+;
+@end group
-redirects:redirect
- | redirects redirect
- ;
+@group
+redirects:
+ redirect
+| redirects redirect
+;
+@end group
@end example
-@node Mystery Conflicts
-@section Mysterious Reduce/Reduce Conflicts
+@node Mysterious Conflicts
+@section Mysterious Conflicts
+@cindex Mysterious Conflicts
Sometimes reduce/reduce conflicts can occur that don't look warranted.
Here is an example:
%token ID
%%
-def: param_spec return_spec ','
- ;
+def: param_spec return_spec ',';
param_spec:
- type
- | name_list ':' type
- ;
+ type
+| name_list ':' type
+;
@end group
@group
return_spec:
- type
- | name ':' type
- ;
+ type
+| name ':' type
+;
@end group
@group
-type: ID
- ;
+type: ID;
@end group
@group
-name: ID
- ;
+name: ID;
name_list:
- name
- | name ',' name_list
- ;
+ name
+| name ',' name_list
+;
@end group
@end example
It would seem that this grammar can be parsed with only a single token
-of look-ahead: when a @code{param_spec} is being read, an @code{ID} is
+of lookahead: when a @code{param_spec} is being read, an @code{ID} is
a @code{name} if a comma or colon follows, or a @code{type} if another
@code{ID} follows. In other words, this grammar is LR(1).
-@cindex LR(1)
-@cindex LALR(1)
-However, Bison, like most parser generators, cannot actually handle all
-LR(1) grammars. In this grammar, two contexts, that after an @code{ID}
-at the beginning of a @code{param_spec} and likewise at the beginning of
-a @code{return_spec}, are similar enough that Bison assumes they are the
-same. They appear similar because the same set of rules would be
+@cindex LR
+@cindex LALR
+However, for historical reasons, Bison cannot by default handle all
+LR(1) grammars.
+In this grammar, two contexts, that after an @code{ID} at the beginning
+of a @code{param_spec} and likewise at the beginning of a
+@code{return_spec}, are similar enough that Bison assumes they are the
+same.
+They appear similar because the same set of rules would be
active---the rule for reducing to a @code{name} and that for reducing to
a @code{type}. Bison is unable to determine at that stage of processing
-that the rules would require different look-ahead tokens in the two
+that the rules would require different lookahead tokens in the two
contexts, so it makes a single parser state for them both. Combining
the two contexts causes a conflict later. In parser terminology, this
occurrence means that the grammar is not LALR(1).
-In general, it is better to fix deficiencies than to document them. But
-this particular deficiency is intrinsically hard to fix; parser
-generators that can handle LR(1) grammars are hard to write and tend to
-produce parsers that are very large. In practice, Bison is more useful
-as it is now.
-
-When the problem arises, you can often fix it by identifying the two
-parser states that are being confused, and adding something to make them
-look distinct. In the above example, adding one rule to
+@cindex IELR
+@cindex canonical LR
+For many practical grammars (specifically those that fall into the non-LR(1)
+class), the limitations of LALR(1) result in difficulties beyond just
+mysterious reduce/reduce conflicts. The best way to fix all these problems
+is to select a different parser table construction algorithm. Either
+IELR(1) or canonical LR(1) would suffice, but the former is more efficient
+and easier to debug during development. @xref{LR Table Construction}, for
+details. (Bison's IELR(1) and canonical LR(1) implementations are
+experimental. More user feedback will help to stabilize them.)
+
+If you instead wish to work around LALR(1)'s limitations, you
+can often fix a mysterious conflict by identifying the two parser states
+that are being confused, and adding something to make them look
+distinct. In the above example, adding one rule to
@code{return_spec} as follows makes the problem go away:
@example
%%
@dots{}
return_spec:
- type
- | name ':' type
- /* This rule is never used. */
- | ID BOGUS
- ;
+ type
+| name ':' type
+| ID BOGUS /* This rule is never used. */
+;
@end group
@end example
@example
param_spec:
- type
- | name_list ':' type
- ;
+ type
+| name_list ':' type
+;
return_spec:
- type
- | ID ':' type
- ;
+ type
+| ID ':' type
+;
+@end example
+
+For a more detailed exposition of LALR(1) parsers and parser
+generators, @pxref{Bibliography,,DeRemer 1982}.
+
+@node Tuning LR
+@section Tuning LR
+
+The default behavior of Bison's LR-based parsers is chosen mostly for
+historical reasons, but that behavior is often not robust. For example, in
+the previous section, we discussed the mysterious conflicts that can be
+produced by LALR(1), Bison's default parser table construction algorithm.
+Another example is Bison's @code{%define parse.error verbose} directive,
+which instructs the generated parser to produce verbose syntax error
+messages, which can sometimes contain incorrect information.
+
+In this section, we explore several modern features of Bison that allow you
+to tune fundamental aspects of the generated LR-based parsers. Some of
+these features easily eliminate shortcomings like those mentioned above.
+Others can be helpful purely for understanding your parser.
+
+Most of the features discussed in this section are still experimental. More
+user feedback will help to stabilize them.
+
+@menu
+* LR Table Construction:: Choose a different construction algorithm.
+* Default Reductions:: Disable default reductions.
+* LAC:: Correct lookahead sets in the parser states.
+* Unreachable States:: Keep unreachable parser states for debugging.
+@end menu
+
+@node LR Table Construction
+@subsection LR Table Construction
+@cindex Mysterious Conflict
+@cindex LALR
+@cindex IELR
+@cindex canonical LR
+@findex %define lr.type
+
+For historical reasons, Bison constructs LALR(1) parser tables by default.
+However, LALR does not possess the full language-recognition power of LR.
+As a result, the behavior of parsers employing LALR parser tables is often
+mysterious. We presented a simple example of this effect in @ref{Mysterious
+Conflicts}.
+
+As we also demonstrated in that example, the traditional approach to
+eliminating such mysterious behavior is to restructure the grammar.
+Unfortunately, doing so correctly is often difficult. Moreover, merely
+discovering that LALR causes mysterious behavior in your parser can be
+difficult as well.
+
+Fortunately, Bison provides an easy way to eliminate the possibility of such
+mysterious behavior altogether. You simply need to activate a more powerful
+parser table construction algorithm by using the @code{%define lr.type}
+directive.
+
+@deffn {Directive} {%define lr.type @var{TYPE}}
+Specify the type of parser tables within the LR(1) family. The accepted
+values for @var{TYPE} are:
+
+@itemize
+@item @code{lalr} (default)
+@item @code{ielr}
+@item @code{canonical-lr}
+@end itemize
+
+(This feature is experimental. More user feedback will help to stabilize
+it.)
+@end deffn
+
+For example, to activate IELR, you might add the following directive to you
+grammar file:
+
+@example
+%define lr.type ielr
@end example
-@node Stack Overflow
-@section Stack Overflow, and How to Avoid It
+@noindent For the example in @ref{Mysterious Conflicts}, the mysterious
+conflict is then eliminated, so there is no need to invest time in
+comprehending the conflict or restructuring the grammar to fix it. If,
+during future development, the grammar evolves such that all mysterious
+behavior would have disappeared using just LALR, you need not fear that
+continuing to use IELR will result in unnecessarily large parser tables.
+That is, IELR generates LALR tables when LALR (using a deterministic parsing
+algorithm) is sufficient to support the full language-recognition power of
+LR. Thus, by enabling IELR at the start of grammar development, you can
+safely and completely eliminate the need to consider LALR's shortcomings.
+
+While IELR is almost always preferable, there are circumstances where LALR
+or the canonical LR parser tables described by Knuth
+(@pxref{Bibliography,,Knuth 1965}) can be useful. Here we summarize the
+relative advantages of each parser table construction algorithm within
+Bison:
+
+@itemize
+@item LALR
+
+There are at least two scenarios where LALR can be worthwhile:
+
+@itemize
+@item GLR without static conflict resolution.
+
+@cindex GLR with LALR
+When employing GLR parsers (@pxref{GLR Parsers}), if you do not resolve any
+conflicts statically (for example, with @code{%left} or @code{%prec}), then
+the parser explores all potential parses of any given input. In this case,
+the choice of parser table construction algorithm is guaranteed not to alter
+the language accepted by the parser. LALR parser tables are the smallest
+parser tables Bison can currently construct, so they may then be preferable.
+Nevertheless, once you begin to resolve conflicts statically, GLR behaves
+more like a deterministic parser in the syntactic contexts where those
+conflicts appear, and so either IELR or canonical LR can then be helpful to
+avoid LALR's mysterious behavior.
+
+@item Malformed grammars.
+
+Occasionally during development, an especially malformed grammar with a
+major recurring flaw may severely impede the IELR or canonical LR parser
+table construction algorithm. LALR can be a quick way to construct parser
+tables in order to investigate such problems while ignoring the more subtle
+differences from IELR and canonical LR.
+@end itemize
+
+@item IELR
+
+IELR (Inadequacy Elimination LR) is a minimal LR algorithm. That is, given
+any grammar (LR or non-LR), parsers using IELR or canonical LR parser tables
+always accept exactly the same set of sentences. However, like LALR, IELR
+merges parser states during parser table construction so that the number of
+parser states is often an order of magnitude less than for canonical LR.
+More importantly, because canonical LR's extra parser states may contain
+duplicate conflicts in the case of non-LR grammars, the number of conflicts
+for IELR is often an order of magnitude less as well. This effect can
+significantly reduce the complexity of developing a grammar.
+
+@item Canonical LR
+
+@cindex delayed syntax error detection
+@cindex LAC
+@findex %nonassoc
+While inefficient, canonical LR parser tables can be an interesting means to
+explore a grammar because they possess a property that IELR and LALR tables
+do not. That is, if @code{%nonassoc} is not used and default reductions are
+left disabled (@pxref{Default Reductions}), then, for every left context of
+every canonical LR state, the set of tokens accepted by that state is
+guaranteed to be the exact set of tokens that is syntactically acceptable in
+that left context. It might then seem that an advantage of canonical LR
+parsers in production is that, under the above constraints, they are
+guaranteed to detect a syntax error as soon as possible without performing
+any unnecessary reductions. However, IELR parsers that use LAC are also
+able to achieve this behavior without sacrificing @code{%nonassoc} or
+default reductions. For details and a few caveats of LAC, @pxref{LAC}.
+@end itemize
+
+For a more detailed exposition of the mysterious behavior in LALR parsers
+and the benefits of IELR, @pxref{Bibliography,,Denny 2008 March}, and
+@ref{Bibliography,,Denny 2010 November}.
+
+@node Default Reductions
+@subsection Default Reductions
+@cindex default reductions
+@findex %define lr.default-reductions
+@findex %nonassoc
+
+After parser table construction, Bison identifies the reduction with the
+largest lookahead set in each parser state. To reduce the size of the
+parser state, traditional Bison behavior is to remove that lookahead set and
+to assign that reduction to be the default parser action. Such a reduction
+is known as a @dfn{default reduction}.
+
+Default reductions affect more than the size of the parser tables. They
+also affect the behavior of the parser:
+
+@itemize
+@item Delayed @code{yylex} invocations.
+
+@cindex delayed yylex invocations
+@cindex consistent states
+@cindex defaulted states
+A @dfn{consistent state} is a state that has only one possible parser
+action. If that action is a reduction and is encoded as a default
+reduction, then that consistent state is called a @dfn{defaulted state}.
+Upon reaching a defaulted state, a Bison-generated parser does not bother to
+invoke @code{yylex} to fetch the next token before performing the reduction.
+In other words, whether default reductions are enabled in consistent states
+determines how soon a Bison-generated parser invokes @code{yylex} for a
+token: immediately when it @emph{reaches} that token in the input or when it
+eventually @emph{needs} that token as a lookahead to determine the next
+parser action. Traditionally, default reductions are enabled, and so the
+parser exhibits the latter behavior.
+
+The presence of defaulted states is an important consideration when
+designing @code{yylex} and the grammar file. That is, if the behavior of
+@code{yylex} can influence or be influenced by the semantic actions
+associated with the reductions in defaulted states, then the delay of the
+next @code{yylex} invocation until after those reductions is significant.
+For example, the semantic actions might pop a scope stack that @code{yylex}
+uses to determine what token to return. Thus, the delay might be necessary
+to ensure that @code{yylex} does not look up the next token in a scope that
+should already be considered closed.
+
+@item Delayed syntax error detection.
+
+@cindex delayed syntax error detection
+When the parser fetches a new token by invoking @code{yylex}, it checks
+whether there is an action for that token in the current parser state. The
+parser detects a syntax error if and only if either (1) there is no action
+for that token or (2) the action for that token is the error action (due to
+the use of @code{%nonassoc}). However, if there is a default reduction in
+that state (which might or might not be a defaulted state), then it is
+impossible for condition 1 to exist. That is, all tokens have an action.
+Thus, the parser sometimes fails to detect the syntax error until it reaches
+a later state.
+
+@cindex LAC
+@c If there's an infinite loop, default reductions can prevent an incorrect
+@c sentence from being rejected.
+While default reductions never cause the parser to accept syntactically
+incorrect sentences, the delay of syntax error detection can have unexpected
+effects on the behavior of the parser. However, the delay can be caused
+anyway by parser state merging and the use of @code{%nonassoc}, and it can
+be fixed by another Bison feature, LAC. We discuss the effects of delayed
+syntax error detection and LAC more in the next section (@pxref{LAC}).
+@end itemize
+
+For canonical LR, the only default reduction that Bison enables by default
+is the accept action, which appears only in the accepting state, which has
+no other action and is thus a defaulted state. However, the default accept
+action does not delay any @code{yylex} invocation or syntax error detection
+because the accept action ends the parse.
+
+For LALR and IELR, Bison enables default reductions in nearly all states by
+default. There are only two exceptions. First, states that have a shift
+action on the @code{error} token do not have default reductions because
+delayed syntax error detection could then prevent the @code{error} token
+from ever being shifted in that state. However, parser state merging can
+cause the same effect anyway, and LAC fixes it in both cases, so future
+versions of Bison might drop this exception when LAC is activated. Second,
+GLR parsers do not record the default reduction as the action on a lookahead
+token for which there is a conflict. The correct action in this case is to
+split the parse instead.
+
+To adjust which states have default reductions enabled, use the
+@code{%define lr.default-reductions} directive.
+
+@deffn {Directive} {%define lr.default-reductions @var{WHERE}}
+Specify the kind of states that are permitted to contain default reductions.
+The accepted values of @var{WHERE} are:
+@itemize
+@item @code{most} (default for LALR and IELR)
+@item @code{consistent}
+@item @code{accepting} (default for canonical LR)
+@end itemize
+
+(The ability to specify where default reductions are permitted is
+experimental. More user feedback will help to stabilize it.)
+@end deffn
+
+@node LAC
+@subsection LAC
+@findex %define parse.lac
+@cindex LAC
+@cindex lookahead correction
+
+Canonical LR, IELR, and LALR can suffer from a couple of problems upon
+encountering a syntax error. First, the parser might perform additional
+parser stack reductions before discovering the syntax error. Such
+reductions can perform user semantic actions that are unexpected because
+they are based on an invalid token, and they cause error recovery to begin
+in a different syntactic context than the one in which the invalid token was
+encountered. Second, when verbose error messages are enabled (@pxref{Error
+Reporting}), the expected token list in the syntax error message can both
+contain invalid tokens and omit valid tokens.
+
+The culprits for the above problems are @code{%nonassoc}, default reductions
+in inconsistent states (@pxref{Default Reductions}), and parser state
+merging. Because IELR and LALR merge parser states, they suffer the most.
+Canonical LR can suffer only if @code{%nonassoc} is used or if default
+reductions are enabled for inconsistent states.
+
+LAC (Lookahead Correction) is a new mechanism within the parsing algorithm
+that solves these problems for canonical LR, IELR, and LALR without
+sacrificing @code{%nonassoc}, default reductions, or state merging. You can
+enable LAC with the @code{%define parse.lac} directive.
+
+@deffn {Directive} {%define parse.lac @var{VALUE}}
+Enable LAC to improve syntax error handling.
+@itemize
+@item @code{none} (default)
+@item @code{full}
+@end itemize
+(This feature is experimental. More user feedback will help to stabilize
+it. Moreover, it is currently only available for deterministic parsers in
+C.)
+@end deffn
+
+Conceptually, the LAC mechanism is straight-forward. Whenever the parser
+fetches a new token from the scanner so that it can determine the next
+parser action, it immediately suspends normal parsing and performs an
+exploratory parse using a temporary copy of the normal parser state stack.
+During this exploratory parse, the parser does not perform user semantic
+actions. If the exploratory parse reaches a shift action, normal parsing
+then resumes on the normal parser stacks. If the exploratory parse reaches
+an error instead, the parser reports a syntax error. If verbose syntax
+error messages are enabled, the parser must then discover the list of
+expected tokens, so it performs a separate exploratory parse for each token
+in the grammar.
+
+There is one subtlety about the use of LAC. That is, when in a consistent
+parser state with a default reduction, the parser will not attempt to fetch
+a token from the scanner because no lookahead is needed to determine the
+next parser action. Thus, whether default reductions are enabled in
+consistent states (@pxref{Default Reductions}) affects how soon the parser
+detects a syntax error: immediately when it @emph{reaches} an erroneous
+token or when it eventually @emph{needs} that token as a lookahead to
+determine the next parser action. The latter behavior is probably more
+intuitive, so Bison currently provides no way to achieve the former behavior
+while default reductions are enabled in consistent states.
+
+Thus, when LAC is in use, for some fixed decision of whether to enable
+default reductions in consistent states, canonical LR and IELR behave almost
+exactly the same for both syntactically acceptable and syntactically
+unacceptable input. While LALR still does not support the full
+language-recognition power of canonical LR and IELR, LAC at least enables
+LALR's syntax error handling to correctly reflect LALR's
+language-recognition power.
+
+There are a few caveats to consider when using LAC:
+
+@itemize
+@item Infinite parsing loops.
+
+IELR plus LAC does have one shortcoming relative to canonical LR. Some
+parsers generated by Bison can loop infinitely. LAC does not fix infinite
+parsing loops that occur between encountering a syntax error and detecting
+it, but enabling canonical LR or disabling default reductions sometimes
+does.
+
+@item Verbose error message limitations.
+
+Because of internationalization considerations, Bison-generated parsers
+limit the size of the expected token list they are willing to report in a
+verbose syntax error message. If the number of expected tokens exceeds that
+limit, the list is simply dropped from the message. Enabling LAC can
+increase the size of the list and thus cause the parser to drop it. Of
+course, dropping the list is better than reporting an incorrect list.
+
+@item Performance.
+
+Because LAC requires many parse actions to be performed twice, it can have a
+performance penalty. However, not all parse actions must be performed
+twice. Specifically, during a series of default reductions in consistent
+states and shift actions, the parser never has to initiate an exploratory
+parse. Moreover, the most time-consuming tasks in a parse are often the
+file I/O, the lexical analysis performed by the scanner, and the user's
+semantic actions, but none of these are performed during the exploratory
+parse. Finally, the base of the temporary stack used during an exploratory
+parse is a pointer into the normal parser state stack so that the stack is
+never physically copied. In our experience, the performance penalty of LAC
+has proved insignificant for practical grammars.
+@end itemize
+
+While the LAC algorithm shares techniques that have been recognized in the
+parser community for years, for the publication that introduces LAC,
+@pxref{Bibliography,,Denny 2010 May}.
+
+@node Unreachable States
+@subsection Unreachable States
+@findex %define lr.keep-unreachable-states
+@cindex unreachable states
+
+If there exists no sequence of transitions from the parser's start state to
+some state @var{s}, then Bison considers @var{s} to be an @dfn{unreachable
+state}. A state can become unreachable during conflict resolution if Bison
+disables a shift action leading to it from a predecessor state.
+
+By default, Bison removes unreachable states from the parser after conflict
+resolution because they are useless in the generated parser. However,
+keeping unreachable states is sometimes useful when trying to understand the
+relationship between the parser and the grammar.
+
+@deffn {Directive} {%define lr.keep-unreachable-states @var{VALUE}}
+Request that Bison allow unreachable states to remain in the parser tables.
+@var{VALUE} must be a Boolean. The default is @code{false}.
+@end deffn
+
+There are a few caveats to consider:
+
+@itemize @bullet
+@item Missing or extraneous warnings.
+
+Unreachable states may contain conflicts and may use rules not used in any
+other state. Thus, keeping unreachable states may induce warnings that are
+irrelevant to your parser's behavior, and it may eliminate warnings that are
+relevant. Of course, the change in warnings may actually be relevant to a
+parser table analysis that wants to keep unreachable states, so this
+behavior will likely remain in future Bison releases.
+
+@item Other useless states.
+
+While Bison is able to remove unreachable states, it is not guaranteed to
+remove other kinds of useless states. Specifically, when Bison disables
+reduce actions during conflict resolution, some goto actions may become
+useless, and thus some additional states may become useless. If Bison were
+to compute which goto actions were useless and then disable those actions,
+it could identify such states as unreachable and then remove those states.
+However, Bison does not compute which goto actions are useless.
+@end itemize
+
+@node Generalized LR Parsing
+@section Generalized LR (GLR) Parsing
+@cindex GLR parsing
+@cindex generalized LR (GLR) parsing
+@cindex ambiguous grammars
+@cindex nondeterministic parsing
+
+Bison produces @emph{deterministic} parsers that choose uniquely
+when to reduce and which reduction to apply
+based on a summary of the preceding input and on one extra token of lookahead.
+As a result, normal Bison handles a proper subset of the family of
+context-free languages.
+Ambiguous grammars, since they have strings with more than one possible
+sequence of reductions cannot have deterministic parsers in this sense.
+The same is true of languages that require more than one symbol of
+lookahead, since the parser lacks the information necessary to make a
+decision at the point it must be made in a shift-reduce parser.
+Finally, as previously mentioned (@pxref{Mysterious Conflicts}),
+there are languages where Bison's default choice of how to
+summarize the input seen so far loses necessary information.
+
+When you use the @samp{%glr-parser} declaration in your grammar file,
+Bison generates a parser that uses a different algorithm, called
+Generalized LR (or GLR). A Bison GLR
+parser uses the same basic
+algorithm for parsing as an ordinary Bison parser, but behaves
+differently in cases where there is a shift-reduce conflict that has not
+been resolved by precedence rules (@pxref{Precedence}) or a
+reduce-reduce conflict. When a GLR parser encounters such a
+situation, it
+effectively @emph{splits} into a several parsers, one for each possible
+shift or reduction. These parsers then proceed as usual, consuming
+tokens in lock-step. Some of the stacks may encounter other conflicts
+and split further, with the result that instead of a sequence of states,
+a Bison GLR parsing stack is what is in effect a tree of states.
+
+In effect, each stack represents a guess as to what the proper parse
+is. Additional input may indicate that a guess was wrong, in which case
+the appropriate stack silently disappears. Otherwise, the semantics
+actions generated in each stack are saved, rather than being executed
+immediately. When a stack disappears, its saved semantic actions never
+get executed. When a reduction causes two stacks to become equivalent,
+their sets of semantic actions are both saved with the state that
+results from the reduction. We say that two stacks are equivalent
+when they both represent the same sequence of states,
+and each pair of corresponding states represents a
+grammar symbol that produces the same segment of the input token
+stream.
+
+Whenever the parser makes a transition from having multiple
+states to having one, it reverts to the normal deterministic parsing
+algorithm, after resolving and executing the saved-up actions.
+At this transition, some of the states on the stack will have semantic
+values that are sets (actually multisets) of possible actions. The
+parser tries to pick one of the actions by first finding one whose rule
+has the highest dynamic precedence, as set by the @samp{%dprec}
+declaration. Otherwise, if the alternative actions are not ordered by
+precedence, but there the same merging function is declared for both
+rules by the @samp{%merge} declaration,
+Bison resolves and evaluates both and then calls the merge function on
+the result. Otherwise, it reports an ambiguity.
+
+It is possible to use a data structure for the GLR parsing tree that
+permits the processing of any LR(1) grammar in linear time (in the
+size of the input), any unambiguous (not necessarily
+LR(1)) grammar in
+quadratic worst-case time, and any general (possibly ambiguous)
+context-free grammar in cubic worst-case time. However, Bison currently
+uses a simpler data structure that requires time proportional to the
+length of the input times the maximum number of stacks required for any
+prefix of the input. Thus, really ambiguous or nondeterministic
+grammars can require exponential time and space to process. Such badly
+behaving examples, however, are not generally of practical interest.
+Usually, nondeterminism in a grammar is local---the parser is ``in
+doubt'' only for a few tokens at a time. Therefore, the current data
+structure should generally be adequate. On LR(1) portions of a
+grammar, in particular, it is only slightly slower than with the
+deterministic LR(1) Bison parser.
+
+For a more detailed exposition of GLR parsers, @pxref{Bibliography,,Scott
+2000}.
+
+@node Memory Management
+@section Memory Management, and How to Avoid Memory Exhaustion
+@cindex memory exhaustion
+@cindex memory management
@cindex stack overflow
@cindex parser stack overflow
@cindex overflow of parser stack
-The Bison parser stack can overflow if too many tokens are shifted and
+The Bison parser stack can run out of memory if too many tokens are shifted and
not reduced. When this happens, the parser function @code{yyparse}
-returns a nonzero value, pausing only to call @code{yyerror} to report
-the overflow.
+calls @code{yyerror} and then returns 2.
+
+Because Bison parsers have growing stacks, hitting the upper limit
+usually results from using a right recursion instead of a left
+recursion, @xref{Recursion, ,Recursive Rules}.
@vindex YYMAXDEPTH
By defining the macro @code{YYMAXDEPTH}, you can control how deep the
-parser stack can become before a stack overflow occurs. Define the
+parser stack can become before memory is exhausted. Define the
macro with a value that is an integer. This value is the maximum number
of tokens that can be shifted (and not reduced) before overflow.
-It must be a constant expression whose value is known at compile time.
The stack space allowed is not necessarily allocated. If you specify a
-large value for @code{YYMAXDEPTH}, the parser actually allocates a small
+large value for @code{YYMAXDEPTH}, the parser normally allocates a small
stack at first, and then makes it bigger by stages as needed. This
increasing allocation happens automatically and silently. Therefore,
you do not need to make @code{YYMAXDEPTH} painfully small merely to save
space for ordinary inputs that do not need much stack.
+However, do not allow @code{YYMAXDEPTH} to be a value so large that
+arithmetic overflow could occur when calculating the size of the stack
+space. Also, do not allow @code{YYMAXDEPTH} to be less than
+@code{YYINITDEPTH}.
+
@cindex default stack limit
The default value of @code{YYMAXDEPTH}, if you do not define it, is
10000.
@vindex YYINITDEPTH
You can control how much stack is allocated initially by defining the
-macro @code{YYINITDEPTH}. This value too must be a compile-time
-constant integer. The default is 200.
+macro @code{YYINITDEPTH} to a positive integer. For the deterministic
+parser in C, this value must be a compile-time constant
+unless you are assuming C99 or some other target language or compiler
+that allows variable-length arrays. The default is 200.
+
+Do not allow @code{YYINITDEPTH} to be greater than @code{YYMAXDEPTH}.
+
+You can generate a deterministic parser containing C++ user code from
+the default (C) skeleton, as well as from the C++ skeleton
+(@pxref{C++ Parsers}). However, if you do use the default skeleton
+and want to allow the parsing stack to grow,
+be careful not to use semantic types or location types that require
+non-trivial copy constructors.
+The C skeleton bypasses these constructors when copying data to
+new, larger stacks.
@node Error Recovery
@chapter Error Recovery
@cindex error recovery
@cindex recovery from errors
-It is not usually acceptable to have a program terminate on a parse
+It is not usually acceptable to have a program terminate on a syntax
error. For example, a compiler should recover sufficiently to parse the
rest of the input file and check it for errors; a calculator should accept
another expression.
For example:
@example
-stmnts: /* empty string */
- | stmnts '\n'
- | stmnts exp '\n'
- | stmnts error '\n'
+stmts:
+ /* empty string */
+| stmts '\n'
+| stmts exp '\n'
+| stmts error '\n'
@end example
The fourth rule in this example says that an error followed by a newline
-makes a valid addition to any @code{stmnts}.
+makes a valid addition to any @code{stmts}.
What happens if a syntax error occurs in the middle of an @code{exp}? The
error recovery rule, interpreted strictly, applies to the precise sequence
-of a @code{stmnts}, an @code{error} and a newline. If an error occurs in
+of a @code{stmts}, an @code{error} and a newline. If an error occurs in
the middle of an @code{exp}, there will probably be some additional tokens
-and subexpressions on the stack after the last @code{stmnts}, and there
+and subexpressions on the stack after the last @code{stmts}, and there
will be tokens to read before the next newline. So the rule is not
applicable in the ordinary way.
But Bison can force the situation to fit the rule, by discarding part of
-the semantic context and part of the input. First it discards states and
-objects from the stack until it gets back to a state in which the
+the semantic context and part of the input. First it discards states
+and objects from the stack until it gets back to a state in which the
@code{error} token is acceptable. (This means that the subexpressions
-already parsed are discarded, back to the last complete @code{stmnts}.) At
-this point the @code{error} token can be shifted. Then, if the old
-look-ahead token is not acceptable to be shifted next, the parser reads
+already parsed are discarded, back to the last complete @code{stmts}.)
+At this point the @code{error} token can be shifted. Then, if the old
+lookahead token is not acceptable to be shifted next, the parser reads
tokens and discards them until it finds a token which is acceptable. In
-this example, Bison reads and discards input until the next newline
-so that the fourth rule can apply.
+this example, Bison reads and discards input until the next newline so
+that the fourth rule can apply. Note that discarded symbols are
+possible sources of memory leaks, see @ref{Destructor Decl, , Freeing
+Discarded Symbols}, for a means to reclaim this memory.
The choice of error rules in the grammar is a choice of strategies for
error recovery. A simple and useful strategy is simply to skip the rest of
the current input line or current statement if an error is detected:
@example
-stmnt: error ';' /* on error, skip until ';' is read */
+stmt: error ';' /* On error, skip until ';' is read. */
@end example
It is also useful to recover to the matching close-delimiter of an
spurious error message:
@example
-primary: '(' expr ')'
- | '(' error ')'
- @dots{}
- ;
+primary:
+ '(' expr ')'
+| '(' error ')'
+@dots{}
+;
@end example
Error recovery strategies are necessarily guesses. When they guess wrong,
one syntax error often leads to another. In the above example, the error
recovery rule guesses that an error is due to bad input within one
-@code{stmnt}. Suppose that instead a spurious semicolon is inserted in the
-middle of a valid @code{stmnt}. After the error recovery rule recovers
+@code{stmt}. Suppose that instead a spurious semicolon is inserted in the
+middle of a valid @code{stmt}. After the error recovery rule recovers
from the first error, another syntax error will be found straightaway,
since the text following the spurious semicolon is also an invalid
-@code{stmnt}.
+@code{stmt}.
To prevent an outpouring of error messages, the parser will output no error
message for another syntax error that happens shortly after the first; only
@samp{yyerrok;} is a valid C statement.
@findex yyclearin
-The previous look-ahead token is reanalyzed immediately after an error. If
+The previous lookahead token is reanalyzed immediately after an error. If
this is unacceptable, then the macro @code{yyclearin} may be used to clear
this token. Write the statement @samp{yyclearin;} in the error rule's
action.
+@xref{Action Features, ,Special Features for Use in Actions}.
-For example, suppose that on a parse error, an error handling routine is
+For example, suppose that on a syntax error, an error handling routine is
called that advances the input stream to some point where parsing should
once again commence. The next symbol returned by the lexical scanner is
-probably correct. The previous look-ahead token ought to be discarded
+probably correct. The previous lookahead token ought to be discarded
with @samp{yyclearin;}.
@vindex YYRECOVERING
-The macro @code{YYRECOVERING} stands for an expression that has the
-value 1 when the parser is recovering from a syntax error, and 0 the
-rest of the time. A value of 1 indicates that error messages are
-currently suppressed for new syntax errors.
+The expression @code{YYRECOVERING ()} yields 1 when the parser
+is recovering from a syntax error, and 0 otherwise.
+Syntax error diagnostics are suppressed while recovering from a syntax
+error.
@node Context Dependency
@chapter Handling Context Dependencies
earlier:
@example
-typedef int foo, bar, lose;
-static foo (bar); /* @r{redeclare @code{bar} as static variable} */
-static int foo (lose); /* @r{redeclare @code{foo} as function} */
+typedef int foo, bar;
+int baz (void)
+@group
+@{
+ static bar (bar); /* @r{redeclare @code{bar} as static variable} */
+ extern foo foo (foo); /* @r{redeclare @code{foo} as function} */
+ return foo (bar);
+@}
+@end group
@end example
Unfortunately, the name being declared is separated from the declaration
duplication, with actions omitted for brevity:
@example
+@group
initdcl:
- declarator maybeasm '='
- init
- | declarator maybeasm
- ;
+ declarator maybeasm '=' init
+| declarator maybeasm
+;
+@end group
+@group
notype_initdcl:
- notype_declarator maybeasm '='
- init
- | notype_declarator maybeasm
- ;
+ notype_declarator maybeasm '=' init
+| notype_declarator maybeasm
+;
+@end group
@end example
@noindent
@example
@group
%@{
-int hexflag;
+ int hexflag;
+ int yylex (void);
+ void yyerror (char const *);
%@}
%%
@dots{}
@end group
@group
-expr: IDENTIFIER
- | constant
- | HEX '('
- @{ hexflag = 1; @}
- expr ')'
- @{ hexflag = 0;
- $$ = $4; @}
- | expr '+' expr
- @{ $$ = make_sum ($1, $3); @}
- @dots{}
- ;
+expr:
+ IDENTIFIER
+| constant
+| HEX '(' @{ hexflag = 1; @}
+ expr ')' @{ hexflag = 0; $$ = $4; @}
+| expr '+' expr @{ $$ = make_sum ($1, $3); @}
+@dots{}
+;
@end group
@group
constant:
- INTEGER
- | STRING
- ;
+ INTEGER
+| STRING
+;
@end group
@end example
it is nonzero, all integers are parsed in hexadecimal, and tokens starting
with letters are parsed as integers if possible.
-The declaration of @code{hexflag} shown in the prologue of the parser file
-is needed to make it accessible to the actions (@pxref{Prologue, ,The Prologue}).
-You must also write the code in @code{yylex} to obey the flag.
+The declaration of @code{hexflag} shown in the prologue of the grammar
+file is needed to make it accessible to the actions (@pxref{Prologue,
+,The Prologue}). You must also write the code in @code{yylex} to obey
+the flag.
@node Tie-in Recovery
@section Lexical Tie-ins and Error Recovery
tokens until the next semicolon, and then start a new statement, like this:
@example
-stmt: expr ';'
- | IF '(' expr ')' stmt @{ @dots{} @}
- @dots{}
- error ';'
- @{ hexflag = 0; @}
- ;
+stmt:
+ expr ';'
+| IF '(' expr ')' stmt @{ @dots{} @}
+@dots{}
+| error ';' @{ hexflag = 0; @}
+;
@end example
If there is a syntax error in the middle of a @samp{hex (@var{expr})}
@example
@group
-expr: @dots{}
- | '(' expr ')'
- @{ $$ = $2; @}
- | '(' error ')'
- @dots{}
+expr:
+ @dots{}
+| '(' expr ')' @{ $$ = $2; @}
+| '(' error ')'
+@dots{}
@end group
@end example
be such that you can be sure that it always will, or always won't, have to
clear the flag.
+@c ================================================== Debugging Your Parser
+
@node Debugging
@chapter Debugging Your Parser
-@findex YYDEBUG
-@findex yydebug
-@cindex debugging
-@cindex tracing the parser
-If a Bison grammar compiles properly but doesn't do what you want when it
-runs, the @code{yydebug} parser-trace feature can help you figure out why.
+Developing a parser can be a challenge, especially if you don't
+understand the algorithm (@pxref{Algorithm, ,The Bison Parser
+Algorithm}). Even so, sometimes a detailed description of the automaton
+can help (@pxref{Understanding, , Understanding Your Parser}), or
+tracing the execution of the parser can give some insight on why it
+behaves improperly (@pxref{Tracing, , Tracing Your Parser}).
-To enable compilation of trace facilities, you must define the macro
-@code{YYDEBUG} when you compile the parser. You could use @samp{-DYYDEBUG=1}
-as a compiler option or you could put @samp{#define YYDEBUG 1} in the prologue
-of the grammar file (@pxref{Prologue, , The Prologue}). Alternatively, use the
-@samp{-t} option when you run Bison (@pxref{Invocation, ,Invoking Bison}).
-We always define @code{YYDEBUG} so that debugging is always possible.
+@menu
+* Understanding:: Understanding the structure of your parser.
+* Tracing:: Tracing the execution of your parser.
+@end menu
-The trace facility uses @code{stderr}, so you must add
-@w{@code{#include <stdio.h>}} to the prologue unless it is already there.
+@node Understanding
+@section Understanding Your Parser
-Once you have compiled the program with trace facilities, the way to
-request a trace is to store a nonzero value in the variable @code{yydebug}.
-You can do this by making the C code do it (in @code{main}, perhaps), or
-you can alter the value with a C debugger.
+As documented elsewhere (@pxref{Algorithm, ,The Bison Parser Algorithm})
+Bison parsers are @dfn{shift/reduce automata}. In some cases (much more
+frequent than one would hope), looking at this automaton is required to
+tune or simply fix a parser. Bison provides two different
+representation of it, either textually or graphically (as a DOT file).
-Each step taken by the parser when @code{yydebug} is nonzero produces a
-line or two of trace information, written on @code{stderr}. The trace
-messages tell you these things:
+The textual file is generated when the options @option{--report} or
+@option{--verbose} are specified, see @xref{Invocation, , Invoking
+Bison}. Its name is made by removing @samp{.tab.c} or @samp{.c} from
+the parser implementation file name, and adding @samp{.output}
+instead. Therefore, if the grammar file is @file{foo.y}, then the
+parser implementation file is called @file{foo.tab.c} by default. As
+a consequence, the verbose output file is called @file{foo.output}.
-@itemize @bullet
-@item
-Each time the parser calls @code{yylex}, what kind of token was read.
+The following grammar file, @file{calc.y}, will be used in the sequel:
-@item
-Each time a token is shifted, the depth and complete contents of the
-state stack (@pxref{Parser States}).
+@example
+%token NUM STR
+%left '+' '-'
+%left '*'
+%%
+exp:
+ exp '+' exp
+| exp '-' exp
+| exp '*' exp
+| exp '/' exp
+| NUM
+;
+useless: STR;
+%%
+@end example
-@item
-Each time a rule is reduced, which rule it is, and the complete contents
-of the state stack afterward.
-@end itemize
+@command{bison} reports:
-To make sense of this information, it helps to refer to the listing file
-produced by the Bison @samp{-v} option (@pxref{Invocation, ,Invoking Bison}). This file
-shows the meaning of each state in terms of positions in various rules, and
-also what each state will do with each possible input token. As you read
-the successive trace messages, you can see that the parser is functioning
-according to its specification in the listing file. Eventually you will
-arrive at the place where something undesirable happens, and you will see
-which parts of the grammar are to blame.
-
-The parser file is a C program and you can use C debuggers on it, but it's
-not easy to interpret what it is doing. The parser function is a
-finite-state machine interpreter, and aside from the actions it executes
-the same code over and over. Only the values of variables show where in
-the grammar it is working.
+@example
+calc.y: warning: 1 nonterminal useless in grammar
+calc.y: warning: 1 rule useless in grammar
+calc.y:11.1-7: warning: nonterminal useless in grammar: useless
+calc.y:11.10-12: warning: rule useless in grammar: useless: STR
+calc.y: conflicts: 7 shift/reduce
+@end example
-@findex YYPRINT
-The debugging information normally gives the token type of each token
-read, but not its semantic value. You can optionally define a macro
-named @code{YYPRINT} to provide a way to print the value. If you define
-@code{YYPRINT}, it should take three arguments. The parser will pass a
-standard I/O stream, the numeric code for the token type, and the token
-value (from @code{yylval}).
+When given @option{--report=state}, in addition to @file{calc.tab.c}, it
+creates a file @file{calc.output} with contents detailed below. The
+order of the output and the exact presentation might vary, but the
+interpretation is the same.
-Here is an example of @code{YYPRINT} suitable for the multi-function
-calculator (@pxref{Mfcalc Decl, ,Declarations for @code{mfcalc}}):
+@noindent
+@cindex token, useless
+@cindex useless token
+@cindex nonterminal, useless
+@cindex useless nonterminal
+@cindex rule, useless
+@cindex useless rule
+The first section reports useless tokens, nonterminals and rules. Useless
+nonterminals and rules are removed in order to produce a smaller parser, but
+useless tokens are preserved, since they might be used by the scanner (note
+the difference between ``useless'' and ``unused'' below):
-@smallexample
-#define YYPRINT(file, type, value) yyprint (file, type, value)
+@example
+Nonterminals useless in grammar
+ useless
-static void
-yyprint (FILE *file, int type, YYSTYPE value)
-@{
- if (type == VAR)
- fprintf (file, " %s", value.tptr->name);
- else if (type == NUM)
- fprintf (file, " %d", value.val);
-@}
-@end smallexample
+Terminals unused in grammar
+ STR
-@node Invocation
-@chapter Invoking Bison
-@cindex invoking Bison
-@cindex Bison invocation
-@cindex options for invoking Bison
+Rules useless in grammar
+ 6 useless: STR
+@end example
-The usual way to invoke Bison is as follows:
+@noindent
+The next section lists states that still have conflicts.
@example
-bison @var{infile}
+State 8 conflicts: 1 shift/reduce
+State 9 conflicts: 1 shift/reduce
+State 10 conflicts: 1 shift/reduce
+State 11 conflicts: 4 shift/reduce
@end example
-Here @var{infile} is the grammar file name, which usually ends in
-@samp{.y}. The parser file's name is made by replacing the @samp{.y}
-with @samp{.tab.c}. Thus, the @samp{bison foo.y} filename yields
-@file{foo.tab.c}, and the @samp{bison hack/foo.y} filename yields
-@file{hack/foo.tab.c}. It's is also possible, in case you are writting
-C++ code instead of C in your grammar file, to name it @file{foo.ypp}
-or @file{foo.y++}. Then, the output files will take an extention like
-the given one as input (repectively @file{foo.tab.cpp} and @file{foo.tab.c++}).
-This feature takes effect with all options that manipulate filenames like
-@samp{-o} or @samp{-d}.
-
-For example :
+@noindent
+Then Bison reproduces the exact grammar it used:
@example
-bison -d @var{infile.yxx}
+Grammar
+
+ 0 $accept: exp $end
+
+ 1 exp: exp '+' exp
+ 2 | exp '-' exp
+ 3 | exp '*' exp
+ 4 | exp '/' exp
+ 5 | NUM
@end example
+
@noindent
-will produce @file{infile.tab.cxx} and @file{infile.tab.hxx}. and
+and reports the uses of the symbols:
@example
-bison -d @var{infile.y} -o @var{output.c++}
-@end example
-@noindent
-will produce @file{output.c++} and @file{outfile.h++}.
+@group
+Terminals, with rules where they appear
+
+$end (0) 0
+'*' (42) 3
+'+' (43) 1
+'-' (45) 2
+'/' (47) 4
+error (256)
+NUM (258) 5
+STR (259)
+@end group
+@group
+Nonterminals, with rules where they appear
-@menu
+$accept (9)
+ on left: 0
+exp (10)
+ on left: 1 2 3 4 5, on right: 0 1 2 3 4
+@end group
+@end example
+
+@noindent
+@cindex item
+@cindex pointed rule
+@cindex rule, pointed
+Bison then proceeds onto the automaton itself, describing each state
+with its set of @dfn{items}, also known as @dfn{pointed rules}. Each
+item is a production rule together with a point (@samp{.}) marking
+the location of the input cursor.
+
+@example
+state 0
+
+ 0 $accept: . exp $end
+
+ NUM shift, and go to state 1
+
+ exp go to state 2
+@end example
+
+This reads as follows: ``state 0 corresponds to being at the very
+beginning of the parsing, in the initial rule, right before the start
+symbol (here, @code{exp}). When the parser returns to this state right
+after having reduced a rule that produced an @code{exp}, the control
+flow jumps to state 2. If there is no such transition on a nonterminal
+symbol, and the lookahead is a @code{NUM}, then this token is shifted onto
+the parse stack, and the control flow jumps to state 1. Any other
+lookahead triggers a syntax error.''
+
+@cindex core, item set
+@cindex item set core
+@cindex kernel, item set
+@cindex item set core
+Even though the only active rule in state 0 seems to be rule 0, the
+report lists @code{NUM} as a lookahead token because @code{NUM} can be
+at the beginning of any rule deriving an @code{exp}. By default Bison
+reports the so-called @dfn{core} or @dfn{kernel} of the item set, but if
+you want to see more detail you can invoke @command{bison} with
+@option{--report=itemset} to list the derived items as well:
+
+@example
+state 0
+
+ 0 $accept: . exp $end
+ 1 exp: . exp '+' exp
+ 2 | . exp '-' exp
+ 3 | . exp '*' exp
+ 4 | . exp '/' exp
+ 5 | . NUM
+
+ NUM shift, and go to state 1
+
+ exp go to state 2
+@end example
+
+@noindent
+In the state 1@dots{}
+
+@example
+state 1
+
+ 5 exp: NUM .
+
+ $default reduce using rule 5 (exp)
+@end example
+
+@noindent
+the rule 5, @samp{exp: NUM;}, is completed. Whatever the lookahead token
+(@samp{$default}), the parser will reduce it. If it was coming from
+state 0, then, after this reduction it will return to state 0, and will
+jump to state 2 (@samp{exp: go to state 2}).
+
+@example
+state 2
+
+ 0 $accept: exp . $end
+ 1 exp: exp . '+' exp
+ 2 | exp . '-' exp
+ 3 | exp . '*' exp
+ 4 | exp . '/' exp
+
+ $end shift, and go to state 3
+ '+' shift, and go to state 4
+ '-' shift, and go to state 5
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
+@end example
+
+@noindent
+In state 2, the automaton can only shift a symbol. For instance,
+because of the item @samp{exp: exp . '+' exp}, if the lookahead is
+@samp{+} it is shifted onto the parse stack, and the automaton
+jumps to state 4, corresponding to the item @samp{exp: exp '+' . exp}.
+Since there is no default action, any lookahead not listed triggers a syntax
+error.
+
+@cindex accepting state
+The state 3 is named the @dfn{final state}, or the @dfn{accepting
+state}:
+
+@example
+state 3
+
+ 0 $accept: exp $end .
+
+ $default accept
+@end example
+
+@noindent
+the initial rule is completed (the start symbol and the end-of-input were
+read), the parsing exits successfully.
+
+The interpretation of states 4 to 7 is straightforward, and is left to
+the reader.
+
+@example
+state 4
+
+ 1 exp: exp '+' . exp
+
+ NUM shift, and go to state 1
+
+ exp go to state 8
+
+
+state 5
+
+ 2 exp: exp '-' . exp
+
+ NUM shift, and go to state 1
+
+ exp go to state 9
+
+
+state 6
+
+ 3 exp: exp '*' . exp
+
+ NUM shift, and go to state 1
+
+ exp go to state 10
+
+
+state 7
+
+ 4 exp: exp '/' . exp
+
+ NUM shift, and go to state 1
+
+ exp go to state 11
+@end example
+
+As was announced in beginning of the report, @samp{State 8 conflicts:
+1 shift/reduce}:
+
+@example
+state 8
+
+ 1 exp: exp . '+' exp
+ 1 | exp '+' exp .
+ 2 | exp . '-' exp
+ 3 | exp . '*' exp
+ 4 | exp . '/' exp
+
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
+
+ '/' [reduce using rule 1 (exp)]
+ $default reduce using rule 1 (exp)
+@end example
+
+Indeed, there are two actions associated to the lookahead @samp{/}:
+either shifting (and going to state 7), or reducing rule 1. The
+conflict means that either the grammar is ambiguous, or the parser lacks
+information to make the right decision. Indeed the grammar is
+ambiguous, as, since we did not specify the precedence of @samp{/}, the
+sentence @samp{NUM + NUM / NUM} can be parsed as @samp{NUM + (NUM /
+NUM)}, which corresponds to shifting @samp{/}, or as @samp{(NUM + NUM) /
+NUM}, which corresponds to reducing rule 1.
+
+Because in deterministic parsing a single decision can be made, Bison
+arbitrarily chose to disable the reduction, see @ref{Shift/Reduce, ,
+Shift/Reduce Conflicts}. Discarded actions are reported between
+square brackets.
+
+Note that all the previous states had a single possible action: either
+shifting the next token and going to the corresponding state, or
+reducing a single rule. In the other cases, i.e., when shifting
+@emph{and} reducing is possible or when @emph{several} reductions are
+possible, the lookahead is required to select the action. State 8 is
+one such state: if the lookahead is @samp{*} or @samp{/} then the action
+is shifting, otherwise the action is reducing rule 1. In other words,
+the first two items, corresponding to rule 1, are not eligible when the
+lookahead token is @samp{*}, since we specified that @samp{*} has higher
+precedence than @samp{+}. More generally, some items are eligible only
+with some set of possible lookahead tokens. When run with
+@option{--report=lookahead}, Bison specifies these lookahead tokens:
+
+@example
+state 8
+
+ 1 exp: exp . '+' exp
+ 1 | exp '+' exp . [$end, '+', '-', '/']
+ 2 | exp . '-' exp
+ 3 | exp . '*' exp
+ 4 | exp . '/' exp
+
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
+
+ '/' [reduce using rule 1 (exp)]
+ $default reduce using rule 1 (exp)
+@end example
+
+Note however that while @samp{NUM + NUM / NUM} is ambiguous (which results in
+the conflicts on @samp{/}), @samp{NUM + NUM * NUM} is not: the conflict was
+solved thanks to associativity and precedence directives. If invoked with
+@option{--report=solved}, Bison includes information about the solved
+conflicts in the report:
+
+@example
+Conflict between rule 1 and token '+' resolved as reduce (%left '+').
+Conflict between rule 1 and token '-' resolved as reduce (%left '-').
+Conflict between rule 1 and token '*' resolved as shift ('+' < '*').
+@end example
+
+
+The remaining states are similar:
+
+@example
+@group
+state 9
+
+ 1 exp: exp . '+' exp
+ 2 | exp . '-' exp
+ 2 | exp '-' exp .
+ 3 | exp . '*' exp
+ 4 | exp . '/' exp
+
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
+
+ '/' [reduce using rule 2 (exp)]
+ $default reduce using rule 2 (exp)
+@end group
+
+@group
+state 10
+
+ 1 exp: exp . '+' exp
+ 2 | exp . '-' exp
+ 3 | exp . '*' exp
+ 3 | exp '*' exp .
+ 4 | exp . '/' exp
+
+ '/' shift, and go to state 7
+
+ '/' [reduce using rule 3 (exp)]
+ $default reduce using rule 3 (exp)
+@end group
+
+@group
+state 11
+
+ 1 exp: exp . '+' exp
+ 2 | exp . '-' exp
+ 3 | exp . '*' exp
+ 4 | exp . '/' exp
+ 4 | exp '/' exp .
+
+ '+' shift, and go to state 4
+ '-' shift, and go to state 5
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
+
+ '+' [reduce using rule 4 (exp)]
+ '-' [reduce using rule 4 (exp)]
+ '*' [reduce using rule 4 (exp)]
+ '/' [reduce using rule 4 (exp)]
+ $default reduce using rule 4 (exp)
+@end group
+@end example
+
+@noindent
+Observe that state 11 contains conflicts not only due to the lack of
+precedence of @samp{/} with respect to @samp{+}, @samp{-}, and
+@samp{*}, but also because the
+associativity of @samp{/} is not specified.
+
+
+@node Tracing
+@section Tracing Your Parser
+@findex yydebug
+@cindex debugging
+@cindex tracing the parser
+
+If a Bison grammar compiles properly but doesn't do what you want when it
+runs, the @code{yydebug} parser-trace feature can help you figure out why.
+
+There are several means to enable compilation of trace facilities:
+
+@table @asis
+@item the macro @code{YYDEBUG}
+@findex YYDEBUG
+Define the macro @code{YYDEBUG} to a nonzero value when you compile the
+parser. This is compliant with POSIX Yacc. You could use
+@samp{-DYYDEBUG=1} as a compiler option or you could put @samp{#define
+YYDEBUG 1} in the prologue of the grammar file (@pxref{Prologue, , The
+Prologue}).
+
+@item the option @option{-t}, @option{--debug}
+Use the @samp{-t} option when you run Bison (@pxref{Invocation,
+,Invoking Bison}). This is POSIX compliant too.
+
+@item the directive @samp{%debug}
+@findex %debug
+Add the @code{%debug} directive (@pxref{Decl Summary, ,Bison Declaration
+Summary}). This Bison extension is maintained for backward
+compatibility with previous versions of Bison.
+
+@item the variable @samp{parse.trace}
+@findex %define parse.trace
+Add the @samp{%define parse.trace} directive (@pxref{%define
+Summary,,parse.trace}), or pass the @option{-Dparse.trace} option
+(@pxref{Bison Options}). This is a Bison extension, which is especially
+useful for languages that don't use a preprocessor. Unless POSIX and Yacc
+portability matter to you, this is the preferred solution.
+@end table
+
+We suggest that you always enable the trace option so that debugging is
+always possible.
+
+The trace facility outputs messages with macro calls of the form
+@code{YYFPRINTF (stderr, @var{format}, @var{args})} where
+@var{format} and @var{args} are the usual @code{printf} format and variadic
+arguments. If you define @code{YYDEBUG} to a nonzero value but do not
+define @code{YYFPRINTF}, @code{<stdio.h>} is automatically included
+and @code{YYFPRINTF} is defined to @code{fprintf}.
+
+Once you have compiled the program with trace facilities, the way to
+request a trace is to store a nonzero value in the variable @code{yydebug}.
+You can do this by making the C code do it (in @code{main}, perhaps), or
+you can alter the value with a C debugger.
+
+Each step taken by the parser when @code{yydebug} is nonzero produces a
+line or two of trace information, written on @code{stderr}. The trace
+messages tell you these things:
+
+@itemize @bullet
+@item
+Each time the parser calls @code{yylex}, what kind of token was read.
+
+@item
+Each time a token is shifted, the depth and complete contents of the
+state stack (@pxref{Parser States}).
+
+@item
+Each time a rule is reduced, which rule it is, and the complete contents
+of the state stack afterward.
+@end itemize
+
+To make sense of this information, it helps to refer to the listing file
+produced by the Bison @samp{-v} option (@pxref{Invocation, ,Invoking
+Bison}). This file shows the meaning of each state in terms of
+positions in various rules, and also what each state will do with each
+possible input token. As you read the successive trace messages, you
+can see that the parser is functioning according to its specification in
+the listing file. Eventually you will arrive at the place where
+something undesirable happens, and you will see which parts of the
+grammar are to blame.
+
+The parser implementation file is a C program and you can use C
+debuggers on it, but it's not easy to interpret what it is doing. The
+parser function is a finite-state machine interpreter, and aside from
+the actions it executes the same code over and over. Only the values
+of variables show where in the grammar it is working.
+
+@findex YYPRINT
+The debugging information normally gives the token type of each token
+read, but not its semantic value. You can optionally define a macro
+named @code{YYPRINT} to provide a way to print the value. If you define
+@code{YYPRINT}, it should take three arguments. The parser will pass a
+standard I/O stream, the numeric code for the token type, and the token
+value (from @code{yylval}).
+
+Here is an example of @code{YYPRINT} suitable for the multi-function
+calculator (@pxref{Mfcalc Declarations, ,Declarations for @code{mfcalc}}):
+
+@example
+%@{
+ static void print_token_value (FILE *, int, YYSTYPE);
+ #define YYPRINT(file, type, value) \
+ print_token_value (file, type, value)
+%@}
+
+@dots{} %% @dots{} %% @dots{}
+
+static void
+print_token_value (FILE *file, int type, YYSTYPE value)
+@{
+ if (type == VAR)
+ fprintf (file, "%s", value.tptr->name);
+ else if (type == NUM)
+ fprintf (file, "%d", value.val);
+@}
+@end example
+
+@c ================================================= Invoking Bison
+
+@node Invocation
+@chapter Invoking Bison
+@cindex invoking Bison
+@cindex Bison invocation
+@cindex options for invoking Bison
+
+The usual way to invoke Bison is as follows:
+
+@example
+bison @var{infile}
+@end example
+
+Here @var{infile} is the grammar file name, which usually ends in
+@samp{.y}. The parser implementation file's name is made by replacing
+the @samp{.y} with @samp{.tab.c} and removing any leading directory.
+Thus, the @samp{bison foo.y} file name yields @file{foo.tab.c}, and
+the @samp{bison hack/foo.y} file name yields @file{foo.tab.c}. It's
+also possible, in case you are writing C++ code instead of C in your
+grammar file, to name it @file{foo.ypp} or @file{foo.y++}. Then, the
+output files will take an extension like the given one as input
+(respectively @file{foo.tab.cpp} and @file{foo.tab.c++}). This
+feature takes effect with all options that manipulate file names like
+@samp{-o} or @samp{-d}.
+
+For example :
+
+@example
+bison -d @var{infile.yxx}
+@end example
+@noindent
+will produce @file{infile.tab.cxx} and @file{infile.tab.hxx}, and
+
+@example
+bison -d -o @var{output.c++} @var{infile.y}
+@end example
+@noindent
+will produce @file{output.c++} and @file{outfile.h++}.
+
+For compatibility with POSIX, the standard Bison
+distribution also contains a shell script called @command{yacc} that
+invokes Bison with the @option{-y} option.
+
+@menu
* Bison Options:: All the options described in detail,
- in alphabetical order by short options.
-* Environment Variables:: Variables which affect Bison execution.
+ in alphabetical order by short options.
* Option Cross Key:: Alphabetical list of long options.
-* VMS Invocation:: Bison command syntax on VMS.
+* Yacc Library:: Yacc-compatible @code{yylex} and @code{main}.
@end menu
-@node Bison Options
-@section Bison Options
+@node Bison Options
+@section Bison Options
+
+Bison supports both traditional single-letter options and mnemonic long
+option names. Long option names are indicated with @samp{--} instead of
+@samp{-}. Abbreviations for option names are allowed as long as they
+are unique. When a long option takes an argument, like
+@samp{--file-prefix}, connect the option name and the argument with
+@samp{=}.
+
+Here is a list of options that can be used with Bison, alphabetized by
+short option. It is followed by a cross key alphabetized by long
+option.
+
+@c Please, keep this ordered as in `bison --help'.
+@noindent
+Operations modes:
+@table @option
+@item -h
+@itemx --help
+Print a summary of the command-line options to Bison and exit.
+
+@item -V
+@itemx --version
+Print the version number of Bison and exit.
+
+@item --print-localedir
+Print the name of the directory containing locale-dependent data.
+
+@item --print-datadir
+Print the name of the directory containing skeletons and XSLT.
+
+@item -y
+@itemx --yacc
+Act more like the traditional Yacc command. This can cause different
+diagnostics to be generated, and may change behavior in other minor
+ways. Most importantly, imitate Yacc's output file name conventions,
+so that the parser implementation file is called @file{y.tab.c}, and
+the other outputs are called @file{y.output} and @file{y.tab.h}.
+Also, if generating a deterministic parser in C, generate
+@code{#define} statements in addition to an @code{enum} to associate
+token numbers with token names. Thus, the following shell script can
+substitute for Yacc, and the Bison distribution contains such a script
+for compatibility with POSIX:
+
+@example
+#! /bin/sh
+bison -y "$@@"
+@end example
+
+The @option{-y}/@option{--yacc} option is intended for use with
+traditional Yacc grammars. If your grammar uses a Bison extension
+like @samp{%glr-parser}, Bison might not be Yacc-compatible even if
+this option is specified.
+
+@item -W [@var{category}]
+@itemx --warnings[=@var{category}]
+Output warnings falling in @var{category}. @var{category} can be one
+of:
+@table @code
+@item midrule-values
+Warn about mid-rule values that are set but not used within any of the actions
+of the parent rule.
+For example, warn about unused @code{$2} in:
+
+@example
+exp: '1' @{ $$ = 1; @} '+' exp @{ $$ = $1 + $4; @};
+@end example
+
+Also warn about mid-rule values that are used but not set.
+For example, warn about unset @code{$$} in the mid-rule action in:
+
+@example
+exp: '1' @{ $1 = 1; @} '+' exp @{ $$ = $2 + $4; @};
+@end example
+
+These warnings are not enabled by default since they sometimes prove to
+be false alarms in existing grammars employing the Yacc constructs
+@code{$0} or @code{$-@var{n}} (where @var{n} is some positive integer).
+
+@item yacc
+Incompatibilities with POSIX Yacc.
+
+@item conflicts-sr
+@itemx conflicts-rr
+S/R and R/R conflicts. These warnings are enabled by default. However, if
+the @code{%expect} or @code{%expect-rr} directive is specified, an
+unexpected number of conflicts is an error, and an expected number of
+conflicts is not reported, so @option{-W} and @option{--warning} then have
+no effect on the conflict report.
+
+@item other
+All warnings not categorized above. These warnings are enabled by default.
+
+This category is provided merely for the sake of completeness. Future
+releases of Bison may move warnings from this category to new, more specific
+categories.
+
+@item all
+All the warnings.
+@item none
+Turn off all the warnings.
+@item error
+Treat warnings as errors.
+@end table
+
+A category can be turned off by prefixing its name with @samp{no-}. For
+instance, @option{-Wno-yacc} will hide the warnings about
+POSIX Yacc incompatibilities.
+@end table
+
+@noindent
+Tuning the parser:
+
+@table @option
+@item -t
+@itemx --debug
+In the parser implementation file, define the macro @code{YYDEBUG} to
+1 if it is not already defined, so that the debugging facilities are
+compiled. @xref{Tracing, ,Tracing Your Parser}.
+
+@item -D @var{name}[=@var{value}]
+@itemx --define=@var{name}[=@var{value}]
+@itemx -F @var{name}[=@var{value}]
+@itemx --force-define=@var{name}[=@var{value}]
+Each of these is equivalent to @samp{%define @var{name} "@var{value}"}
+(@pxref{%define Summary}) except that Bison processes multiple
+definitions for the same @var{name} as follows:
+
+@itemize
+@item
+Bison quietly ignores all command-line definitions for @var{name} except
+the last.
+@item
+If that command-line definition is specified by a @code{-D} or
+@code{--define}, Bison reports an error for any @code{%define}
+definition for @var{name}.
+@item
+If that command-line definition is specified by a @code{-F} or
+@code{--force-define} instead, Bison quietly ignores all @code{%define}
+definitions for @var{name}.
+@item
+Otherwise, Bison reports an error if there are multiple @code{%define}
+definitions for @var{name}.
+@end itemize
+
+You should avoid using @code{-F} and @code{--force-define} in your
+make files unless you are confident that it is safe to quietly ignore
+any conflicting @code{%define} that may be added to the grammar file.
+
+@item -L @var{language}
+@itemx --language=@var{language}
+Specify the programming language for the generated parser, as if
+@code{%language} was specified (@pxref{Decl Summary, , Bison Declaration
+Summary}). Currently supported languages include C, C++, and Java.
+@var{language} is case-insensitive.
+
+This option is experimental and its effect may be modified in future
+releases.
+
+@item --locations
+Pretend that @code{%locations} was specified. @xref{Decl Summary}.
+
+@item -p @var{prefix}
+@itemx --name-prefix=@var{prefix}
+Pretend that @code{%name-prefix "@var{prefix}"} was specified.
+@xref{Decl Summary}.
+
+@item -l
+@itemx --no-lines
+Don't put any @code{#line} preprocessor commands in the parser
+implementation file. Ordinarily Bison puts them in the parser
+implementation file so that the C compiler and debuggers will
+associate errors with your source file, the grammar file. This option
+causes them to associate errors with the parser implementation file,
+treating it as an independent source file in its own right.
+
+@item -S @var{file}
+@itemx --skeleton=@var{file}
+Specify the skeleton to use, similar to @code{%skeleton}
+(@pxref{Decl Summary, , Bison Declaration Summary}).
+
+@c You probably don't need this option unless you are developing Bison.
+@c You should use @option{--language} if you want to specify the skeleton for a
+@c different language, because it is clearer and because it will always
+@c choose the correct skeleton for non-deterministic or push parsers.
+
+If @var{file} does not contain a @code{/}, @var{file} is the name of a skeleton
+file in the Bison installation directory.
+If it does, @var{file} is an absolute file name or a file name relative to the
+current working directory.
+This is similar to how most shells resolve commands.
+
+@item -k
+@itemx --token-table
+Pretend that @code{%token-table} was specified. @xref{Decl Summary}.
+@end table
+
+@noindent
+Adjust the output:
+
+@table @option
+@item --defines[=@var{file}]
+Pretend that @code{%defines} was specified, i.e., write an extra output
+file containing macro definitions for the token type names defined in
+the grammar, as well as a few other declarations. @xref{Decl Summary}.
+
+@item -d
+This is the same as @code{--defines} except @code{-d} does not accept a
+@var{file} argument since POSIX Yacc requires that @code{-d} can be bundled
+with other short options.
+
+@item -b @var{file-prefix}
+@itemx --file-prefix=@var{prefix}
+Pretend that @code{%file-prefix} was specified, i.e., specify prefix to use
+for all Bison output file names. @xref{Decl Summary}.
+
+@item -r @var{things}
+@itemx --report=@var{things}
+Write an extra output file containing verbose description of the comma
+separated list of @var{things} among:
+
+@table @code
+@item state
+Description of the grammar, conflicts (resolved and unresolved), and
+parser's automaton.
+
+@item lookahead
+Implies @code{state} and augments the description of the automaton with
+each rule's lookahead set.
+
+@item itemset
+Implies @code{state} and augments the description of the automaton with
+the full set of items for each state, instead of its core only.
+@end table
+
+@item --report-file=@var{file}
+Specify the @var{file} for the verbose description.
+
+@item -v
+@itemx --verbose
+Pretend that @code{%verbose} was specified, i.e., write an extra output
+file containing verbose descriptions of the grammar and
+parser. @xref{Decl Summary}.
+
+@item -o @var{file}
+@itemx --output=@var{file}
+Specify the @var{file} for the parser implementation file.
+
+The other output files' names are constructed from @var{file} as
+described under the @samp{-v} and @samp{-d} options.
+
+@item -g [@var{file}]
+@itemx --graph[=@var{file}]
+Output a graphical representation of the parser's
+automaton computed by Bison, in @uref{http://www.graphviz.org/, Graphviz}
+@uref{http://www.graphviz.org/doc/info/lang.html, DOT} format.
+@code{@var{file}} is optional.
+If omitted and the grammar file is @file{foo.y}, the output file will be
+@file{foo.dot}.
+
+@item -x [@var{file}]
+@itemx --xml[=@var{file}]
+Output an XML report of the parser's automaton computed by Bison.
+@code{@var{file}} is optional.
+If omitted and the grammar file is @file{foo.y}, the output file will be
+@file{foo.xml}.
+(The current XML schema is experimental and may evolve.
+More user feedback will help to stabilize it.)
+@end table
+
+@node Option Cross Key
+@section Option Cross Key
+
+Here is a list of options, alphabetized by long option, to help you find
+the corresponding short option and directive.
+
+@multitable {@option{--force-define=@var{name}[=@var{value}]}} {@option{-F @var{name}[=@var{value}]}} {@code{%nondeterministic-parser}}
+@headitem Long Option @tab Short Option @tab Bison Directive
+@include cross-options.texi
+@end multitable
+
+@node Yacc Library
+@section Yacc Library
+
+The Yacc library contains default implementations of the
+@code{yyerror} and @code{main} functions. These default
+implementations are normally not useful, but POSIX requires
+them. To use the Yacc library, link your program with the
+@option{-ly} option. Note that Bison's implementation of the Yacc
+library is distributed under the terms of the GNU General
+Public License (@pxref{Copying}).
+
+If you use the Yacc library's @code{yyerror} function, you should
+declare @code{yyerror} as follows:
+
+@example
+int yyerror (char const *);
+@end example
+
+Bison ignores the @code{int} value returned by this @code{yyerror}.
+If you use the Yacc library's @code{main} function, your
+@code{yyparse} function should have the following type signature:
+
+@example
+int yyparse (void);
+@end example
+
+@c ================================================= C++ Bison
+
+@node Other Languages
+@chapter Parsers Written In Other Languages
+
+@menu
+* C++ Parsers:: The interface to generate C++ parser classes
+* Java Parsers:: The interface to generate Java parser classes
+@end menu
+
+@node C++ Parsers
+@section C++ Parsers
+
+@menu
+* C++ Bison Interface:: Asking for C++ parser generation
+* C++ Semantic Values:: %union vs. C++
+* C++ Location Values:: The position and location classes
+* C++ Parser Interface:: Instantiating and running the parser
+* C++ Scanner Interface:: Exchanges between yylex and parse
+* A Complete C++ Example:: Demonstrating their use
+@end menu
+
+@node C++ Bison Interface
+@subsection C++ Bison Interface
+@c - %skeleton "lalr1.cc"
+@c - Always pure
+@c - initial action
+
+The C++ deterministic parser is selected using the skeleton directive,
+@samp{%skeleton "lalr1.cc"}, or the synonymous command-line option
+@option{--skeleton=lalr1.cc}.
+@xref{Decl Summary}.
+
+When run, @command{bison} will create several entities in the @samp{yy}
+namespace.
+@findex %define api.namespace
+Use the @samp{%define api.namespace} directive to change the namespace name,
+see @ref{%define Summary,,api.namespace}. The various classes are generated
+in the following files:
+
+@table @file
+@item position.hh
+@itemx location.hh
+The definition of the classes @code{position} and @code{location},
+used for location tracking when enabled. @xref{C++ Location Values}.
+
+@item stack.hh
+An auxiliary class @code{stack} used by the parser.
+
+@item @var{file}.hh
+@itemx @var{file}.cc
+(Assuming the extension of the grammar file was @samp{.yy}.) The
+declaration and implementation of the C++ parser class. The basename
+and extension of these two files follow the same rules as with regular C
+parsers (@pxref{Invocation}).
+
+The header is @emph{mandatory}; you must either pass
+@option{-d}/@option{--defines} to @command{bison}, or use the
+@samp{%defines} directive.
+@end table
+
+All these files are documented using Doxygen; run @command{doxygen}
+for a complete and accurate documentation.
+
+@node C++ Semantic Values
+@subsection C++ Semantic Values
+@c - No objects in unions
+@c - YYSTYPE
+@c - Printer and destructor
+
+Bison supports two different means to handle semantic values in C++. One is
+alike the C interface, and relies on unions (@pxref{C++ Unions}). As C++
+practitioners know, unions are inconvenient in C++, therefore another
+approach is provided, based on variants (@pxref{C++ Variants}).
+
+@menu
+* C++ Unions:: Semantic values cannot be objects
+* C++ Variants:: Using objects as semantic values
+@end menu
+
+@node C++ Unions
+@subsubsection C++ Unions
+
+The @code{%union} directive works as for C, see @ref{Union Decl, ,The
+Collection of Value Types}. In particular it produces a genuine
+@code{union}, which have a few specific features in C++.
+@itemize @minus
+@item
+The type @code{YYSTYPE} is defined but its use is discouraged: rather
+you should refer to the parser's encapsulated type
+@code{yy::parser::semantic_type}.
+@item
+Non POD (Plain Old Data) types cannot be used. C++ forbids any
+instance of classes with constructors in unions: only @emph{pointers}
+to such objects are allowed.
+@end itemize
+
+Because objects have to be stored via pointers, memory is not
+reclaimed automatically: using the @code{%destructor} directive is the
+only means to avoid leaks. @xref{Destructor Decl, , Freeing Discarded
+Symbols}.
+
+@node C++ Variants
+@subsubsection C++ Variants
+
+Starting with version 2.6, Bison provides a @emph{variant} based
+implementation of semantic values for C++. This alleviates all the
+limitations reported in the previous section, and in particular, object
+types can be used without pointers.
+
+To enable variant-based semantic values, set @code{%define} variable
+@code{variant} (@pxref{%define Summary,, variant}). Once this defined,
+@code{%union} is ignored, and instead of using the name of the fields of the
+@code{%union} to ``type'' the symbols, use genuine types.
+
+For instance, instead of
+
+@example
+%union
+@{
+ int ival;
+ std::string* sval;
+@}
+%token <ival> NUMBER;
+%token <sval> STRING;
+@end example
+
+@noindent
+write
+
+@example
+%token <int> NUMBER;
+%token <std::string> STRING;
+@end example
+
+@code{STRING} is no longer a pointer, which should fairly simplify the user
+actions in the grammar and in the scanner (in particular the memory
+management).
+
+Since C++ features destructors, and since it is customary to specialize
+@code{operator<<} to support uniform printing of values, variants also
+typically simplify Bison printers and destructors.
+
+Variants are stricter than unions. When based on unions, you may play any
+dirty game with @code{yylval}, say storing an @code{int}, reading a
+@code{char*}, and then storing a @code{double} in it. This is no longer
+possible with variants: they must be initialized, then assigned to, and
+eventually, destroyed.
+
+@deftypemethod {semantic_type} {T&} build<T> ()
+Initialize, but leave empty. Returns the address where the actual value may
+be stored. Requires that the variant was not initialized yet.
+@end deftypemethod
+
+@deftypemethod {semantic_type} {T&} build<T> (const T& @var{t})
+Initialize, and copy-construct from @var{t}.
+@end deftypemethod
+
+
+@strong{Warning}: We do not use Boost.Variant, for two reasons. First, it
+appeared unacceptable to require Boost on the user's machine (i.e., the
+machine on which the generated parser will be compiled, not the machine on
+which @command{bison} was run). Second, for each possible semantic value,
+Boost.Variant not only stores the value, but also a tag specifying its
+type. But the parser already ``knows'' the type of the semantic value, so
+that would be duplicating the information.
+
+Therefore we developed light-weight variants whose type tag is external (so
+they are really like @code{unions} for C++ actually). But our code is much
+less mature that Boost.Variant. So there is a number of limitations in
+(the current implementation of) variants:
+@itemize
+@item
+Alignment must be enforced: values should be aligned in memory according to
+the most demanding type. Computing the smallest alignment possible requires
+meta-programming techniques that are not currently implemented in Bison, and
+therefore, since, as far as we know, @code{double} is the most demanding
+type on all platforms, alignments are enforced for @code{double} whatever
+types are actually used. This may waste space in some cases.
+
+@item
+Our implementation is not conforming with strict aliasing rules. Alias
+analysis is a technique used in optimizing compilers to detect when two
+pointers are disjoint (they cannot ``meet''). Our implementation breaks
+some of the rules that G++ 4.4 uses in its alias analysis, so @emph{strict
+alias analysis must be disabled}. Use the option
+@option{-fno-strict-aliasing} to compile the generated parser.
+
+@item
+There might be portability issues we are not aware of.
+@end itemize
+
+As far as we know, these limitations @emph{can} be alleviated. All it takes
+is some time and/or some talented C++ hacker willing to contribute to Bison.
+
+@node C++ Location Values
+@subsection C++ Location Values
+@c - %locations
+@c - class Position
+@c - class Location
+@c - %define filename_type "const symbol::Symbol"
+
+When the directive @code{%locations} is used, the C++ parser supports
+location tracking, see @ref{Tracking Locations}. Two auxiliary classes
+define a @code{position}, a single point in a file, and a @code{location}, a
+range composed of a pair of @code{position}s (possibly spanning several
+files).
+
+@tindex uint
+In this section @code{uint} is an abbreviation for @code{unsigned int}: in
+genuine code only the latter is used.
+
+@menu
+* C++ position:: One point in the source file
+* C++ location:: Two points in the source file
+@end menu
+
+@node C++ position
+@subsubsection C++ @code{position}
+
+@deftypeop {Constructor} {position} {} position (std::string* @var{file} = 0, uint @var{line} = 1, uint @var{col} = 1)
+Create a @code{position} denoting a given point. Note that @code{file} is
+not reclaimed when the @code{position} is destroyed: memory managed must be
+handled elsewhere.
+@end deftypeop
+
+@deftypemethod {position} {void} initialize (std::string* @var{file} = 0, uint @var{line} = 1, uint @var{col} = 1)
+Reset the position to the given values.
+@end deftypemethod
+
+@deftypeivar {position} {std::string*} file
+The name of the file. It will always be handled as a pointer, the
+parser will never duplicate nor deallocate it. As an experimental
+feature you may change it to @samp{@var{type}*} using @samp{%define
+filename_type "@var{type}"}.
+@end deftypeivar
+
+@deftypeivar {position} {uint} line
+The line, starting at 1.
+@end deftypeivar
+
+@deftypemethod {position} {uint} lines (int @var{height} = 1)
+Advance by @var{height} lines, resetting the column number.
+@end deftypemethod
+
+@deftypeivar {position} {uint} column
+The column, starting at 1.
+@end deftypeivar
+
+@deftypemethod {position} {uint} columns (int @var{width} = 1)
+Advance by @var{width} columns, without changing the line number.
+@end deftypemethod
+
+@deftypemethod {position} {position&} operator+= (int @var{width})
+@deftypemethodx {position} {position} operator+ (int @var{width})
+@deftypemethodx {position} {position&} operator-= (int @var{width})
+@deftypemethodx {position} {position} operator- (int @var{width})
+Various forms of syntactic sugar for @code{columns}.
+@end deftypemethod
+
+@deftypemethod {position} {bool} operator== (const position& @var{that})
+@deftypemethodx {position} {bool} operator!= (const position& @var{that})
+Whether @code{*this} and @code{that} denote equal/different positions.
+@end deftypemethod
+
+@deftypefun {std::ostream&} operator<< (std::ostream& @var{o}, const position& @var{p})
+Report @var{p} on @var{o} like this:
+@samp{@var{file}:@var{line}.@var{column}}, or
+@samp{@var{line}.@var{column}} if @var{file} is null.
+@end deftypefun
+
+@node C++ location
+@subsubsection C++ @code{location}
+
+@deftypeop {Constructor} {location} {} location (const position& @var{begin}, const position& @var{end})
+Create a @code{Location} from the endpoints of the range.
+@end deftypeop
+
+@deftypeop {Constructor} {location} {} location (const position& @var{pos} = position())
+@deftypeopx {Constructor} {location} {} location (std::string* @var{file}, uint @var{line}, uint @var{col})
+Create a @code{Location} denoting an empty range located at a given point.
+@end deftypeop
+
+@deftypemethod {location} {void} initialize (std::string* @var{file} = 0, uint @var{line} = 1, uint @var{col} = 1)
+Reset the location to an empty range at the given values.
+@end deftypemethod
+
+@deftypeivar {location} {position} begin
+@deftypeivarx {location} {position} end
+The first, inclusive, position of the range, and the first beyond.
+@end deftypeivar
+
+@deftypemethod {location} {uint} columns (int @var{width} = 1)
+@deftypemethodx {location} {uint} lines (int @var{height} = 1)
+Advance the @code{end} position.
+@end deftypemethod
+
+@deftypemethod {location} {location} operator+ (const location& @var{end})
+@deftypemethodx {location} {location} operator+ (int @var{width})
+@deftypemethodx {location} {location} operator+= (int @var{width})
+Various forms of syntactic sugar.
+@end deftypemethod
+
+@deftypemethod {location} {void} step ()
+Move @code{begin} onto @code{end}.
+@end deftypemethod
+
+@deftypemethod {location} {bool} operator== (const location& @var{that})
+@deftypemethodx {location} {bool} operator!= (const location& @var{that})
+Whether @code{*this} and @code{that} denote equal/different ranges of
+positions.
+@end deftypemethod
+
+@deftypefun {std::ostream&} operator<< (std::ostream& @var{o}, const location& @var{p})
+Report @var{p} on @var{o}, taking care of special cases such as: no
+@code{filename} defined, or equal filename/line or column.
+@end deftypefun
+
+@node C++ Parser Interface
+@subsection C++ Parser Interface
+@c - define parser_class_name
+@c - Ctor
+@c - parse, error, set_debug_level, debug_level, set_debug_stream,
+@c debug_stream.
+@c - Reporting errors
+
+The output files @file{@var{output}.hh} and @file{@var{output}.cc}
+declare and define the parser class in the namespace @code{yy}. The
+class name defaults to @code{parser}, but may be changed using
+@samp{%define parser_class_name "@var{name}"}. The interface of
+this class is detailed below. It can be extended using the
+@code{%parse-param} feature: its semantics is slightly changed since
+it describes an additional member of the parser class, and an
+additional argument for its constructor.
+
+@defcv {Type} {parser} {semantic_type}
+@defcvx {Type} {parser} {location_type}
+The types for semantic values and locations (if enabled).
+@end defcv
+
+@defcv {Type} {parser} {token}
+A structure that contains (only) the @code{yytokentype} enumeration, which
+defines the tokens. To refer to the token @code{FOO},
+use @code{yy::parser::token::FOO}. The scanner can use
+@samp{typedef yy::parser::token token;} to ``import'' the token enumeration
+(@pxref{Calc++ Scanner}).
+@end defcv
+
+@defcv {Type} {parser} {syntax_error}
+This class derives from @code{std::runtime_error}. Throw instances of it
+from the scanner or from the user actions to raise parse errors. This is
+equivalent with first
+invoking @code{error} to report the location and message of the syntax
+error, and then to invoke @code{YYERROR} to enter the error-recovery mode.
+But contrary to @code{YYERROR} which can only be invoked from user actions
+(i.e., written in the action itself), the exception can be thrown from
+function invoked from the user action.
+@end defcv
+
+@deftypemethod {parser} {} parser (@var{type1} @var{arg1}, ...)
+Build a new parser object. There are no arguments by default, unless
+@samp{%parse-param @{@var{type1} @var{arg1}@}} was used.
+@end deftypemethod
+
+@deftypemethod {syntax_error} {} syntax_error (const location_type& @var{l}, const std::string& @var{m})
+@deftypemethodx {syntax_error} {} syntax_error (const std::string& @var{m})
+Instantiate a syntax-error exception.
+@end deftypemethod
+
+@deftypemethod {parser} {int} parse ()
+Run the syntactic analysis, and return 0 on success, 1 otherwise.
+@end deftypemethod
+
+@deftypemethod {parser} {std::ostream&} debug_stream ()
+@deftypemethodx {parser} {void} set_debug_stream (std::ostream& @var{o})
+Get or set the stream used for tracing the parsing. It defaults to
+@code{std::cerr}.
+@end deftypemethod
+
+@deftypemethod {parser} {debug_level_type} debug_level ()
+@deftypemethodx {parser} {void} set_debug_level (debug_level @var{l})
+Get or set the tracing level. Currently its value is either 0, no trace,
+or nonzero, full tracing.
+@end deftypemethod
+
+@deftypemethod {parser} {void} error (const location_type& @var{l}, const std::string& @var{m})
+@deftypemethodx {parser} {void} error (const std::string& @var{m})
+The definition for this member function must be supplied by the user:
+the parser uses it to report a parser error occurring at @var{l},
+described by @var{m}. If location tracking is not enabled, the second
+signature is used.
+@end deftypemethod
+
+
+@node C++ Scanner Interface
+@subsection C++ Scanner Interface
+@c - prefix for yylex.
+@c - Pure interface to yylex
+@c - %lex-param
+
+The parser invokes the scanner by calling @code{yylex}. Contrary to C
+parsers, C++ parsers are always pure: there is no point in using the
+@samp{%define api.pure} directive. The actual interface with @code{yylex}
+depends whether you use unions, or variants.
+
+@menu
+* Split Symbols:: Passing symbols as two/three components
+* Complete Symbols:: Making symbols a whole
+@end menu
+
+@node Split Symbols
+@subsubsection Split Symbols
+
+Therefore the interface is as follows.
+
+@deftypemethod {parser} {int} yylex (semantic_type* @var{yylval}, location_type* @var{yylloc}, @var{type1} @var{arg1}, ...)
+@deftypemethodx {parser} {int} yylex (semantic_type* @var{yylval}, @var{type1} @var{arg1}, ...)
+Return the next token. Its type is the return value, its semantic value and
+location (if enabled) being @var{yylval} and @var{yylloc}. Invocations of
+@samp{%lex-param @{@var{type1} @var{arg1}@}} yield additional arguments.
+@end deftypemethod
+
+Note that when using variants, the interface for @code{yylex} is the same,
+but @code{yylval} is handled differently.
+
+Regular union-based code in Lex scanner typically look like:
+
+@example
+[0-9]+ @{
+ yylval.ival = text_to_int (yytext);
+ return yy::parser::INTEGER;
+ @}
+[a-z]+ @{
+ yylval.sval = new std::string (yytext);
+ return yy::parser::IDENTIFIER;
+ @}
+@end example
+
+Using variants, @code{yylval} is already constructed, but it is not
+initialized. So the code would look like:
+
+@example
+[0-9]+ @{
+ yylval.build<int>() = text_to_int (yytext);
+ return yy::parser::INTEGER;
+ @}
+[a-z]+ @{
+ yylval.build<std::string> = yytext;
+ return yy::parser::IDENTIFIER;
+ @}
+@end example
+
+@noindent
+or
+
+@example
+[0-9]+ @{
+ yylval.build(text_to_int (yytext));
+ return yy::parser::INTEGER;
+ @}
+[a-z]+ @{
+ yylval.build(yytext);
+ return yy::parser::IDENTIFIER;
+ @}
+@end example
+
+
+@node Complete Symbols
+@subsubsection Complete Symbols
+
+If you specified both @code{%define variant} and @code{%define lex_symbol},
+the @code{parser} class also defines the class @code{parser::symbol_type}
+which defines a @emph{complete} symbol, aggregating its type (i.e., the
+traditional value returned by @code{yylex}), its semantic value (i.e., the
+value passed in @code{yylval}, and possibly its location (@code{yylloc}).
+
+@deftypemethod {symbol_type} {} symbol_type (token_type @var{type}, const semantic_type& @var{value}, const location_type& @var{location})
+Build a complete terminal symbol which token type is @var{type}, and which
+semantic value is @var{value}. If location tracking is enabled, also pass
+the @var{location}.
+@end deftypemethod
+
+This interface is low-level and should not be used for two reasons. First,
+it is inconvenient, as you still have to build the semantic value, which is
+a variant, and second, because consistency is not enforced: as with unions,
+it is still possible to give an integer as semantic value for a string.
+
+So for each token type, Bison generates named constructors as follows.
+
+@deftypemethod {symbol_type} {} make_@var{token} (const @var{value_type}& @var{value}, const location_type& @var{location})
+@deftypemethodx {symbol_type} {} make_@var{token} (const location_type& @var{location})
+Build a complete terminal symbol for the token type @var{token} (not
+including the @code{api.tokens.prefix}) whose possible semantic value is
+@var{value} of adequate @var{value_type}. If location tracking is enabled,
+also pass the @var{location}.
+@end deftypemethod
+
+For instance, given the following declarations:
+
+@example
+%define api.tokens.prefix "TOK_"
+%token <std::string> IDENTIFIER;
+%token <int> INTEGER;
+%token COLON;
+@end example
+
+@noindent
+Bison generates the following functions:
+
+@example
+symbol_type make_IDENTIFIER(const std::string& v,
+ const location_type& l);
+symbol_type make_INTEGER(const int& v,
+ const location_type& loc);
+symbol_type make_COLON(const location_type& loc);
+@end example
+
+@noindent
+which should be used in a Lex-scanner as follows.
+
+@example
+[0-9]+ return yy::parser::make_INTEGER(text_to_int (yytext), loc);
+[a-z]+ return yy::parser::make_IDENTIFIER(yytext, loc);
+":" return yy::parser::make_COLON(loc);
+@end example
+
+Tokens that do not have an identifier are not accessible: you cannot simply
+use characters such as @code{':'}, they must be declared with @code{%token}.
+
+@node A Complete C++ Example
+@subsection A Complete C++ Example
+
+This section demonstrates the use of a C++ parser with a simple but
+complete example. This example should be available on your system,
+ready to compile, in the directory @dfn{.../bison/examples/calc++}. It
+focuses on the use of Bison, therefore the design of the various C++
+classes is very naive: no accessors, no encapsulation of members etc.
+We will use a Lex scanner, and more precisely, a Flex scanner, to
+demonstrate the various interactions. A hand-written scanner is
+actually easier to interface with.
+
+@menu
+* Calc++ --- C++ Calculator:: The specifications
+* Calc++ Parsing Driver:: An active parsing context
+* Calc++ Parser:: A parser class
+* Calc++ Scanner:: A pure C++ Flex scanner
+* Calc++ Top Level:: Conducting the band
+@end menu
+
+@node Calc++ --- C++ Calculator
+@subsubsection Calc++ --- C++ Calculator
+
+Of course the grammar is dedicated to arithmetics, a single
+expression, possibly preceded by variable assignments. An
+environment containing possibly predefined variables such as
+@code{one} and @code{two}, is exchanged with the parser. An example
+of valid input follows.
+
+@example
+three := 3
+seven := one + two * three
+seven * seven
+@end example
+
+@node Calc++ Parsing Driver
+@subsubsection Calc++ Parsing Driver
+@c - An env
+@c - A place to store error messages
+@c - A place for the result
+
+To support a pure interface with the parser (and the scanner) the
+technique of the ``parsing context'' is convenient: a structure
+containing all the data to exchange. Since, in addition to simply
+launch the parsing, there are several auxiliary tasks to execute (open
+the file for parsing, instantiate the parser etc.), we recommend
+transforming the simple parsing context structure into a fully blown
+@dfn{parsing driver} class.
+
+The declaration of this driver class, @file{calc++-driver.hh}, is as
+follows. The first part includes the CPP guard and imports the
+required standard library components, and the declaration of the parser
+class.
+
+@comment file: calc++-driver.hh
+@example
+#ifndef CALCXX_DRIVER_HH
+# define CALCXX_DRIVER_HH
+# include <string>
+# include <map>
+# include "calc++-parser.hh"
+@end example
+
+
+@noindent
+Then comes the declaration of the scanning function. Flex expects
+the signature of @code{yylex} to be defined in the macro
+@code{YY_DECL}, and the C++ parser expects it to be declared. We can
+factor both as follows.
+
+@comment file: calc++-driver.hh
+@example
+// Tell Flex the lexer's prototype ...
+# define YY_DECL \
+ yy::calcxx_parser::symbol_type yylex (calcxx_driver& driver)
+// ... and declare it for the parser's sake.
+YY_DECL;
+@end example
+
+@noindent
+The @code{calcxx_driver} class is then declared with its most obvious
+members.
+
+@comment file: calc++-driver.hh
+@example
+// Conducting the whole scanning and parsing of Calc++.
+class calcxx_driver
+@{
+public:
+ calcxx_driver ();
+ virtual ~calcxx_driver ();
+
+ std::map<std::string, int> variables;
+
+ int result;
+@end example
+
+@noindent
+To encapsulate the coordination with the Flex scanner, it is useful to have
+member functions to open and close the scanning phase.
+
+@comment file: calc++-driver.hh
+@example
+ // Handling the scanner.
+ void scan_begin ();
+ void scan_end ();
+ bool trace_scanning;
+@end example
+
+@noindent
+Similarly for the parser itself.
+
+@comment file: calc++-driver.hh
+@example
+ // Run the parser on file F.
+ // Return 0 on success.
+ int parse (const std::string& f);
+ // The name of the file being parsed.
+ // Used later to pass the file name to the location tracker.
+ std::string file;
+ // Whether parser traces should be generated.
+ bool trace_parsing;
+@end example
+
+@noindent
+To demonstrate pure handling of parse errors, instead of simply
+dumping them on the standard error output, we will pass them to the
+compiler driver using the following two member functions. Finally, we
+close the class declaration and CPP guard.
+
+@comment file: calc++-driver.hh
+@example
+ // Error handling.
+ void error (const yy::location& l, const std::string& m);
+ void error (const std::string& m);
+@};
+#endif // ! CALCXX_DRIVER_HH
+@end example
+
+The implementation of the driver is straightforward. The @code{parse}
+member function deserves some attention. The @code{error} functions
+are simple stubs, they should actually register the located error
+messages and set error state.
+
+@comment file: calc++-driver.cc
+@example
+#include "calc++-driver.hh"
+#include "calc++-parser.hh"
+
+calcxx_driver::calcxx_driver ()
+ : trace_scanning (false), trace_parsing (false)
+@{
+ variables["one"] = 1;
+ variables["two"] = 2;
+@}
+
+calcxx_driver::~calcxx_driver ()
+@{
+@}
+
+int
+calcxx_driver::parse (const std::string &f)
+@{
+ file = f;
+ scan_begin ();
+ yy::calcxx_parser parser (*this);
+ parser.set_debug_level (trace_parsing);
+ int res = parser.parse ();
+ scan_end ();
+ return res;
+@}
+
+void
+calcxx_driver::error (const yy::location& l, const std::string& m)
+@{
+ std::cerr << l << ": " << m << std::endl;
+@}
+
+void
+calcxx_driver::error (const std::string& m)
+@{
+ std::cerr << m << std::endl;
+@}
+@end example
+
+@node Calc++ Parser
+@subsubsection Calc++ Parser
+
+The grammar file @file{calc++-parser.yy} starts by asking for the C++
+deterministic parser skeleton, the creation of the parser header file,
+and specifies the name of the parser class. Because the C++ skeleton
+changed several times, it is safer to require the version you designed
+the grammar for.
+
+@comment file: calc++-parser.yy
+@example
+%skeleton "lalr1.cc" /* -*- C++ -*- */
+%require "@value{VERSION}"
+%defines
+%define parser_class_name "calcxx_parser"
+@end example
+
+@noindent
+@findex %define variant
+@findex %define lex_symbol
+This example will use genuine C++ objects as semantic values, therefore, we
+require the variant-based interface. To make sure we properly use it, we
+enable assertions. To fully benefit from type-safety and more natural
+definition of ``symbol'', we enable @code{lex_symbol}.
+
+@comment file: calc++-parser.yy
+@example
+%define variant
+%define parse.assert
+%define lex_symbol
+@end example
+
+@noindent
+@findex %code requires
+Then come the declarations/inclusions needed by the semantic values.
+Because the parser uses the parsing driver and reciprocally, both would like
+to include the header of the other, which is, of course, insane. This
+mutual dependency will be broken using forward declarations. Because the
+driver's header needs detailed knowledge about the parser class (in
+particular its inner types), it is the parser's header which will use a
+forward declaration of the driver. @xref{%code Summary}.
+
+@comment file: calc++-parser.yy
+@example
+%code requires
+@{
+# include <string>
+class calcxx_driver;
+@}
+@end example
+
+@noindent
+The driver is passed by reference to the parser and to the scanner.
+This provides a simple but effective pure interface, not relying on
+global variables.
+
+@comment file: calc++-parser.yy
+@example
+// The parsing context.
+%param @{ calcxx_driver& driver @}
+@end example
+
+@noindent
+Then we request location tracking, and initialize the
+first location's file name. Afterward new locations are computed
+relatively to the previous locations: the file name will be
+propagated.
+
+@comment file: calc++-parser.yy
+@example
+%locations
+%initial-action
+@{
+ // Initialize the initial location.
+ @@$.begin.filename = @@$.end.filename = &driver.file;
+@};
+@end example
+
+@noindent
+Use the following two directives to enable parser tracing and verbose error
+messages. However, verbose error messages can contain incorrect information
+(@pxref{LAC}).
+
+@comment file: calc++-parser.yy
+@example
+%define parse.trace
+%define parse.error verbose
+@end example
+
+@noindent
+@findex %code
+The code between @samp{%code @{} and @samp{@}} is output in the
+@file{*.cc} file; it needs detailed knowledge about the driver.
+
+@comment file: calc++-parser.yy
+@example
+%code
+@{
+# include "calc++-driver.hh"
+@}
+@end example
+
+
+@noindent
+The token numbered as 0 corresponds to end of file; the following line
+allows for nicer error messages referring to ``end of file'' instead of
+``$end''. Similarly user friendly names are provided for each symbol. To
+avoid name clashes in the generated files (@pxref{Calc++ Scanner}), prefix
+tokens with @code{TOK_} (@pxref{%define Summary,,api.tokens.prefix}).
+
+@comment file: calc++-parser.yy
+@example
+%define api.tokens.prefix "TOK_"
+%token
+ END 0 "end of file"
+ ASSIGN ":="
+ MINUS "-"
+ PLUS "+"
+ STAR "*"
+ SLASH "/"
+ LPAREN "("
+ RPAREN ")"
+;
+@end example
+
+@noindent
+Since we use variant-based semantic values, @code{%union} is not used, and
+both @code{%type} and @code{%token} expect genuine types, as opposed to type
+tags.
+
+@comment file: calc++-parser.yy
+@example
+%token <std::string> IDENTIFIER "identifier"
+%token <int> NUMBER "number"
+%type <int> exp
+@end example
+
+@noindent
+No @code{%destructor} is needed to enable memory deallocation during error
+recovery; the memory, for strings for instance, will be reclaimed by the
+regular destructors. All the values are printed using their
+@code{operator<<}.
+
+@c FIXME: Document %printer, and mention that it takes a braced-code operand.
+@comment file: calc++-parser.yy
+@example
+%printer @{ debug_stream () << $$; @} <*>;
+@end example
+
+@noindent
+The grammar itself is straightforward (@pxref{Location Tracking Calc, ,
+Location Tracking Calculator: @code{ltcalc}}).
+
+@comment file: calc++-parser.yy
+@example
+%%
+%start unit;
+unit: assignments exp @{ driver.result = $2; @};
+
+assignments:
+ /* Nothing. */ @{@}
+| assignments assignment @{@};
+
+assignment:
+ "identifier" ":=" exp @{ driver.variables[$1] = $3; @};
+
+%left "+" "-";
+%left "*" "/";
+exp:
+ exp "+" exp @{ $$ = $1 + $3; @}
+| exp "-" exp @{ $$ = $1 - $3; @}
+| exp "*" exp @{ $$ = $1 * $3; @}
+| exp "/" exp @{ $$ = $1 / $3; @}
+| "(" exp ")" @{ std::swap ($$, $2); @}
+| "identifier" @{ $$ = driver.variables[$1]; @}
+| "number" @{ std::swap ($$, $1); @};
+%%
+@end example
+
+@noindent
+Finally the @code{error} member function registers the errors to the
+driver.
+
+@comment file: calc++-parser.yy
+@example
+void
+yy::calcxx_parser::error (const location_type& l,
+ const std::string& m)
+@{
+ driver.error (l, m);
+@}
+@end example
+
+@node Calc++ Scanner
+@subsubsection Calc++ Scanner
+
+The Flex scanner first includes the driver declaration, then the
+parser's to get the set of defined tokens.
+
+@comment file: calc++-scanner.ll
+@example
+%@{ /* -*- C++ -*- */
+# include <cerrno>
+# include <climits>
+# include <cstdlib>
+# include <string>
+# include "calc++-driver.hh"
+# include "calc++-parser.hh"
+
+// Work around an incompatibility in flex (at least versions
+// 2.5.31 through 2.5.33): it generates code that does
+// not conform to C89. See Debian bug 333231
+// <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=333231>.
+# undef yywrap
+# define yywrap() 1
+
+// The location of the current token.
+static yy::location loc;
+%@}
+@end example
+
+@noindent
+Because there is no @code{#include}-like feature we don't need
+@code{yywrap}, we don't need @code{unput} either, and we parse an
+actual file, this is not an interactive session with the user.
+Finally, we enable scanner tracing.
+
+@comment file: calc++-scanner.ll
+@example
+%option noyywrap nounput batch debug
+@end example
+
+@noindent
+Abbreviations allow for more readable rules.
+
+@comment file: calc++-scanner.ll
+@example
+id [a-zA-Z][a-zA-Z_0-9]*
+int [0-9]+
+blank [ \t]
+@end example
+
+@noindent
+The following paragraph suffices to track locations accurately. Each
+time @code{yylex} is invoked, the begin position is moved onto the end
+position. Then when a pattern is matched, its width is added to the end
+column. When matching ends of lines, the end
+cursor is adjusted, and each time blanks are matched, the begin cursor
+is moved onto the end cursor to effectively ignore the blanks
+preceding tokens. Comments would be treated equally.
+
+@comment file: calc++-scanner.ll
+@example
+@group
+%@{
+ // Code run each time a pattern is matched.
+ # define YY_USER_ACTION loc.columns (yyleng);
+%@}
+@end group
+%%
+@group
+%@{
+ // Code run each time yylex is called.
+ loc.step ();
+%@}
+@end group
+@{blank@}+ loc.step ();
+[\n]+ loc.lines (yyleng); loc.step ();
+@end example
+
+@noindent
+The rules are simple. The driver is used to report errors.
+
+@comment file: calc++-scanner.ll
+@example
+"-" return yy::calcxx_parser::make_MINUS(loc);
+"+" return yy::calcxx_parser::make_PLUS(loc);
+"*" return yy::calcxx_parser::make_STAR(loc);
+"/" return yy::calcxx_parser::make_SLASH(loc);
+"(" return yy::calcxx_parser::make_LPAREN(loc);
+")" return yy::calcxx_parser::make_RPAREN(loc);
+":=" return yy::calcxx_parser::make_ASSIGN(loc);
+
+@group
+@{int@} @{
+ errno = 0;
+ long n = strtol (yytext, NULL, 10);
+ if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE))
+ driver.error (loc, "integer is out of range");
+ return yy::calcxx_parser::make_NUMBER(n, loc);
+@}
+@end group
+@{id@} return yy::calcxx_parser::make_IDENTIFIER(yytext, loc);
+. driver.error (loc, "invalid character");
+<<EOF>> return yy::calcxx_parser::make_END(loc);
+%%
+@end example
+
+@noindent
+Finally, because the scanner-related driver's member-functions depend
+on the scanner's data, it is simpler to implement them in this file.
+
+@comment file: calc++-scanner.ll
+@example
+@group
+void
+calcxx_driver::scan_begin ()
+@{
+ yy_flex_debug = trace_scanning;
+ if (file == "-")
+ yyin = stdin;
+ else if (!(yyin = fopen (file.c_str (), "r")))
+ @{
+ error ("cannot open " + file + ": " + strerror(errno));
+ exit (EXIT_FAILURE);
+ @}
+@}
+@end group
+
+@group
+void
+calcxx_driver::scan_end ()
+@{
+ fclose (yyin);
+@}
+@end group
+@end example
+
+@node Calc++ Top Level
+@subsubsection Calc++ Top Level
+
+The top level file, @file{calc++.cc}, poses no problem.
+
+@comment file: calc++.cc
+@example
+#include <iostream>
+#include "calc++-driver.hh"
+
+@group
+int
+main (int argc, char *argv[])
+@{
+ int res = 0;
+ calcxx_driver driver;
+ for (++argv; argv[0]; ++argv)
+ if (*argv == std::string ("-p"))
+ driver.trace_parsing = true;
+ else if (*argv == std::string ("-s"))
+ driver.trace_scanning = true;
+ else if (!driver.parse (*argv))
+ std::cout << driver.result << std::endl;
+ else
+ res = 1;
+ return res;
+@}
+@end group
+@end example
+
+@node Java Parsers
+@section Java Parsers
+
+@menu
+* Java Bison Interface:: Asking for Java parser generation
+* Java Semantic Values:: %type and %token vs. Java
+* Java Location Values:: The position and location classes
+* Java Parser Interface:: Instantiating and running the parser
+* Java Scanner Interface:: Specifying the scanner for the parser
+* Java Action Features:: Special features for use in actions
+* Java Differences:: Differences between C/C++ and Java Grammars
+* Java Declarations Summary:: List of Bison declarations used with Java
+@end menu
+
+@node Java Bison Interface
+@subsection Java Bison Interface
+@c - %language "Java"
+
+(The current Java interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+
+The Java parser skeletons are selected using the @code{%language "Java"}
+directive or the @option{-L java}/@option{--language=java} option.
+
+@c FIXME: Documented bug.
+When generating a Java parser, @code{bison @var{basename}.y} will
+create a single Java source file named @file{@var{basename}.java}
+containing the parser implementation. Using a grammar file without a
+@file{.y} suffix is currently broken. The basename of the parser
+implementation file can be changed by the @code{%file-prefix}
+directive or the @option{-p}/@option{--name-prefix} option. The
+entire parser implementation file name can be changed by the
+@code{%output} directive or the @option{-o}/@option{--output} option.
+The parser implementation file contains a single class for the parser.
+
+You can create documentation for generated parsers using Javadoc.
+
+Contrary to C parsers, Java parsers do not use global variables; the
+state of the parser is always local to an instance of the parser class.
+Therefore, all Java parsers are ``pure'', and the @code{%pure-parser}
+and @samp{%define api.pure} directives does not do anything when used in
+Java.
+
+Push parsers are currently unsupported in Java and @code{%define
+api.push-pull} have no effect.
+
+GLR parsers are currently unsupported in Java. Do not use the
+@code{glr-parser} directive.
+
+No header file can be generated for Java parsers. Do not use the
+@code{%defines} directive or the @option{-d}/@option{--defines} options.
+
+@c FIXME: Possible code change.
+Currently, support for tracing is always compiled
+in. Thus the @samp{%define parse.trace} and @samp{%token-table}
+directives and the
+@option{-t}/@option{--debug} and @option{-k}/@option{--token-table}
+options have no effect. This may change in the future to eliminate
+unused code in the generated parser, so use @samp{%define parse.trace}
+explicitly
+if needed. Also, in the future the
+@code{%token-table} directive might enable a public interface to
+access the token names and codes.
+
+Getting a ``code too large'' error from the Java compiler means the code
+hit the 64KB bytecode per method limitation of the Java class file.
+Try reducing the amount of code in actions and static initializers;
+otherwise, report a bug so that the parser skeleton will be improved.
+
+
+@node Java Semantic Values
+@subsection Java Semantic Values
+@c - No %union, specify type in %type/%token.
+@c - YYSTYPE
+@c - Printer and destructor
+
+There is no @code{%union} directive in Java parsers. Instead, the
+semantic values' types (class names) should be specified in the
+@code{%type} or @code{%token} directive:
+
+@example
+%type <Expression> expr assignment_expr term factor
+%type <Integer> number
+@end example
+
+By default, the semantic stack is declared to have @code{Object} members,
+which means that the class types you specify can be of any class.
+To improve the type safety of the parser, you can declare the common
+superclass of all the semantic values using the @samp{%define stype}
+directive. For example, after the following declaration:
+
+@example
+%define stype "ASTNode"
+@end example
+
+@noindent
+any @code{%type} or @code{%token} specifying a semantic type which
+is not a subclass of ASTNode, will cause a compile-time error.
+
+@c FIXME: Documented bug.
+Types used in the directives may be qualified with a package name.
+Primitive data types are accepted for Java version 1.5 or later. Note
+that in this case the autoboxing feature of Java 1.5 will be used.
+Generic types may not be used; this is due to a limitation in the
+implementation of Bison, and may change in future releases.
+
+Java parsers do not support @code{%destructor}, since the language
+adopts garbage collection. The parser will try to hold references
+to semantic values for as little time as needed.
+
+Java parsers do not support @code{%printer}, as @code{toString()}
+can be used to print the semantic values. This however may change
+(in a backwards-compatible way) in future versions of Bison.
+
+
+@node Java Location Values
+@subsection Java Location Values
+@c - %locations
+@c - class Position
+@c - class Location
+
+When the directive @code{%locations} is used, the Java parser supports
+location tracking, see @ref{Tracking Locations}. An auxiliary user-defined
+class defines a @dfn{position}, a single point in a file; Bison itself
+defines a class representing a @dfn{location}, a range composed of a pair of
+positions (possibly spanning several files). The location class is an inner
+class of the parser; the name is @code{Location} by default, and may also be
+renamed using @samp{%define location_type "@var{class-name}"}.
+
+The location class treats the position as a completely opaque value.
+By default, the class name is @code{Position}, but this can be changed
+with @samp{%define position_type "@var{class-name}"}. This class must
+be supplied by the user.
+
+
+@deftypeivar {Location} {Position} begin
+@deftypeivarx {Location} {Position} end
+The first, inclusive, position of the range, and the first beyond.
+@end deftypeivar
+
+@deftypeop {Constructor} {Location} {} Location (Position @var{loc})
+Create a @code{Location} denoting an empty range located at a given point.
+@end deftypeop
+
+@deftypeop {Constructor} {Location} {} Location (Position @var{begin}, Position @var{end})
+Create a @code{Location} from the endpoints of the range.
+@end deftypeop
+
+@deftypemethod {Location} {String} toString ()
+Prints the range represented by the location. For this to work
+properly, the position class should override the @code{equals} and
+@code{toString} methods appropriately.
+@end deftypemethod
+
+
+@node Java Parser Interface
+@subsection Java Parser Interface
+@c - define parser_class_name
+@c - Ctor
+@c - parse, error, set_debug_level, debug_level, set_debug_stream,
+@c debug_stream.
+@c - Reporting errors
+
+The name of the generated parser class defaults to @code{YYParser}. The
+@code{YY} prefix may be changed using the @code{%name-prefix} directive
+or the @option{-p}/@option{--name-prefix} option. Alternatively, use
+@samp{%define parser_class_name "@var{name}"} to give a custom name to
+the class. The interface of this class is detailed below.
+
+By default, the parser class has package visibility. A declaration
+@samp{%define public} will change to public visibility. Remember that,
+according to the Java language specification, the name of the @file{.java}
+file should match the name of the class in this case. Similarly, you can
+use @code{abstract}, @code{final} and @code{strictfp} with the
+@code{%define} declaration to add other modifiers to the parser class.
+A single @samp{%define annotations "@var{annotations}"} directive can
+be used to add any number of annotations to the parser class.
+
+The Java package name of the parser class can be specified using the
+@samp{%define package} directive. The superclass and the implemented
+interfaces of the parser class can be specified with the @code{%define
+extends} and @samp{%define implements} directives.
+
+The parser class defines an inner class, @code{Location}, that is used
+for location tracking (see @ref{Java Location Values}), and a inner
+interface, @code{Lexer} (see @ref{Java Scanner Interface}). Other than
+these inner class/interface, and the members described in the interface
+below, all the other members and fields are preceded with a @code{yy} or
+@code{YY} prefix to avoid clashes with user code.
+
+The parser class can be extended using the @code{%parse-param}
+directive. Each occurrence of the directive will add a @code{protected
+final} field to the parser class, and an argument to its constructor,
+which initialize them automatically.
+
+@deftypeop {Constructor} {YYParser} {} YYParser (@var{lex_param}, @dots{}, @var{parse_param}, @dots{})
+Build a new parser object with embedded @code{%code lexer}. There are
+no parameters, unless @code{%param}s and/or @code{%parse-param}s and/or
+@code{%lex-param}s are used.
+
+Use @code{%code init} for code added to the start of the constructor
+body. This is especially useful to initialize superclasses. Use
+@samp{%define init_throws} to specify any uncaught exceptions.
+@end deftypeop
+
+@deftypeop {Constructor} {YYParser} {} YYParser (Lexer @var{lexer}, @var{parse_param}, @dots{})
+Build a new parser object using the specified scanner. There are no
+additional parameters unless @code{%param}s and/or @code{%parse-param}s are
+used.
+
+If the scanner is defined by @code{%code lexer}, this constructor is
+declared @code{protected} and is called automatically with a scanner
+created with the correct @code{%param}s and/or @code{%lex-param}s.
+
+Use @code{%code init} for code added to the start of the constructor
+body. This is especially useful to initialize superclasses. Use
+@samp{%define init_throws} to specify any uncaught exceptions.
+@end deftypeop
+
+@deftypemethod {YYParser} {boolean} parse ()
+Run the syntactic analysis, and return @code{true} on success,
+@code{false} otherwise.
+@end deftypemethod
+
+@deftypemethod {YYParser} {boolean} getErrorVerbose ()
+@deftypemethodx {YYParser} {void} setErrorVerbose (boolean @var{verbose})
+Get or set the option to produce verbose error messages. These are only
+available with @samp{%define parse.error verbose}, which also turns on
+verbose error messages.
+@end deftypemethod
+
+@deftypemethod {YYParser} {void} yyerror (String @var{msg})
+@deftypemethodx {YYParser} {void} yyerror (Position @var{pos}, String @var{msg})
+@deftypemethodx {YYParser} {void} yyerror (Location @var{loc}, String @var{msg})
+Print an error message using the @code{yyerror} method of the scanner
+instance in use. The @code{Location} and @code{Position} parameters are
+available only if location tracking is active.
+@end deftypemethod
+
+@deftypemethod {YYParser} {boolean} recovering ()
+During the syntactic analysis, return @code{true} if recovering
+from a syntax error.
+@xref{Error Recovery}.
+@end deftypemethod
+
+@deftypemethod {YYParser} {java.io.PrintStream} getDebugStream ()
+@deftypemethodx {YYParser} {void} setDebugStream (java.io.printStream @var{o})
+Get or set the stream used for tracing the parsing. It defaults to
+@code{System.err}.
+@end deftypemethod
+
+@deftypemethod {YYParser} {int} getDebugLevel ()
+@deftypemethodx {YYParser} {void} setDebugLevel (int @var{l})
+Get or set the tracing level. Currently its value is either 0, no trace,
+or nonzero, full tracing.
+@end deftypemethod
+
+@deftypecv {Constant} {YYParser} {String} {bisonVersion}
+@deftypecvx {Constant} {YYParser} {String} {bisonSkeleton}
+Identify the Bison version and skeleton used to generate this parser.
+@end deftypecv
+
+
+@node Java Scanner Interface
+@subsection Java Scanner Interface
+@c - %code lexer
+@c - %lex-param
+@c - Lexer interface
+
+There are two possible ways to interface a Bison-generated Java parser
+with a scanner: the scanner may be defined by @code{%code lexer}, or
+defined elsewhere. In either case, the scanner has to implement the
+@code{Lexer} inner interface of the parser class. This interface also
+contain constants for all user-defined token names and the predefined
+@code{EOF} token.
+
+In the first case, the body of the scanner class is placed in
+@code{%code lexer} blocks. If you want to pass parameters from the
+parser constructor to the scanner constructor, specify them with
+@code{%lex-param}; they are passed before @code{%parse-param}s to the
+constructor.
+
+In the second case, the scanner has to implement the @code{Lexer} interface,
+which is defined within the parser class (e.g., @code{YYParser.Lexer}).
+The constructor of the parser object will then accept an object
+implementing the interface; @code{%lex-param} is not used in this
+case.
+
+In both cases, the scanner has to implement the following methods.
+
+@deftypemethod {Lexer} {void} yyerror (Location @var{loc}, String @var{msg})
+This method is defined by the user to emit an error message. The first
+parameter is omitted if location tracking is not active. Its type can be
+changed using @samp{%define location_type "@var{class-name}".}
+@end deftypemethod
+
+@deftypemethod {Lexer} {int} yylex ()
+Return the next token. Its type is the return value, its semantic
+value and location are saved and returned by the their methods in the
+interface.
+
+Use @samp{%define lex_throws} to specify any uncaught exceptions.
+Default is @code{java.io.IOException}.
+@end deftypemethod
+
+@deftypemethod {Lexer} {Position} getStartPos ()
+@deftypemethodx {Lexer} {Position} getEndPos ()
+Return respectively the first position of the last token that
+@code{yylex} returned, and the first position beyond it. These
+methods are not needed unless location tracking is active.
+
+The return type can be changed using @samp{%define position_type
+"@var{class-name}".}
+@end deftypemethod
+
+@deftypemethod {Lexer} {Object} getLVal ()
+Return the semantic value of the last token that yylex returned.
+
+The return type can be changed using @samp{%define stype
+"@var{class-name}".}
+@end deftypemethod
+
+
+@node Java Action Features
+@subsection Special Features for Use in Java Actions
+
+The following special constructs can be uses in Java actions.
+Other analogous C action features are currently unavailable for Java.
+
+Use @samp{%define throws} to specify any uncaught exceptions from parser
+actions, and initial actions specified by @code{%initial-action}.
+
+@defvar $@var{n}
+The semantic value for the @var{n}th component of the current rule.
+This may not be assigned to.
+@xref{Java Semantic Values}.
+@end defvar
+
+@defvar $<@var{typealt}>@var{n}
+Like @code{$@var{n}} but specifies a alternative type @var{typealt}.
+@xref{Java Semantic Values}.
+@end defvar
+
+@defvar $$
+The semantic value for the grouping made by the current rule. As a
+value, this is in the base type (@code{Object} or as specified by
+@samp{%define stype}) as in not cast to the declared subtype because
+casts are not allowed on the left-hand side of Java assignments.
+Use an explicit Java cast if the correct subtype is needed.
+@xref{Java Semantic Values}.
+@end defvar
+
+@defvar $<@var{typealt}>$
+Same as @code{$$} since Java always allow assigning to the base type.
+Perhaps we should use this and @code{$<>$} for the value and @code{$$}
+for setting the value but there is currently no easy way to distinguish
+these constructs.
+@xref{Java Semantic Values}.
+@end defvar
+
+@defvar @@@var{n}
+The location information of the @var{n}th component of the current rule.
+This may not be assigned to.
+@xref{Java Location Values}.
+@end defvar
+
+@defvar @@$
+The location information of the grouping made by the current rule.
+@xref{Java Location Values}.
+@end defvar
+
+@deffn {Statement} {return YYABORT;}
+Return immediately from the parser, indicating failure.
+@xref{Java Parser Interface}.
+@end deffn
+
+@deffn {Statement} {return YYACCEPT;}
+Return immediately from the parser, indicating success.
+@xref{Java Parser Interface}.
+@end deffn
+
+@deffn {Statement} {return YYERROR;}
+Start error recovery without printing an error message.
+@xref{Error Recovery}.
+@end deffn
+
+@deftypefn {Function} {boolean} recovering ()
+Return whether error recovery is being done. In this state, the parser
+reads token until it reaches a known state, and then restarts normal
+operation.
+@xref{Error Recovery}.
+@end deftypefn
+
+@deftypefn {Function} {void} yyerror (String @var{msg})
+@deftypefnx {Function} {void} yyerror (Position @var{loc}, String @var{msg})
+@deftypefnx {Function} {void} yyerror (Location @var{loc}, String @var{msg})
+Print an error message using the @code{yyerror} method of the scanner
+instance in use. The @code{Location} and @code{Position} parameters are
+available only if location tracking is active.
+@end deftypefn
+
+
+@node Java Differences
+@subsection Differences between C/C++ and Java Grammars
+
+The different structure of the Java language forces several differences
+between C/C++ grammars, and grammars designed for Java parsers. This
+section summarizes these differences.
+
+@itemize
+@item
+Java lacks a preprocessor, so the @code{YYERROR}, @code{YYACCEPT},
+@code{YYABORT} symbols (@pxref{Table of Symbols}) cannot obviously be
+macros. Instead, they should be preceded by @code{return} when they
+appear in an action. The actual definition of these symbols is
+opaque to the Bison grammar, and it might change in the future. The
+only meaningful operation that you can do, is to return them.
+See @pxref{Java Action Features}.
+
+Note that of these three symbols, only @code{YYACCEPT} and
+@code{YYABORT} will cause a return from the @code{yyparse}
+method@footnote{Java parsers include the actions in a separate
+method than @code{yyparse} in order to have an intuitive syntax that
+corresponds to these C macros.}.
+
+@item
+Java lacks unions, so @code{%union} has no effect. Instead, semantic
+values have a common base type: @code{Object} or as specified by
+@samp{%define stype}. Angle brackets on @code{%token}, @code{type},
+@code{$@var{n}} and @code{$$} specify subtypes rather than fields of
+an union. The type of @code{$$}, even with angle brackets, is the base
+type since Java casts are not allow on the left-hand side of assignments.
+Also, @code{$@var{n}} and @code{@@@var{n}} are not allowed on the
+left-hand side of assignments. See @pxref{Java Semantic Values} and
+@pxref{Java Action Features}.
+
+@item
+The prologue declarations have a different meaning than in C/C++ code.
+@table @asis
+@item @code{%code imports}
+blocks are placed at the beginning of the Java source code. They may
+include copyright notices. For a @code{package} declarations, it is
+suggested to use @samp{%define package} instead.
+
+@item unqualified @code{%code}
+blocks are placed inside the parser class.
+
+@item @code{%code lexer}
+blocks, if specified, should include the implementation of the
+scanner. If there is no such block, the scanner can be any class
+that implements the appropriate interface (see @pxref{Java Scanner
+Interface}).
+@end table
+
+Other @code{%code} blocks are not supported in Java parsers.
+In particular, @code{%@{ @dots{} %@}} blocks should not be used
+and may give an error in future versions of Bison.
+
+The epilogue has the same meaning as in C/C++ code and it can
+be used to define other classes used by the parser @emph{outside}
+the parser class.
+@end itemize
+
+
+@node Java Declarations Summary
+@subsection Java Declarations Summary
+
+This summary only include declarations specific to Java or have special
+meaning when used in a Java parser.
+
+@deffn {Directive} {%language "Java"}
+Generate a Java class for the parser.
+@end deffn
+
+@deffn {Directive} %lex-param @{@var{type} @var{name}@}
+A parameter for the lexer class defined by @code{%code lexer}
+@emph{only}, added as parameters to the lexer constructor and the parser
+constructor that @emph{creates} a lexer. Default is none.
+@xref{Java Scanner Interface}.
+@end deffn
+
+@deffn {Directive} %name-prefix "@var{prefix}"
+The prefix of the parser class name @code{@var{prefix}Parser} if
+@samp{%define parser_class_name} is not used. Default is @code{YY}.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} %parse-param @{@var{type} @var{name}@}
+A parameter for the parser class added as parameters to constructor(s)
+and as fields initialized by the constructor(s). Default is none.
+@xref{Java Parser Interface}.
+@end deffn
+
+@deffn {Directive} %token <@var{type}> @var{token} @dots{}
+Declare tokens. Note that the angle brackets enclose a Java @emph{type}.
+@xref{Java Semantic Values}.
+@end deffn
+
+@deffn {Directive} %type <@var{type}> @var{nonterminal} @dots{}
+Declare the type of nonterminals. Note that the angle brackets enclose
+a Java @emph{type}.
+@xref{Java Semantic Values}.
+@end deffn
+
+@deffn {Directive} %code @{ @var{code} @dots{} @}
+Code appended to the inside of the parser class.
+@xref{Java Differences}.
+@end deffn
+
+@deffn {Directive} {%code imports} @{ @var{code} @dots{} @}
+Code inserted just after the @code{package} declaration.
+@xref{Java Differences}.
+@end deffn
+
+@deffn {Directive} {%code init} @{ @var{code} @dots{} @}
+Code inserted at the beginning of the parser constructor body.
+@xref{Java Parser Interface}.
+@end deffn
+
+@deffn {Directive} {%code lexer} @{ @var{code} @dots{} @}
+Code added to the body of a inner lexer class within the parser class.
+@xref{Java Scanner Interface}.
+@end deffn
+
+@deffn {Directive} %% @var{code} @dots{}
+Code (after the second @code{%%}) appended to the end of the file,
+@emph{outside} the parser class.
+@xref{Java Differences}.
+@end deffn
+
+@deffn {Directive} %@{ @var{code} @dots{} %@}
+Not supported. Use @code{%code imports} instead.
+@xref{Java Differences}.
+@end deffn
+
+@deffn {Directive} {%define abstract}
+Whether the parser class is declared @code{abstract}. Default is false.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define annotations} "@var{annotations}"
+The Java annotations for the parser class. Default is none.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define extends} "@var{superclass}"
+The superclass of the parser class. Default is none.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define final}
+Whether the parser class is declared @code{final}. Default is false.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define implements} "@var{interfaces}"
+The implemented interfaces of the parser class, a comma-separated list.
+Default is none.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define init_throws} "@var{exceptions}"
+The exceptions thrown by @code{%code init} from the parser class
+constructor. Default is none.
+@xref{Java Parser Interface}.
+@end deffn
+
+@deffn {Directive} {%define lex_throws} "@var{exceptions}"
+The exceptions thrown by the @code{yylex} method of the lexer, a
+comma-separated list. Default is @code{java.io.IOException}.
+@xref{Java Scanner Interface}.
+@end deffn
+
+@deffn {Directive} {%define location_type} "@var{class}"
+The name of the class used for locations (a range between two
+positions). This class is generated as an inner class of the parser
+class by @command{bison}. Default is @code{Location}.
+@xref{Java Location Values}.
+@end deffn
+
+@deffn {Directive} {%define package} "@var{package}"
+The package to put the parser class in. Default is none.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define parser_class_name} "@var{name}"
+The name of the parser class. Default is @code{YYParser} or
+@code{@var{name-prefix}Parser}.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define position_type} "@var{class}"
+The name of the class used for positions. This class must be supplied by
+the user. Default is @code{Position}.
+@xref{Java Location Values}.
+@end deffn
+
+@deffn {Directive} {%define public}
+Whether the parser class is declared @code{public}. Default is false.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define stype} "@var{class}"
+The base type of semantic values. Default is @code{Object}.
+@xref{Java Semantic Values}.
+@end deffn
+
+@deffn {Directive} {%define strictfp}
+Whether the parser class is declared @code{strictfp}. Default is false.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define throws} "@var{exceptions}"
+The exceptions thrown by user-supplied parser actions and
+@code{%initial-action}, a comma-separated list. Default is none.
+@xref{Java Parser Interface}.
+@end deffn
+
+
+@c ================================================= FAQ
+
+@node FAQ
+@chapter Frequently Asked Questions
+@cindex frequently asked questions
+@cindex questions
+
+Several questions about Bison come up occasionally. Here some of them
+are addressed.
+
+@menu
+* Memory Exhausted:: Breaking the Stack Limits
+* How Can I Reset the Parser:: @code{yyparse} Keeps some State
+* Strings are Destroyed:: @code{yylval} Loses Track of Strings
+* Implementing Gotos/Loops:: Control Flow in the Calculator
+* Multiple start-symbols:: Factoring closely related grammars
+* Secure? Conform?:: Is Bison POSIX safe?
+* I can't build Bison:: Troubleshooting
+* Where can I find help?:: Troubleshouting
+* Bug Reports:: Troublereporting
+* More Languages:: Parsers in C++, Java, and so on
+* Beta Testing:: Experimenting development versions
+* Mailing Lists:: Meeting other Bison users
+@end menu
+
+@node Memory Exhausted
+@section Memory Exhausted
+
+@quotation
+My parser returns with error with a @samp{memory exhausted}
+message. What can I do?
+@end quotation
+
+This question is already addressed elsewhere, @xref{Recursion,
+,Recursive Rules}.
+
+@node How Can I Reset the Parser
+@section How Can I Reset the Parser
+
+The following phenomenon has several symptoms, resulting in the
+following typical questions:
+
+@quotation
+I invoke @code{yyparse} several times, and on correct input it works
+properly; but when a parse error is found, all the other calls fail
+too. How can I reset the error flag of @code{yyparse}?
+@end quotation
+
+@noindent
+or
+
+@quotation
+My parser includes support for an @samp{#include}-like feature, in
+which case I run @code{yyparse} from @code{yyparse}. This fails
+although I did specify @samp{%define api.pure}.
+@end quotation
+
+These problems typically come not from Bison itself, but from
+Lex-generated scanners. Because these scanners use large buffers for
+speed, they might not notice a change of input file. As a
+demonstration, consider the following source file,
+@file{first-line.l}:
+
+@example
+@group
+%@{
+#include <stdio.h>
+#include <stdlib.h>
+%@}
+@end group
+%%
+.*\n ECHO; return 1;
+%%
+@group
+int
+yyparse (char const *file)
+@{
+ yyin = fopen (file, "r");
+ if (!yyin)
+ @{
+ perror ("fopen");
+ exit (EXIT_FAILURE);
+ @}
+@end group
+@group
+ /* One token only. */
+ yylex ();
+ if (fclose (yyin) != 0)
+ @{
+ perror ("fclose");
+ exit (EXIT_FAILURE);
+ @}
+ return 0;
+@}
+@end group
+
+@group
+int
+main (void)
+@{
+ yyparse ("input");
+ yyparse ("input");
+ return 0;
+@}
+@end group
+@end example
+
+@noindent
+If the file @file{input} contains
+
+@example
+input:1: Hello,
+input:2: World!
+@end example
+
+@noindent
+then instead of getting the first line twice, you get:
+
+@example
+$ @kbd{flex -ofirst-line.c first-line.l}
+$ @kbd{gcc -ofirst-line first-line.c -ll}
+$ @kbd{./first-line}
+input:1: Hello,
+input:2: World!
+@end example
+
+Therefore, whenever you change @code{yyin}, you must tell the
+Lex-generated scanner to discard its current buffer and switch to the
+new one. This depends upon your implementation of Lex; see its
+documentation for more. For Flex, it suffices to call
+@samp{YY_FLUSH_BUFFER} after each change to @code{yyin}. If your
+Flex-generated scanner needs to read from several input streams to
+handle features like include files, you might consider using Flex
+functions like @samp{yy_switch_to_buffer} that manipulate multiple
+input buffers.
+
+If your Flex-generated scanner uses start conditions (@pxref{Start
+conditions, , Start conditions, flex, The Flex Manual}), you might
+also want to reset the scanner's state, i.e., go back to the initial
+start condition, through a call to @samp{BEGIN (0)}.
+
+@node Strings are Destroyed
+@section Strings are Destroyed
+
+@quotation
+My parser seems to destroy old strings, or maybe it loses track of
+them. Instead of reporting @samp{"foo", "bar"}, it reports
+@samp{"bar", "bar"}, or even @samp{"foo\nbar", "bar"}.
+@end quotation
+
+This error is probably the single most frequent ``bug report'' sent to
+Bison lists, but is only concerned with a misunderstanding of the role
+of the scanner. Consider the following Lex code:
+
+@example
+@group
+%@{
+#include <stdio.h>
+char *yylval = NULL;
+%@}
+@end group
+@group
+%%
+.* yylval = yytext; return 1;
+\n /* IGNORE */
+%%
+@end group
+@group
+int
+main ()
+@{
+ /* Similar to using $1, $2 in a Bison action. */
+ char *fst = (yylex (), yylval);
+ char *snd = (yylex (), yylval);
+ printf ("\"%s\", \"%s\"\n", fst, snd);
+ return 0;
+@}
+@end group
+@end example
+
+If you compile and run this code, you get:
+
+@example
+$ @kbd{flex -osplit-lines.c split-lines.l}
+$ @kbd{gcc -osplit-lines split-lines.c -ll}
+$ @kbd{printf 'one\ntwo\n' | ./split-lines}
+"one
+two", "two"
+@end example
+
+@noindent
+this is because @code{yytext} is a buffer provided for @emph{reading}
+in the action, but if you want to keep it, you have to duplicate it
+(e.g., using @code{strdup}). Note that the output may depend on how
+your implementation of Lex handles @code{yytext}. For instance, when
+given the Lex compatibility option @option{-l} (which triggers the
+option @samp{%array}) Flex generates a different behavior:
+
+@example
+$ @kbd{flex -l -osplit-lines.c split-lines.l}
+$ @kbd{gcc -osplit-lines split-lines.c -ll}
+$ @kbd{printf 'one\ntwo\n' | ./split-lines}
+"two", "two"
+@end example
+
+
+@node Implementing Gotos/Loops
+@section Implementing Gotos/Loops
+
+@quotation
+My simple calculator supports variables, assignments, and functions,
+but how can I implement gotos, or loops?
+@end quotation
+
+Although very pedagogical, the examples included in the document blur
+the distinction to make between the parser---whose job is to recover
+the structure of a text and to transmit it to subsequent modules of
+the program---and the processing (such as the execution) of this
+structure. This works well with so called straight line programs,
+i.e., precisely those that have a straightforward execution model:
+execute simple instructions one after the others.
+
+@cindex abstract syntax tree
+@cindex AST
+If you want a richer model, you will probably need to use the parser
+to construct a tree that does represent the structure it has
+recovered; this tree is usually called the @dfn{abstract syntax tree},
+or @dfn{AST} for short. Then, walking through this tree,
+traversing it in various ways, will enable treatments such as its
+execution or its translation, which will result in an interpreter or a
+compiler.
+
+This topic is way beyond the scope of this manual, and the reader is
+invited to consult the dedicated literature.
+
+
+@node Multiple start-symbols
+@section Multiple start-symbols
-Bison supports both traditional single-letter options and mnemonic long
-option names. Long option names are indicated with @samp{--} instead of
-@samp{-}. Abbreviations for option names are allowed as long as they
-are unique. When a long option takes an argument, like
-@samp{--file-prefix}, connect the option name and the argument with
-@samp{=}.
+@quotation
+I have several closely related grammars, and I would like to share their
+implementations. In fact, I could use a single grammar but with
+multiple entry points.
+@end quotation
-Here is a list of options that can be used with Bison, alphabetized by
-short option. It is followed by a cross key alphabetized by long
-option.
+Bison does not support multiple start-symbols, but there is a very
+simple means to simulate them. If @code{foo} and @code{bar} are the two
+pseudo start-symbols, then introduce two new tokens, say
+@code{START_FOO} and @code{START_BAR}, and use them as switches from the
+real start-symbol:
-@c Please, keep this ordered as in `bison --help'.
-@noindent
-Operations modes:
-@table @option
-@item -h
-@itemx --help
-Print a summary of the command-line options to Bison and exit.
+@example
+%token START_FOO START_BAR;
+%start start;
+start:
+ START_FOO foo
+| START_BAR bar;
+@end example
-@item -V
-@itemx --version
-Print the version number of Bison and exit.
+These tokens prevents the introduction of new conflicts. As far as the
+parser goes, that is all that is needed.
-@need 1750
-@item -y
-@itemx --yacc
-@itemx --fixed-output-files
-Equivalent to @samp{-o y.tab.c}; the parser output file is called
-@file{y.tab.c}, and the other outputs are called @file{y.output} and
-@file{y.tab.h}. The purpose of this option is to imitate Yacc's output
-file name conventions. Thus, the following shell script can substitute
-for Yacc:@refill
+Now the difficult part is ensuring that the scanner will send these
+tokens first. If your scanner is hand-written, that should be
+straightforward. If your scanner is generated by Lex, them there is
+simple means to do it: recall that anything between @samp{%@{ ... %@}}
+after the first @code{%%} is copied verbatim in the top of the generated
+@code{yylex} function. Make sure a variable @code{start_token} is
+available in the scanner (e.g., a global variable or using
+@code{%lex-param} etc.), and use the following:
@example
-bison -y $*
+ /* @r{Prologue.} */
+%%
+%@{
+ if (start_token)
+ @{
+ int t = start_token;
+ start_token = 0;
+ return t;
+ @}
+%@}
+ /* @r{The rules.} */
@end example
-@end table
-@noindent
-Tuning the parser:
-@table @option
-@item -S @var{file}
-@itemx --skeleton=@var{file}
-Specify the skeleton to use. You probably don't need this option unless
-you are developing Bison.
+@node Secure? Conform?
+@section Secure? Conform?
-@item -t
-@itemx --debug
-Output a definition of the macro @code{YYDEBUG} into the parser file, so
-that the debugging facilities are compiled. @xref{Debugging, ,Debugging
-Your Parser}.
+@quotation
+Is Bison secure? Does it conform to POSIX?
+@end quotation
-@item --locations
-Pretend that @code{%locations} was specified. @xref{Decl Summary}.
+If you're looking for a guarantee or certification, we don't provide it.
+However, Bison is intended to be a reliable program that conforms to the
+POSIX specification for Yacc. If you run into problems,
+please send us a bug report.
-@item -p @var{prefix}
-@itemx --name-prefix=@var{prefix}
-Pretend that @code{%name-prefix="@var{prefix}"} was specified.
-@xref{Decl Summary}.
+@node I can't build Bison
+@section I can't build Bison
-@item -l
-@itemx --no-lines
-Don't put any @code{#line} preprocessor commands in the parser file.
-Ordinarily Bison puts them in the parser file so that the C compiler
-and debuggers will associate errors with your source file, the
-grammar file. This option causes them to associate errors with the
-parser file, treating it as an independent source file in its own right.
+@quotation
+I can't build Bison because @command{make} complains that
+@code{msgfmt} is not found.
+What should I do?
+@end quotation
-@item -n
-@itemx --no-parser
-Pretend that @code{%no-parser} was specified. @xref{Decl Summary}.
+Like most GNU packages with internationalization support, that feature
+is turned on by default. If you have problems building in the @file{po}
+subdirectory, it indicates that your system's internationalization
+support is lacking. You can re-configure Bison with
+@option{--disable-nls} to turn off this support, or you can install GNU
+gettext from @url{ftp://ftp.gnu.org/gnu/gettext/} and re-configure
+Bison. See the file @file{ABOUT-NLS} for more information.
-@item -k
-@itemx --token-table
-Pretend that @code{%token-table} was specified. @xref{Decl Summary}.
-@end table
-@noindent
-Adjust the output:
+@node Where can I find help?
+@section Where can I find help?
-@table @option
-@item -d
-@itemx --defines
-Pretend that @code{%defines} was specified, i.e., write an extra output
-file containing macro definitions for the token type names defined in
-the grammar and the semantic value type @code{YYSTYPE}, as well as a few
-@code{extern} variable declarations. @xref{Decl Summary}.
+@quotation
+I'm having trouble using Bison. Where can I find help?
+@end quotation
-@item --defines=@var{defines-file}
-Same as above, but save in the file @var{defines-file}.
+First, read this fine manual. Beyond that, you can send mail to
+@email{help-bison@@gnu.org}. This mailing list is intended to be
+populated with people who are willing to answer questions about using
+and installing Bison. Please keep in mind that (most of) the people on
+the list have aspects of their lives which are not related to Bison (!),
+so you may not receive an answer to your question right away. This can
+be frustrating, but please try not to honk them off; remember that any
+help they provide is purely voluntary and out of the kindness of their
+hearts.
-@item -b @var{file-prefix}
-@itemx --file-prefix=@var{prefix}
-Pretend that @code{%verbose} was specified, i.e, specify prefix to use
-for all Bison output file names. @xref{Decl Summary}.
+@node Bug Reports
+@section Bug Reports
-@item -v
-@itemx --verbose
-Pretend that @code{%verbose} was specified, i.e, write an extra output
-file containing verbose descriptions of the grammar and
-parser. @xref{Decl Summary}.
+@quotation
+I found a bug. What should I include in the bug report?
+@end quotation
-@item -o @var{filename}
-@itemx --output=@var{filename}
-Specify the @var{filename} for the parser file.
+Before you send a bug report, make sure you are using the latest
+version. Check @url{ftp://ftp.gnu.org/pub/gnu/bison/} or one of its
+mirrors. Be sure to include the version number in your bug report. If
+the bug is present in the latest version but not in a previous version,
+try to determine the most recent version which did not contain the bug.
-The other output files' names are constructed from @var{filename} as
-described under the @samp{-v} and @samp{-d} options.
+If the bug is parser-related, you should include the smallest grammar
+you can which demonstrates the bug. The grammar file should also be
+complete (i.e., I should be able to run it through Bison without having
+to edit or add anything). The smaller and simpler the grammar, the
+easier it will be to fix the bug.
-@item -g
-Output a VCG definition of the LALR(1) grammar automaton computed by
-Bison. If the grammar file is @file{foo.y}, the VCG output file will
-be @file{foo.vcg}.
+Include information about your compilation environment, including your
+operating system's name and version and your compiler's name and
+version. If you have trouble compiling, you should also include a
+transcript of the build session, starting with the invocation of
+`configure'. Depending on the nature of the bug, you may be asked to
+send additional files as well (such as `config.h' or `config.cache').
-@item --graph=@var{graph-file}
-The behaviour of @var{--graph} is the same than @samp{-g}. The only
-difference is that it has an optionnal argument which is the name of
-the output graph filename.
-@end table
+Patches are most welcome, but not required. That is, do not hesitate to
+send a bug report just because you cannot provide a fix.
-@node Environment Variables
-@section Environment Variables
-@cindex environment variables
-@cindex BISON_HAIRY
-@cindex BISON_SIMPLE
-
-Here is a list of environment variables which affect the way Bison
-runs.
-
-@table @samp
-@item BISON_SIMPLE
-@itemx BISON_HAIRY
-Much of the parser generated by Bison is copied verbatim from a file
-called @file{bison.simple}. If Bison cannot find that file, or if you
-would like to direct Bison to use a different copy, setting the
-environment variable @code{BISON_SIMPLE} to the path of the file will
-cause Bison to use that copy instead.
-
-When the @samp{%semantic_parser} declaration is used, Bison copies from
-a file called @file{bison.hairy} instead. The location of this file can
-also be specified or overridden in a similar fashion, with the
-@code{BISON_HAIRY} environment variable.
+Send bug reports to @email{bug-bison@@gnu.org}.
-@end table
+@node More Languages
+@section More Languages
-@node Option Cross Key
-@section Option Cross Key
+@quotation
+Will Bison ever have C++ and Java support? How about @var{insert your
+favorite language here}?
+@end quotation
-Here is a list of options, alphabetized by long option, to help you find
-the corresponding short option.
-
-@tex
-\def\leaderfill{\leaders\hbox to 1em{\hss.\hss}\hfill}
-
-{\tt
-\line{ --debug \leaderfill -t}
-\line{ --defines \leaderfill -d}
-\line{ --file-prefix \leaderfill -b}
-\line{ --fixed-output-files \leaderfill -y}
-\line{ --graph \leaderfill -g}
-\line{ --help \leaderfill -h}
-\line{ --name-prefix \leaderfill -p}
-\line{ --no-lines \leaderfill -l}
-\line{ --no-parser \leaderfill -n}
-\line{ --output \leaderfill -o}
-\line{ --token-table \leaderfill -k}
-\line{ --verbose \leaderfill -v}
-\line{ --version \leaderfill -V}
-\line{ --yacc \leaderfill -y}
-}
-@end tex
+C++ and Java support is there now, and is documented. We'd love to add other
+languages; contributions are welcome.
-@ifinfo
-@example
---debug -t
---defines=@var{defines-file} -d
---file-prefix=@var{prefix} -b @var{file-prefix}
---fixed-output-files --yacc -y
---graph=@var{graph-file} -d
---help -h
---name-prefix=@var{prefix} -p @var{name-prefix}
---no-lines -l
---no-parser -n
---output=@var{outfile} -o @var{outfile}
---token-table -k
---verbose -v
---version -V
-@end example
-@end ifinfo
+@node Beta Testing
+@section Beta Testing
-@node VMS Invocation
-@section Invoking Bison under VMS
-@cindex invoking Bison under VMS
-@cindex VMS
+@quotation
+What is involved in being a beta tester?
+@end quotation
-The command line syntax for Bison on VMS is a variant of the usual
-Bison command syntax---adapted to fit VMS conventions.
+It's not terribly involved. Basically, you would download a test
+release, compile it, and use it to build and run a parser or two. After
+that, you would submit either a bug report or a message saying that
+everything is okay. It is important to report successes as well as
+failures because test releases eventually become mainstream releases,
+but only if they are adequately tested. If no one tests, development is
+essentially halted.
-To find the VMS equivalent for any Bison option, start with the long
-option, and substitute a @samp{/} for the leading @samp{--}, and
-substitute a @samp{_} for each @samp{-} in the name of the long option.
-For example, the following invocation under VMS:
+Beta testers are particularly needed for operating systems to which the
+developers do not have easy access. They currently have easy access to
+recent GNU/Linux and Solaris versions. Reports about other operating
+systems are especially welcome.
-@example
-bison /debug/name_prefix=bar foo.y
-@end example
+@node Mailing Lists
+@section Mailing Lists
-@noindent
-is equivalent to the following command under POSIX.
+@quotation
+How do I join the help-bison and bug-bison mailing lists?
+@end quotation
-@example
-bison --debug --name-prefix=bar foo.y
-@end example
+See @url{http://lists.gnu.org/}.
-The VMS file system does not permit filenames such as
-@file{foo.tab.c}. In the above example, the output file
-would instead be named @file{foo_tab.c}.
+@c ================================================= Table of Symbols
@node Table of Symbols
@appendix Bison Symbols
@cindex Bison symbols, table of
@cindex symbols in Bison, table of
-@table @code
-@item error
-A token name reserved for error recovery. This token may be used in
-grammar rules so as to allow the Bison parser to recognize an error in
-the grammar without halting the process. In effect, a sentence
-containing an error may be recognized as valid. On a parse error, the
-token @code{error} becomes the current look-ahead token. Actions
-corresponding to @code{error} are then executed, and the look-ahead
-token is reset to the token that originally caused the violation.
-@xref{Error Recovery}.
-
-@item YYABORT
-Macro to pretend that an unrecoverable syntax error has occurred, by
-making @code{yyparse} return 1 immediately. The error reporting
-function @code{yyerror} is not called. @xref{Parser Function, ,The
-Parser Function @code{yyparse}}.
+@deffn {Variable} @@$
+In an action, the location of the left-hand side of the rule.
+@xref{Tracking Locations}.
+@end deffn
-@item YYACCEPT
-Macro to pretend that a complete utterance of the language has been
-read, by making @code{yyparse} return 0 immediately.
-@xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@deffn {Variable} @@@var{n}
+In an action, the location of the @var{n}-th symbol of the right-hand side
+of the rule. @xref{Tracking Locations}.
+@end deffn
-@item YYBACKUP
-Macro to discard a value from the parser stack and fake a look-ahead
-token. @xref{Action Features, ,Special Features for Use in Actions}.
+@deffn {Variable} @@@var{name}
+In an action, the location of a symbol addressed by name. @xref{Tracking
+Locations}.
+@end deffn
-@item YYERROR
-Macro to pretend that a syntax error has just been detected: call
-@code{yyerror} and then perform normal error recovery if possible
-(@pxref{Error Recovery}), or (if recovery is impossible) make
-@code{yyparse} return 1. @xref{Error Recovery}.
+@deffn {Variable} @@[@var{name}]
+In an action, the location of a symbol addressed by name. @xref{Tracking
+Locations}.
+@end deffn
-@item YYERROR_VERBOSE
-Macro that you define with @code{#define} in the Bison declarations
-section to request verbose, specific error message strings when
-@code{yyerror} is called.
+@deffn {Variable} $$
+In an action, the semantic value of the left-hand side of the rule.
+@xref{Actions}.
+@end deffn
-@item YYINITDEPTH
-Macro for specifying the initial size of the parser stack.
-@xref{Stack Overflow}.
+@deffn {Variable} $@var{n}
+In an action, the semantic value of the @var{n}-th symbol of the
+right-hand side of the rule. @xref{Actions}.
+@end deffn
-@item YYLEX_PARAM
-Macro for specifying an extra argument (or list of extra arguments) for
-@code{yyparse} to pass to @code{yylex}. @xref{Pure Calling,, Calling
-Conventions for Pure Parsers}.
+@deffn {Variable} $@var{name}
+In an action, the semantic value of a symbol addressed by name.
+@xref{Actions}.
+@end deffn
-@item YYLTYPE
-Macro for the data type of @code{yylloc}; a structure with four
-members. @xref{Location Type, , Data Types of Locations}.
+@deffn {Variable} $[@var{name}]
+In an action, the semantic value of a symbol addressed by name.
+@xref{Actions}.
+@end deffn
-@item yyltype
-Default value for YYLTYPE.
+@deffn {Delimiter} %%
+Delimiter used to separate the grammar rule section from the
+Bison declarations section or the epilogue.
+@xref{Grammar Layout, ,The Overall Layout of a Bison Grammar}.
+@end deffn
-@item YYMAXDEPTH
-Macro for specifying the maximum size of the parser stack.
-@xref{Stack Overflow}.
+@c Don't insert spaces, or check the DVI output.
+@deffn {Delimiter} %@{@var{code}%@}
+All code listed between @samp{%@{} and @samp{%@}} is copied verbatim
+to the parser implementation file. Such code forms the prologue of
+the grammar file. @xref{Grammar Outline, ,Outline of a Bison
+Grammar}.
+@end deffn
+
+@deffn {Directive} %?@{@var{expression}@}
+Predicate actions. This is a type of action clause that may appear in
+rules. The expression is evaluated, and if false, causes a syntax error. In
+GLR parsers during nondeterministic operation,
+this silently causes an alternative parse to die. During deterministic
+operation, it is the same as the effect of YYERROR.
+@xref{Semantic Predicates}.
+
+This feature is experimental.
+More user feedback will help to determine whether it should become a permanent
+feature.
+@end deffn
+
+@deffn {Construct} /*@dots{}*/
+Comment delimiters, as in C.
+@end deffn
-@item YYPARSE_PARAM
-Macro for specifying the name of a parameter that @code{yyparse} should
-accept. @xref{Pure Calling,, Calling Conventions for Pure Parsers}.
+@deffn {Delimiter} :
+Separates a rule's result from its components. @xref{Rules, ,Syntax of
+Grammar Rules}.
+@end deffn
-@item YYRECOVERING
-Macro whose value indicates whether the parser is recovering from a
-syntax error. @xref{Action Features, ,Special Features for Use in Actions}.
+@deffn {Delimiter} ;
+Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}.
+@end deffn
-@item YYSTACK_USE_ALLOCA
-Macro used to control the use of @code{alloca}. If defined to @samp{0},
-the parser will not use @code{alloca} but @code{malloc} when trying to
-grow its internal stacks. Do @emph{not} define @code{YYSTACK_USE_ALLOCA}
-to anything else.
+@deffn {Delimiter} |
+Separates alternate rules for the same result nonterminal.
+@xref{Rules, ,Syntax of Grammar Rules}.
+@end deffn
-@item YYSTYPE
-Macro for the data type of semantic values; @code{int} by default.
-@xref{Value Type, ,Data Types of Semantic Values}.
+@deffn {Directive} <*>
+Used to define a default tagged @code{%destructor} or default tagged
+@code{%printer}.
-@item yychar
-External integer variable that contains the integer value of the current
-look-ahead token. (In a pure parser, it is a local variable within
-@code{yyparse}.) Error-recovery rule actions may examine this variable.
-@xref{Action Features, ,Special Features for Use in Actions}.
+This feature is experimental.
+More user feedback will help to determine whether it should become a permanent
+feature.
-@item yyclearin
-Macro used in error-recovery rule actions. It clears the previous
-look-ahead token. @xref{Error Recovery}.
+@xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
-@item yydebug
-External integer variable set to zero by default. If @code{yydebug}
-is given a nonzero value, the parser will output information on input
-symbols and parser action. @xref{Debugging, ,Debugging Your Parser}.
+@deffn {Directive} <>
+Used to define a default tagless @code{%destructor} or default tagless
+@code{%printer}.
-@item yyerrok
-Macro to cause parser to recover immediately to its normal mode
-after a parse error. @xref{Error Recovery}.
+This feature is experimental.
+More user feedback will help to determine whether it should become a permanent
+feature.
-@item yyerror
-User-supplied function to be called by @code{yyparse} on error. The
-function receives one argument, a pointer to a character string
-containing an error message. @xref{Error Reporting, ,The Error
-Reporting Function @code{yyerror}}.
+@xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
-@item yylex
-User-supplied lexical analyzer function, called with no arguments
-to get the next token. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
+@deffn {Symbol} $accept
+The predefined nonterminal whose only rule is @samp{$accept: @var{start}
+$end}, where @var{start} is the start symbol. @xref{Start Decl, , The
+Start-Symbol}. It cannot be used in the grammar.
+@end deffn
-@item yylval
-External variable in which @code{yylex} should place the semantic
-value associated with a token. (In a pure parser, it is a local
-variable within @code{yyparse}, and its address is passed to
-@code{yylex}.) @xref{Token Values, ,Semantic Values of Tokens}.
+@deffn {Directive} %code @{@var{code}@}
+@deffnx {Directive} %code @var{qualifier} @{@var{code}@}
+Insert @var{code} verbatim into the output parser source at the
+default location or at the location specified by @var{qualifier}.
+@xref{%code Summary}.
+@end deffn
-@item yylloc
-External variable in which @code{yylex} should place the line and column
-numbers associated with a token. (In a pure parser, it is a local
-variable within @code{yyparse}, and its address is passed to
-@code{yylex}.) You can ignore this variable if you don't use the
-@samp{@@} feature in the grammar actions. @xref{Token Positions,
-,Textual Positions of Tokens}.
+@deffn {Directive} %debug
+Equip the parser for debugging. @xref{Decl Summary}.
+@end deffn
-@item yynerrs
-Global variable which Bison increments each time there is a parse error.
-(In a pure parser, it is a local variable within @code{yyparse}.)
-@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
+@ifset defaultprec
+@deffn {Directive} %default-prec
+Assign a precedence to rules that lack an explicit @samp{%prec}
+modifier. @xref{Contextual Precedence, ,Context-Dependent
+Precedence}.
+@end deffn
+@end ifset
-@item yyparse
-The parser function produced by Bison; call this function to start
-parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@deffn {Directive} %define @var{variable}
+@deffnx {Directive} %define @var{variable} @var{value}
+@deffnx {Directive} %define @var{variable} "@var{value}"
+Define a variable to adjust Bison's behavior. @xref{%define Summary}.
+@end deffn
-@item %debug
-Equip the parser for debugging. @xref{Decl Summary}.
+@deffn {Directive} %defines
+Bison declaration to create a parser header file, which is usually
+meant for the scanner. @xref{Decl Summary}.
+@end deffn
-@item %defines
-Bison declaration to create a header file meant for the scanner.
+@deffn {Directive} %defines @var{defines-file}
+Same as above, but save in the file @var{defines-file}.
@xref{Decl Summary}.
+@end deffn
+
+@deffn {Directive} %destructor
+Specify how the parser should reclaim the memory associated to
+discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
+
+@deffn {Directive} %dprec
+Bison declaration to assign a precedence to a rule that is used at parse
+time to resolve reduce/reduce conflicts. @xref{GLR Parsers, ,Writing
+GLR Parsers}.
+@end deffn
+
+@deffn {Symbol} $end
+The predefined token marking the end of the token stream. It cannot be
+used in the grammar.
+@end deffn
-@item %file-prefix="@var{prefix}"
-Bison declaration to set tge prefix of the output files. @xref{Decl
+@deffn {Symbol} error
+A token name reserved for error recovery. This token may be used in
+grammar rules so as to allow the Bison parser to recognize an error in
+the grammar without halting the process. In effect, a sentence
+containing an error may be recognized as valid. On a syntax error, the
+token @code{error} becomes the current lookahead token. Actions
+corresponding to @code{error} are then executed, and the lookahead
+token is reset to the token that originally caused the violation.
+@xref{Error Recovery}.
+@end deffn
+
+@deffn {Directive} %error-verbose
+An obsolete directive standing for @samp{%define parse.error verbose}
+(@pxref{Error Reporting, ,The Error Reporting Function @code{yyerror}}).
+@end deffn
+
+@deffn {Directive} %file-prefix "@var{prefix}"
+Bison declaration to set the prefix of the output files. @xref{Decl
Summary}.
+@end deffn
-@c @item %source_extension
-@c Bison declaration to specify the generated parser output file extension.
-@c @xref{Decl Summary}.
-@c
-@c @item %header_extension
-@c Bison declaration to specify the generated parser header file extension
-@c if required. @xref{Decl Summary}.
+@deffn {Directive} %glr-parser
+Bison declaration to produce a GLR parser. @xref{GLR
+Parsers, ,Writing GLR Parsers}.
+@end deffn
-@item %left
-Bison declaration to assign left associativity to token(s).
-@xref{Precedence Decl, ,Operator Precedence}.
+@deffn {Directive} %initial-action
+Run user code before parsing. @xref{Initial Action Decl, , Performing Actions before Parsing}.
+@end deffn
-@item %name-prefix="@var{prefix}"
-Bison declaration to rename the external symbols. @xref{Decl Summary}.
+@deffn {Directive} %language
+Specify the programming language for the generated parser.
+@xref{Decl Summary}.
+@end deffn
+
+@deffn {Directive} %left
+Bison declaration to assign precedence and left associativity to token(s).
+@xref{Precedence Decl, ,Operator Precedence}.
+@end deffn
+
+@deffn {Directive} %lex-param @{@var{argument-declaration}@} @dots{}
+Bison declaration to specifying additional arguments that
+@code{yylex} should accept. @xref{Pure Calling,, Calling Conventions
+for Pure Parsers}.
+@end deffn
+
+@deffn {Directive} %merge
+Bison declaration to assign a merging function to a rule. If there is a
+reduce/reduce conflict with a rule having the same merging function, the
+function is applied to the two semantic values to get a single result.
+@xref{GLR Parsers, ,Writing GLR Parsers}.
+@end deffn
+
+@deffn {Directive} %name-prefix "@var{prefix}"
+Bison declaration to rename the external symbols. @xref{Decl Summary}.
+@end deffn
+
+@ifset defaultprec
+@deffn {Directive} %no-default-prec
+Do not assign a precedence to rules that lack an explicit @samp{%prec}
+modifier. @xref{Contextual Precedence, ,Context-Dependent
+Precedence}.
+@end deffn
+@end ifset
-@item %no-lines
+@deffn {Directive} %no-lines
Bison declaration to avoid generating @code{#line} directives in the
-parser file. @xref{Decl Summary}.
+parser implementation file. @xref{Decl Summary}.
+@end deffn
-@item %nonassoc
-Bison declaration to assign non-associativity to token(s).
+@deffn {Directive} %nonassoc
+Bison declaration to assign precedence and nonassociativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
+@end deffn
-@item %output="@var{filename}"
-Bison declaration to set the name of the parser file. @xref{Decl
-Summary}.
+@deffn {Directive} %output "@var{file}"
+Bison declaration to set the name of the parser implementation file.
+@xref{Decl Summary}.
+@end deffn
+
+@deffn {Directive} %param @{@var{argument-declaration}@} @dots{}
+Bison declaration to specify additional arguments that both
+@code{yylex} and @code{yyparse} should accept. @xref{Parser Function,, The
+Parser Function @code{yyparse}}.
+@end deffn
+
+@deffn {Directive} %parse-param @{@var{argument-declaration}@} @dots{}
+Bison declaration to specify additional arguments that @code{yyparse}
+should accept. @xref{Parser Function,, The Parser Function @code{yyparse}}.
+@end deffn
-@item %prec
+@deffn {Directive} %prec
Bison declaration to assign a precedence to a specific rule.
@xref{Contextual Precedence, ,Context-Dependent Precedence}.
+@end deffn
-@item %pure-parser
-Bison declaration to request a pure (reentrant) parser.
-@xref{Pure Decl, ,A Pure (Reentrant) Parser}.
+@deffn {Directive} %precedence
+Bison declaration to assign precedence to token(s), but no associativity
+@xref{Precedence Decl, ,Operator Precedence}.
+@end deffn
-@item %right
-Bison declaration to assign right associativity to token(s).
+@deffn {Directive} %pure-parser
+Deprecated version of @samp{%define api.pure} (@pxref{%define
+Summary,,api.pure}), for which Bison is more careful to warn about
+unreasonable usage.
+@end deffn
+
+@deffn {Directive} %require "@var{version}"
+Require version @var{version} or higher of Bison. @xref{Require Decl, ,
+Require a Version of Bison}.
+@end deffn
+
+@deffn {Directive} %right
+Bison declaration to assign precedence and right associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
+@end deffn
+
+@deffn {Directive} %skeleton
+Specify the skeleton to use; usually for development.
+@xref{Decl Summary}.
+@end deffn
-@item %start
-Bison declaration to specify the start symbol. @xref{Start Decl, ,The Start-Symbol}.
+@deffn {Directive} %start
+Bison declaration to specify the start symbol. @xref{Start Decl, ,The
+Start-Symbol}.
+@end deffn
-@item %token
+@deffn {Directive} %token
Bison declaration to declare token(s) without specifying precedence.
@xref{Token Decl, ,Token Type Names}.
+@end deffn
-@item %token-table
-Bison declaration to include a token name table in the parser file.
-@xref{Decl Summary}.
+@deffn {Directive} %token-table
+Bison declaration to include a token name table in the parser
+implementation file. @xref{Decl Summary}.
+@end deffn
+
+@deffn {Directive} %type
+Bison declaration to declare nonterminals. @xref{Type Decl,
+,Nonterminal Symbols}.
+@end deffn
-@item %type
-Bison declaration to declare nonterminals. @xref{Type Decl, ,Nonterminal Symbols}.
+@deffn {Symbol} $undefined
+The predefined token onto which all undefined values returned by
+@code{yylex} are mapped. It cannot be used in the grammar, rather, use
+@code{error}.
+@end deffn
-@item %union
+@deffn {Directive} %union
Bison declaration to specify several possible data types for semantic
values. @xref{Union Decl, ,The Collection of Value Types}.
-@end table
+@end deffn
-These are the punctuation and delimiters used in Bison input:
+@deffn {Macro} YYABORT
+Macro to pretend that an unrecoverable syntax error has occurred, by
+making @code{yyparse} return 1 immediately. The error reporting
+function @code{yyerror} is not called. @xref{Parser Function, ,The
+Parser Function @code{yyparse}}.
-@table @samp
-@item %%
-Delimiter used to separate the grammar rule section from the
-Bison declarations section or the epilogue.
-@xref{Grammar Layout, ,The Overall Layout of a Bison Grammar}.
+For Java parsers, this functionality is invoked using @code{return YYABORT;}
+instead.
+@end deffn
-@item %@{ %@}
-All code listed between @samp{%@{} and @samp{%@}} is copied directly to
-the output file uninterpreted. Such code forms the prologue of the input
-file. @xref{Grammar Outline, ,Outline of a Bison
-Grammar}.
+@deffn {Macro} YYACCEPT
+Macro to pretend that a complete utterance of the language has been
+read, by making @code{yyparse} return 0 immediately.
+@xref{Parser Function, ,The Parser Function @code{yyparse}}.
-@item /*@dots{}*/
-Comment delimiters, as in C.
+For Java parsers, this functionality is invoked using @code{return YYACCEPT;}
+instead.
+@end deffn
-@item :
-Separates a rule's result from its components. @xref{Rules, ,Syntax of
-Grammar Rules}.
+@deffn {Macro} YYBACKUP
+Macro to discard a value from the parser stack and fake a lookahead
+token. @xref{Action Features, ,Special Features for Use in Actions}.
+@end deffn
-@item ;
-Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}.
+@deffn {Variable} yychar
+External integer variable that contains the integer value of the
+lookahead token. (In a pure parser, it is a local variable within
+@code{yyparse}.) Error-recovery rule actions may examine this variable.
+@xref{Action Features, ,Special Features for Use in Actions}.
+@end deffn
-@item |
-Separates alternate rules for the same result nonterminal.
-@xref{Rules, ,Syntax of Grammar Rules}.
-@end table
+@deffn {Variable} yyclearin
+Macro used in error-recovery rule actions. It clears the previous
+lookahead token. @xref{Error Recovery}.
+@end deffn
+
+@deffn {Macro} YYDEBUG
+Macro to define to equip the parser with tracing code. @xref{Tracing,
+,Tracing Your Parser}.
+@end deffn
+
+@deffn {Variable} yydebug
+External integer variable set to zero by default. If @code{yydebug}
+is given a nonzero value, the parser will output information on input
+symbols and parser action. @xref{Tracing, ,Tracing Your Parser}.
+@end deffn
+
+@deffn {Macro} yyerrok
+Macro to cause parser to recover immediately to its normal mode
+after a syntax error. @xref{Error Recovery}.
+@end deffn
+
+@deffn {Macro} YYERROR
+Macro to pretend that a syntax error has just been detected: call
+@code{yyerror} and then perform normal error recovery if possible
+(@pxref{Error Recovery}), or (if recovery is impossible) make
+@code{yyparse} return 1. @xref{Error Recovery}.
+
+For Java parsers, this functionality is invoked using @code{return YYERROR;}
+instead.
+@end deffn
+
+@deffn {Function} yyerror
+User-supplied function to be called by @code{yyparse} on error.
+@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
+@end deffn
+
+@deffn {Macro} YYERROR_VERBOSE
+An obsolete macro used in the @file{yacc.c} skeleton, that you define
+with @code{#define} in the prologue to request verbose, specific error
+message strings when @code{yyerror} is called. It doesn't matter what
+definition you use for @code{YYERROR_VERBOSE}, just whether you define
+it. Using @samp{%define parse.error verbose} is preferred
+(@pxref{Error Reporting, ,The Error Reporting Function @code{yyerror}}).
+@end deffn
+
+@deffn {Macro} YYINITDEPTH
+Macro for specifying the initial size of the parser stack.
+@xref{Memory Management}.
+@end deffn
+
+@deffn {Function} yylex
+User-supplied lexical analyzer function, called with no arguments to get
+the next token. @xref{Lexical, ,The Lexical Analyzer Function
+@code{yylex}}.
+@end deffn
+
+@deffn {Macro} YYLEX_PARAM
+An obsolete macro for specifying an extra argument (or list of extra
+arguments) for @code{yyparse} to pass to @code{yylex}. The use of this
+macro is deprecated, and is supported only for Yacc like parsers.
+@xref{Pure Calling,, Calling Conventions for Pure Parsers}.
+@end deffn
+
+@deffn {Variable} yylloc
+External variable in which @code{yylex} should place the line and column
+numbers associated with a token. (In a pure parser, it is a local
+variable within @code{yyparse}, and its address is passed to
+@code{yylex}.)
+You can ignore this variable if you don't use the @samp{@@} feature in the
+grammar actions.
+@xref{Token Locations, ,Textual Locations of Tokens}.
+In semantic actions, it stores the location of the lookahead token.
+@xref{Actions and Locations, ,Actions and Locations}.
+@end deffn
+
+@deffn {Type} YYLTYPE
+Data type of @code{yylloc}; by default, a structure with four
+members. @xref{Location Type, , Data Types of Locations}.
+@end deffn
+
+@deffn {Variable} yylval
+External variable in which @code{yylex} should place the semantic
+value associated with a token. (In a pure parser, it is a local
+variable within @code{yyparse}, and its address is passed to
+@code{yylex}.)
+@xref{Token Values, ,Semantic Values of Tokens}.
+In semantic actions, it stores the semantic value of the lookahead token.
+@xref{Actions, ,Actions}.
+@end deffn
+
+@deffn {Macro} YYMAXDEPTH
+Macro for specifying the maximum size of the parser stack. @xref{Memory
+Management}.
+@end deffn
+
+@deffn {Variable} yynerrs
+Global variable which Bison increments each time it reports a syntax error.
+(In a pure parser, it is a local variable within @code{yyparse}. In a
+pure push parser, it is a member of yypstate.)
+@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
+@end deffn
+
+@deffn {Function} yyparse
+The parser function produced by Bison; call this function to start
+parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@end deffn
+
+@deffn {Function} yypstate_delete
+The function to delete a parser instance, produced by Bison in push mode;
+call this function to delete the memory associated with a parser.
+@xref{Parser Delete Function, ,The Parser Delete Function
+@code{yypstate_delete}}.
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+@end deffn
+
+@deffn {Function} yypstate_new
+The function to create a parser instance, produced by Bison in push mode;
+call this function to create a new parser.
+@xref{Parser Create Function, ,The Parser Create Function
+@code{yypstate_new}}.
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+@end deffn
+
+@deffn {Function} yypull_parse
+The parser function produced by Bison in push mode; call this function to
+parse the rest of the input stream.
+@xref{Pull Parser Function, ,The Pull Parser Function
+@code{yypull_parse}}.
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+@end deffn
+
+@deffn {Function} yypush_parse
+The parser function produced by Bison in push mode; call this function to
+parse a single token. @xref{Push Parser Function, ,The Push Parser Function
+@code{yypush_parse}}.
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+@end deffn
+
+@deffn {Macro} YYPARSE_PARAM
+An obsolete macro for specifying the name of a parameter that
+@code{yyparse} should accept. The use of this macro is deprecated, and
+is supported only for Yacc like parsers. @xref{Pure Calling,, Calling
+Conventions for Pure Parsers}.
+@end deffn
+
+@deffn {Macro} YYRECOVERING
+The expression @code{YYRECOVERING ()} yields 1 when the parser
+is recovering from a syntax error, and 0 otherwise.
+@xref{Action Features, ,Special Features for Use in Actions}.
+@end deffn
+
+@deffn {Macro} YYSTACK_USE_ALLOCA
+Macro used to control the use of @code{alloca} when the
+deterministic parser in C needs to extend its stacks. If defined to 0,
+the parser will use @code{malloc} to extend its stacks. If defined to
+1, the parser will use @code{alloca}. Values other than 0 and 1 are
+reserved for future Bison extensions. If not defined,
+@code{YYSTACK_USE_ALLOCA} defaults to 0.
+
+In the all-too-common case where your code may run on a host with a
+limited stack and with unreliable stack-overflow checking, you should
+set @code{YYMAXDEPTH} to a value that cannot possibly result in
+unchecked stack overflow on any of your target hosts when
+@code{alloca} is called. You can inspect the code that Bison
+generates in order to determine the proper numeric values. This will
+require some expertise in low-level implementation details.
+@end deffn
+
+@deffn {Type} YYSTYPE
+Data type of semantic values; @code{int} by default.
+@xref{Value Type, ,Data Types of Semantic Values}.
+@end deffn
@node Glossary
@appendix Glossary
@cindex glossary
@table @asis
-@item Backus-Naur Form (BNF)
-Formal method of specifying context-free grammars. BNF was first used
-in the @cite{ALGOL-60} report, 1963. @xref{Language and Grammar,
-,Languages and Context-Free Grammars}.
+@item Accepting state
+A state whose only action is the accept action.
+The accepting state is thus a consistent state.
+@xref{Understanding,,}.
+
+@item Backus-Naur Form (BNF; also called ``Backus Normal Form'')
+Formal method of specifying context-free grammars originally proposed
+by John Backus, and slightly improved by Peter Naur in his 1960-01-02
+committee document contributing to what became the Algol 60 report.
+@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
+
+@item Consistent state
+A state containing only one possible action. @xref{Default Reductions}.
@item Context-free grammars
Grammars specified as rules that can be applied regardless of context.
permitted. @xref{Language and Grammar, ,Languages and Context-Free
Grammars}.
+@item Default reduction
+The reduction that a parser should perform if the current parser state
+contains no other action for the lookahead token. In permitted parser
+states, Bison declares the reduction with the largest lookahead set to be
+the default reduction and removes that lookahead set. @xref{Default
+Reductions}.
+
+@item Defaulted state
+A consistent state with a default reduction. @xref{Default Reductions}.
+
@item Dynamic allocation
Allocation of memory that occurs during execution, rather than at
compile time or on entry to a function.
machine moves from state to state as specified by the logic of the
machine. In the case of the parser, the input is the language being
parsed, and the states correspond to various stages in the grammar
-rules. @xref{Algorithm, ,The Bison Parser Algorithm }.
+rules. @xref{Algorithm, ,The Bison Parser Algorithm}.
+
+@item Generalized LR (GLR)
+A parsing algorithm that can handle all context-free grammars, including those
+that are not LR(1). It resolves situations that Bison's
+deterministic parsing
+algorithm cannot by effectively splitting off multiple parsers, trying all
+possible parsers, and discarding those that fail in the light of additional
+right context. @xref{Generalized LR Parsing, ,Generalized
+LR Parsing}.
@item Grouping
A language construct that is (in general) grammatically divisible;
-for example, `expression' or `declaration' in C.
+for example, `expression' or `declaration' in C@.
@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
+@item IELR(1) (Inadequacy Elimination LR(1))
+A minimal LR(1) parser table construction algorithm. That is, given any
+context-free grammar, IELR(1) generates parser tables with the full
+language-recognition power of canonical LR(1) but with nearly the same
+number of parser states as LALR(1). This reduction in parser states is
+often an order of magnitude. More importantly, because canonical LR(1)'s
+extra parser states may contain duplicate conflicts in the case of non-LR(1)
+grammars, the number of conflicts for IELR(1) is often an order of magnitude
+less as well. This can significantly reduce the complexity of developing a
+grammar. @xref{LR Table Construction}.
+
@item Infix operator
An arithmetic operator that is placed between the operands on which it
performs some operation.
@item Input stream
A continuous flow of data between devices or programs.
+@item LAC (Lookahead Correction)
+A parsing mechanism that fixes the problem of delayed syntax error
+detection, which is caused by LR state merging, default reductions, and the
+use of @code{%nonassoc}. Delayed syntax error detection results in
+unexpected semantic actions, initiation of error recovery in the wrong
+syntactic context, and an incorrect list of expected tokens in a verbose
+syntax error message. @xref{LAC}.
+
@item Language construct
One of the typical usage schemas of the language. For example, one of
the constructs of the C language is the @code{if} statement.
@item Left-to-right parsing
Parsing a sentence of a language by analyzing it token by token from
-left to right. @xref{Algorithm, ,The Bison Parser Algorithm }.
+left to right. @xref{Algorithm, ,The Bison Parser Algorithm}.
@item Lexical analyzer (scanner)
A function that reads an input stream and returns tokens one by one.
@item Literal string token
A token which consists of two or more fixed characters. @xref{Symbols}.
-@item Look-ahead token
-A token already read but not yet shifted. @xref{Look-Ahead, ,Look-Ahead
+@item Lookahead token
+A token already read but not yet shifted. @xref{Lookahead, ,Lookahead
Tokens}.
@item LALR(1)
The class of context-free grammars that Bison (like most other parser
-generators) can handle; a subset of LR(1). @xref{Mystery Conflicts, ,
-Mysterious Reduce/Reduce Conflicts}.
+generators) can handle by default; a subset of LR(1).
+@xref{Mysterious Conflicts}.
@item LR(1)
The class of context-free grammars in which at most one token of
-look-ahead is needed to disambiguate the parsing of any piece of input.
+lookahead is needed to disambiguate the parsing of any piece of input.
@item Nonterminal symbol
A grammar symbol standing for a grammatical construct that can
be expressed through rules in terms of smaller constructs; in other
words, a construct that is not a token. @xref{Symbols}.
-@item Parse error
-An error encountered during parsing of an input stream due to invalid
-syntax. @xref{Error Recovery}.
-
@item Parser
A function that recognizes valid sentences of a language by analyzing
the syntax structure of a set of tokens passed to it from a lexical
@item Reduction
Replacing a string of nonterminals and/or terminals with a single
nonterminal, according to a grammar rule. @xref{Algorithm, ,The Bison
-Parser Algorithm }.
+Parser Algorithm}.
@item Reentrant
A reentrant subprogram is a subprogram which can be in invoked any
@item Shift
A parser is said to shift when it makes the choice of analyzing
further input from the stream rather than reducing immediately some
-already-recognized rule. @xref{Algorithm, ,The Bison Parser Algorithm }.
+already-recognized rule. @xref{Algorithm, ,The Bison Parser Algorithm}.
@item Single-character literal
A single character that is recognized and interpreted as is.
during parsing to allow for recognition and use of existing
information in repeated uses of a symbol. @xref{Multi-function Calc}.
+@item Syntax error
+An error encountered during parsing of an input stream due to invalid
+syntax. @xref{Error Recovery}.
+
@item Token
A basic, grammatically indivisible unit of a language. The symbol
that describes a token in the grammar is a terminal symbol.
A grammar symbol that has no rules in the grammar and therefore is
grammatically indivisible. The piece of text it represents is a token.
@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
+
+@item Unreachable state
+A parser state to which there does not exist a sequence of transitions from
+the parser's start state. A state can become unreachable during conflict
+resolution. @xref{Unreachable States}.
@end table
@node Copying This Manual
@appendix Copying This Manual
+@include fdl.texi
-@menu
-* GNU Free Documentation License:: License for copying this manual.
-@end menu
+@node Bibliography
+@unnumbered Bibliography
-@include fdl.texi
+@table @asis
+@item [Denny 2008]
+Joel E. Denny and Brian A. Malloy, IELR(1): Practical LR(1) Parser Tables
+for Non-LR(1) Grammars with Conflict Resolution, in @cite{Proceedings of the
+2008 ACM Symposium on Applied Computing} (SAC'08), ACM, New York, NY, USA,
+pp.@: 240--245. @uref{http://dx.doi.org/10.1145/1363686.1363747}
+
+@item [Denny 2010 May]
+Joel E. Denny, PSLR(1): Pseudo-Scannerless Minimal LR(1) for the
+Deterministic Parsing of Composite Languages, Ph.D. Dissertation, Clemson
+University, Clemson, SC, USA (May 2010).
+@uref{http://proquest.umi.com/pqdlink?did=2041473591&Fmt=7&clientId=79356&RQT=309&VName=PQD}
+
+@item [Denny 2010 November]
+Joel E. Denny and Brian A. Malloy, The IELR(1) Algorithm for Generating
+Minimal LR(1) Parser Tables for Non-LR(1) Grammars with Conflict Resolution,
+in @cite{Science of Computer Programming}, Vol.@: 75, Issue 11 (November
+2010), pp.@: 943--979. @uref{http://dx.doi.org/10.1016/j.scico.2009.08.001}
+
+@item [DeRemer 1982]
+Frank DeRemer and Thomas Pennello, Efficient Computation of LALR(1)
+Look-Ahead Sets, in @cite{ACM Transactions on Programming Languages and
+Systems}, Vol.@: 4, No.@: 4 (October 1982), pp.@:
+615--649. @uref{http://dx.doi.org/10.1145/69622.357187}
+
+@item [Knuth 1965]
+Donald E. Knuth, On the Translation of Languages from Left to Right, in
+@cite{Information and Control}, Vol.@: 8, Issue 6 (December 1965), pp.@:
+607--639. @uref{http://dx.doi.org/10.1016/S0019-9958(65)90426-2}
+
+@item [Scott 2000]
+Elizabeth Scott, Adrian Johnstone, and Shamsa Sadaf Hussain,
+@cite{Tomita-Style Generalised LR Parsers}, Royal Holloway, University of
+London, Department of Computer Science, TR-00-12 (December 2000).
+@uref{http://www.cs.rhul.ac.uk/research/languages/publications/tomita_style_1.ps}
+@end table
@node Index
@unnumbered Index
@printindex cp
@bye
+
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projects / bison.git / blobdiff