+++ /dev/null
-\input texinfo @c -*-texinfo-*-
-@comment %**start of header
-@setfilename bison.info
-@include version.texi
-@settitle Bison @value{VERSION}
-@setchapternewpage odd
-
-@finalout
-
-@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 want to document %default-prec and %no-default-prec.
-@c This feature is experimental and may change in future Bison versions.
-@c @set defaultprec
-
-@ifnotinfo
-@syncodeindex fn cp
-@syncodeindex vr cp
-@syncodeindex tp cp
-@end ifnotinfo
-@ifinfo
-@synindex fn cp
-@synindex vr cp
-@synindex tp cp
-@end ifinfo
-@comment %**end of header
-
-@copying
-
-This manual (@value{UPDATED}) is for GNU Bison (version
-@value{VERSION}), the GNU parser generator.
-
-Copyright @copyright{} 1988-1993, 1995, 1998-2012 Free Software
-Foundation, Inc.
-
-@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
-
-@dircategory Software development
-@direntry
-* bison: (bison). GNU parser generator (Yacc replacement).
-@end direntry
-
-@titlepage
-@title Bison
-@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
-@insertcopying
-@sp 2
-Published by the Free Software Foundation @*
-51 Franklin Street, Fifth Floor @*
-Boston, MA 02110-1301 USA @*
-Printed copies are available from the Free Software Foundation.@*
-ISBN 1-882114-44-2
-@sp 2
-Cover art by Etienne Suvasa.
-@end titlepage
-
-@contents
-
-@ifnottex
-@node Top
-@top Bison
-@insertcopying
-@end ifnottex
-
-@menu
-* Introduction::
-* Conditions::
-* 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.
-
-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.
-* Context Dependency:: What to do if your language syntax is too
- 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.
-* 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:: Deferred semantic actions have special concerns.
-* 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.
-* 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.
-
-Reverse Polish Notation Calculator
-
-* 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::
-
-Location Tracking Calculator: @code{ltcalc}
-
-* 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}
-
-* Mfcalc Declarations:: Bison declarations for multi-function calculator.
-* Mfcalc Rules:: Grammar rules for the calculator.
-* Mfcalc Symbol Table:: Symbol table management subroutines.
-
-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.
-* 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.
-* 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
-
-* Value Type:: Specifying one data type for all semantic values.
-* Multiple Types:: Specifying several alternative data types.
-* Actions:: An action is the semantic definition of a grammar rule.
-* Action Types:: Specifying data types for actions to operate on.
-* Mid-Rule Actions:: Most actions go at the end of a rule.
- 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.
-* Initial Action Decl:: Code run before parsing starts.
-* Destructor Decl:: Declaring how symbols are freed.
-* Printer Decl:: Declaring how symbol values are displayed.
-* 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.
-* 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 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
-
-* 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.
-* 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.
-* 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.
-* Lexical Tie-ins:: Token parsing can depend on the syntactic 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.
-
-Tracing Your Parser
-
-* Enabling Traces:: Activating run-time trace support
-* Mfcalc Traces:: Extending @code{mfcalc} to support traces
-* The YYPRINT Macro:: Obsolete interface for semantic value reports
-
-Invoking Bison
-
-* Bison Options:: All the options described in detail,
- in alphabetical order by short options.
-* Option Cross Key:: Alphabetical list of long options.
-* 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
-
-* Copying This Manual:: License for copying this manual.
-
-@end detailmenu
-@end menu
-
-@node Introduction
-@unnumbered Introduction
-@cindex introduction
-
-@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
-
-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
-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 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
-make software proprietary. @strong{Software should be free.} But we
-concluded that limiting Bison's use to free software was doing little to
-encourage people to make other software free. So we decided to make the
-practical conditions for using Bison match the practical conditions for
-using the other GNU tools.
-
-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
-
-This chapter introduces many of the basic concepts without which the
-details of Bison will not make sense. If you do not already know how to
-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.
-* 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
-@section Languages and Context-Free Grammars
-
-@cindex context-free grammar
-@cindex grammar, context-free
-In order for Bison to parse a language, it must be described by a
-@dfn{context-free grammar}. This means that you specify one or more
-@dfn{syntactic groupings} and give rules for constructing them from their
-parts. For example, in the C language, one kind of grouping is called an
-`expression'. One rule for making an expression might be, ``An expression
-can be made of a minus sign and another expression''. Another would be,
-``An expression can be an integer''. As you can see, rules are often
-recursive, but there must be at least one rule which leads out of the
-recursion.
-
-@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.
-
-@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
-@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}.
-
-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
-lexicography, not grammar.)
-
-Here is a simple C function subdivided into tokens:
-
-@example
-int /* @r{keyword `int'} */
-square (int x) /* @r{identifier, open-paren, keyword `int',}
- @r{identifier, close-paren} */
-@{ /* @r{open-brace} */
- return x * x; /* @r{keyword `return', identifier, asterisk,}
- @r{identifier, semicolon} */
-@} /* @r{close-brace} */
-@end example
-
-The syntactic groupings of C include the expression, the statement, the
-declaration, and the function definition. These are represented in the
-grammar of C by nonterminal symbols `expression', `statement',
-`declaration' and `function definition'. The full grammar uses dozens of
-additional language constructs, each with its own nonterminal symbol, in
-order to express the meanings of these four. The example above is a
-function definition; it contains one declaration, and one statement. In
-the statement, each @samp{x} is an expression and so is @samp{x * x}.
-
-Each nonterminal symbol must have grammatical rules showing how it is made
-out of simpler constructs. For example, one kind of C statement is the
-@code{return} statement; this would be described with a grammar rule which
-reads informally as follows:
-
-@quotation
-A `statement' can be made of a `return' keyword, an `expression' and a
-`semicolon'.
-@end quotation
-
-@noindent
-There would be many other rules for `statement', one for each kind of
-statement in C.
-
-@cindex start symbol
-One nonterminal symbol must be distinguished as the special one which
-defines a complete utterance in the language. It is called the @dfn{start
-symbol}. In a compiler, this means a complete input program. In the C
-language, the nonterminal symbol `sequence of definitions and declarations'
-plays this role.
-
-For example, @samp{1 + 2} is a valid C expression---a valid part of a C
-program---but it is not valid as an @emph{entire} C program. In the
-context-free grammar of C, this follows from the fact that `expression' is
-not the start symbol.
-
-The Bison parser reads a sequence of tokens as its input, and groups the
-tokens using the grammar rules. If the input is valid, the end result is
-that the entire token sequence reduces to a single grouping whose symbol is
-the grammar's start symbol. If we use a grammar for C, the entire input
-must be a `sequence of definitions and declarations'. If not, the parser
-reports a syntax error.
-
-@node Grammar in Bison
-@section From Formal Rules to Bison Input
-@cindex Bison grammar
-@cindex grammar, Bison
-@cindex formal grammar
-
-A formal grammar is a mathematical construct. To define the language
-for Bison, you must write a file expressing the grammar in Bison syntax:
-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
-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
-type}. Token types as well can be represented as C-like identifiers. By
-convention, these identifiers should be upper case to distinguish them from
-nonterminals: for example, @code{INTEGER}, @code{IDENTIFIER}, @code{IF} or
-@code{RETURN}. A terminal symbol that stands for a particular keyword in
-the language should be named after that keyword converted to upper case.
-The terminal symbol @code{error} is reserved for error recovery.
-@xref{Symbols}.
-
-A terminal symbol can also be represented as a character literal, just like
-a C character constant. You should do this whenever a token is just a
-single character (parenthesis, plus-sign, etc.): use that same character in
-a literal as the terminal symbol for that token.
-
-A third way to represent a terminal symbol is with a C string constant
-containing several characters. @xref{Symbols}, for more information.
-
-The grammar rules also have an expression in Bison syntax. For example,
-here is the Bison rule for a C @code{return} statement. The semicolon in
-quotes is a literal character token, representing part of the C syntax for
-the statement; the naked semicolon, and the colon, are Bison punctuation
-used in every rule.
-
-@example
-stmt: RETURN expr ';' ;
-@end example
-
-@noindent
-@xref{Rules, ,Syntax of Grammar Rules}.
-
-@node Semantic Values
-@section Semantic Values
-@cindex semantic value
-@cindex value, semantic
-
-A formal grammar selects tokens only by their classifications: for example,
-if a rule mentions the terminal symbol `integer constant', it means that
-@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.
-
-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},
-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.
-
-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
-identifier. (A token such as @code{','} which is just punctuation doesn't
-need to have any semantic value.)
-
-For example, an input token might be classified as token type
-@code{INTEGER} and have the semantic value 4. Another input token might
-have the same token type @code{INTEGER} but value 3989. When a grammar
-rule says that @code{INTEGER} is allowed, either of these tokens is
-acceptable because each is an @code{INTEGER}. When the parser accepts the
-token, it keeps track of the token's semantic value.
-
-Each grouping can also have a semantic value as well as its nonterminal
-symbol. For example, in a calculator, an expression typically has a
-semantic value that is a number. In a compiler for a programming
-language, an expression typically has a semantic value that is a tree
-structure describing the meaning of the expression.
-
-@node Semantic Actions
-@section Semantic Actions
-@cindex semantic actions
-@cindex actions, semantic
-
-In order to be useful, a program must do more than parse input; it must
-also produce some output based on the input. In a Bison grammar, a grammar
-rule can have an @dfn{action} made up of C statements. Each time the
-parser recognizes a match for that rule, the action is executed.
-@xref{Actions}.
-
-Most of the time, the purpose of an action is to compute the semantic value
-of the whole construct from the semantic values of its parts. For example,
-suppose we have a rule which says an expression can be the sum of two
-expressions. When the parser recognizes such a sum, each of the
-subexpressions has a semantic value which describes how it was built up.
-The action for this rule should create a similar sort of value for the
-newly recognized larger expression.
-
-For example, here is a rule that says an expression can be the sum of
-two subexpressions:
-
-@example
-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 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:: Deferred semantic actions have special concerns.
-* 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
-
-@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}.
-
-@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.
-In a deferred semantic action, its effect is undefined.
-@c The effect is probably a syntax error at the split point.
-
-Also, see @ref{Location Default Action, ,Default Action for Locations}, which
-describes a special usage of @code{YYLLOC_DEFAULT} in GLR parsers.
-
-@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 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 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{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
-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
-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
-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 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
-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 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 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
-@cindex stages in using Bison
-@cindex using Bison
-
-The actual language-design process using Bison, from grammar specification
-to a working compiler or interpreter, has these parts:
-
-@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.
-
-@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.
-
-@item
-Write a controlling function that calls the Bison-produced parser.
-
-@item
-Write error-reporting routines.
-@end enumerate
-
-To turn this source code as written into a runnable program, you
-must follow these steps:
-
-@enumerate
-@item
-Run Bison on the grammar to produce the parser.
-
-@item
-Compile the code output by Bison, as well as any other source files.
-
-@item
-Link the object files to produce the finished product.
-@end enumerate
-
-@node Grammar Layout
-@section The Overall Layout of a Bison Grammar
-@cindex grammar file
-@cindex file format
-@cindex format of grammar file
-@cindex layout of Bison grammar
-
-The input file for the Bison utility is a @dfn{Bison grammar file}. The
-general form of a Bison grammar file is as follows:
-
-@example
-%@{
-@var{Prologue}
-%@}
-
-@var{Bison declarations}
-
-%%
-@var{Grammar rules}
-%%
-@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
-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
-semantic values of various symbols.
-
-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
-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 several sample programs written using Bison: a
-reverse polish notation calculator, an algebraic (infix) notation
-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. 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.
-* 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.
-@end menu
-
-@node RPN Calc
-@section Reverse Polish Notation Calculator
-@cindex reverse polish notation
-@cindex polish notation calculator
-@cindex @code{rpcalc}
-@cindex calculator, simple
-
-The first example is that of a simple double-precision @dfn{reverse polish
-notation} calculator (a calculator using postfix operators). This example
-provides a good starting point, since operator precedence is not an issue.
-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 grammar files.
-
-@menu
-* 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 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{}*/}.
-
-@example
-/* Reverse polish notation calculator. */
-
-%@{
- #define YYSTYPE double
- #include <math.h>
- int yylex (void);
- void yyerror (char const *);
-%@}
-
-%token NUM
-
-%% /* Grammar rules and actions follow. */
-@end example
-
-The declarations section (@pxref{Prologue, , The prologue}) contains two
-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
-groupings (@pxref{Value Type, ,Data Types of Semantic Values}). The
-Bison parser will use whatever type @code{YYSTYPE} is defined as; if you
-don't define it, @code{int} is the default. Because we specify
-@code{double}, each token and each expression has an associated value,
-which is a floating point number.
-
-The @code{#include} directive is used to declare the exponentiation
-function @code{pow}.
-
-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
-type for numeric constants.
-
-@node Rpcalc Rules
-@subsection Grammar Rules for @code{rpcalc}
-
-Here are the grammar rules for the reverse polish notation calculator.
-
-@example
-@group
-input:
- /* empty */
-| input line
-;
-@end group
-
-@group
-line:
- '\n'
-| exp '\n' @{ printf ("%.10g\n", $1); @}
-;
-@end group
-
-@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' @{ $$ = -$1; @} /* Unary minus */
-;
-@end group
-%%
-@end example
-
-The groupings of the rpcalc ``language'' defined here are the expression
-(given the name @code{exp}), the line of input (@code{line}), and the
-complete input transcript (@code{input}). Each of these nonterminal
-symbols has several alternate rules, joined by the vertical bar @samp{|}
-which is read as ``or''. The following sections explain what these rules
-mean.
-
-The semantics of the language is determined by the actions taken when a
-grouping is recognized. The actions are the C code that appears inside
-braces. @xref{Actions}.
-
-You must specify these actions in C, but Bison provides the means for
-passing semantic values between the rules. In each action, the
-pseudo-variable @code{$$} stands for the semantic value for the grouping
-that the rule is going to construct. Assigning a value to @code{$$} is the
-main job of most actions. The semantic values of the components of the
-rule are referred to as @code{$1}, @code{$2}, and so on.
-
-@menu
-* Rpcalc Input::
-* Rpcalc Line::
-* Rpcalc Expr::
-@end menu
-
-@node Rpcalc Input
-@subsubsection Explanation of @code{input}
-
-Consider the definition of @code{input}:
-
-@example
-input:
- /* empty */
-| input line
-;
-@end example
-
-This definition reads as follows: ``A complete input is either an empty
-string, or a complete input followed by an input line''. Notice that
-``complete input'' is defined in terms of itself. This definition is said
-to be @dfn{left recursive} since @code{input} appears always as the
-leftmost symbol in the sequence. @xref{Recursion, ,Recursive Rules}.
-
-The first alternative is empty because there are no symbols between the
-colon and the first @samp{|}; this means that @code{input} can match an
-empty string of input (no tokens). We write the rules this way because it
-is legitimate to type @kbd{Ctrl-d} right after you start the calculator.
-It's conventional to put an empty alternative first and write the comment
-@samp{/* empty */} in it.
-
-The second alternate rule (@code{input line}) handles all nontrivial input.
-It means, ``After reading any number of lines, read one more line if
-possible.'' The left recursion makes this rule into a loop. Since the
-first alternative matches empty input, the loop can be executed zero or
-more times.
-
-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-input.
-
-@node Rpcalc Line
-@subsubsection Explanation of @code{line}
-
-Now consider the definition of @code{line}:
-
-@example
-line:
- '\n'
-| exp '\n' @{ printf ("%.10g\n", $1); @}
-;
-@end example
-
-The first alternative is a token which is a newline character; this means
-that rpcalc accepts a blank line (and ignores it, since there is no
-action). The second alternative is an expression followed by a newline.
-This is the alternative that makes rpcalc useful. The semantic value of
-the @code{exp} grouping is the value of @code{$1} because the @code{exp} in
-question is the first symbol in the alternative. The action prints this
-value, which is the result of the computation the user asked for.
-
-This action is unusual because it does not assign a value to @code{$$}. As
-a consequence, the semantic value associated with the @code{line} is
-uninitialized (its value will be unpredictable). This would be a bug if
-that value were ever used, but we don't use it: once rpcalc has printed the
-value of the user's input line, that value is no longer needed.
-
-@node Rpcalc Expr
-@subsubsection Explanation of @code{expr}
-
-The @code{exp} grouping has several rules, one for each kind of expression.
-The first rule handles the simplest expressions: those that are just numbers.
-The second handles an addition-expression, which looks like two expressions
-followed by a plus-sign. The third handles subtraction, and so on.
-
-@example
-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{}
-@end example
-
-Most of the rules have actions that compute the value of the expression in
-terms of the value of its parts. For example, in the rule for addition,
-@code{$1} refers to the first component @code{exp} and @code{$2} refers to
-the second one. The third component, @code{'+'}, has no meaningful
-associated semantic value, but if it had one you could refer to it as
-@code{$3}. When @code{yyparse} recognizes a sum expression using this
-rule, the sum of the two subexpressions' values is produced as the value of
-the entire expression. @xref{Actions}.
-
-You don't have to give an action for every rule. When a rule has no
-action, Bison by default copies the value of @code{$1} into @code{$$}.
-This is what happens in the first rule (the one that uses @code{NUM}).
-
-The formatting shown here is the recommended convention, but Bison does
-not require it. You can add or change white space as much as you wish.
-For example, this:
-
-@example
-exp: NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{} ;
-@end example
-
-@noindent
-means the same thing as this:
-
-@example
-exp:
- NUM
-| exp exp '+' @{ $$ = $1 + $2; @}
-| @dots{}
-;
-@end example
-
-@noindent
-The latter, however, is much more readable.
-
-@node Rpcalc Lexer
-@subsection The @code{rpcalc} Lexical Analyzer
-@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}}.
-
-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
-for such a single-character token is the character itself.
-
-The return value of the lexical analyzer function is a numeric code which
-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 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,
-therefore, @code{NUM} becomes a macro for @code{yylex} to use.
-
-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 Declarations,
-,Declarations for @code{rpcalc}}.)
-
-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:
-
-@example
-@group
-/* 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
-
-@group
-int
-yylex (void)
-@{
- int c;
-
- /* Skip white space. */
- while ((c = getchar ()) == ' ' || c == '\t')
- continue;
-@end group
-@group
- /* Process numbers. */
- if (c == '.' || isdigit (c))
- @{
- ungetc (c, stdin);
- scanf ("%lf", &yylval);
- return NUM;
- @}
-@end group
-@group
- /* Return end-of-input. */
- if (c == EOF)
- return 0;
- /* Return a single char. */
- return c;
-@}
-@end group
-@end example
-
-@node Rpcalc Main
-@subsection The Controlling Function
-@cindex controlling function
-@cindex main function in simple example
-
-In keeping with the spirit of this example, the controlling function is
-kept to the bare minimum. The only requirement is that it call
-@code{yyparse} to start the process of parsing.
-
-@example
-@group
-int
-main (void)
-@{
- return yyparse ();
-@}
-@end group
-@end example
-
-@node Rpcalc Error
-@subsection The Error Reporting Routine
-@cindex error reporting routine
-
-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{"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:
-
-@example
-@group
-#include <stdio.h>
-@end group
-
-@group
-/* Called by yyparse on error. */
-void
-yyerror (char const *s)
-@{
- fprintf (stderr, "%s\n", s);
-@}
-@end group
-@end example
-
-After @code{yyerror} returns, the Bison parser may recover from the error
-and continue parsing if the grammar contains a suitable error rule
-(@pxref{Error Recovery}). Otherwise, @code{yyparse} returns nonzero. We
-have not written any error rules in this example, so any invalid input will
-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 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 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 the grammar file, you use the following command
-to convert it into a parser implementation file:
-
-@example
-bison @var{file}.y
-@end example
-
-@noindent
-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 Implementation File
-@cindex compiling the parser
-
-Here is how to compile and run the parser implementation file:
-
-@example
-@group
-# @r{List files in current directory.}
-$ @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}.}
-$ @kbd{cc -lm -o rpcalc rpcalc.tab.c}
-@end group
-
-@group
-# @r{List files again.}
-$ @kbd{ls}
-rpcalc rpcalc.tab.c rpcalc.y
-@end group
-@end example
-
-The file @file{rpcalc} now contains the executable code. Here is an
-example session using @code{rpcalc}.
-
-@example
-$ @kbd{rpcalc}
-@kbd{4 9 +}
-13
-@kbd{3 7 + 3 4 5 *+-}
--13
-@kbd{3 7 + 3 4 5 * + - n} @r{Note the unary minus, @samp{n}}
-13
-@kbd{5 6 / 4 n +}
--3.166666667
-@kbd{3 4 ^} @r{Exponentiation}
-81
-@kbd{^D} @r{End-of-file indicator}
-$
-@end example
-
-@node Infix Calc
-@section Infix Notation Calculator: @code{calc}
-@cindex infix notation calculator
-@cindex @code{calc}
-@cindex calculator, infix notation
-
-We now modify rpcalc to handle infix operators instead of postfix. Infix
-notation involves the concept of operator precedence and the need for
-parentheses nested to arbitrary depth. Here is the Bison code for
-@file{calc.y}, an infix desk-top calculator.
-
-@example
-/* Infix notation calculator. */
-
-@group
-%@{
- #define YYSTYPE double
- #include <math.h>
- #include <stdio.h>
- int yylex (void);
- void yyerror (char const *);
-%@}
-@end group
-
-@group
-/* Bison declarations. */
-%token NUM
-%left '-' '+'
-%left '*' '/'
-%left NEG /* negation--unary minus */
-%right '^' /* exponentiation */
-@end group
-
-%% /* The grammar follows. */
-@group
-input:
- /* empty */
-| input line
-;
-@end group
-
-@group
-line:
- '\n'
-| exp '\n' @{ printf ("\t%.10g\n", $1); @}
-;
-@end group
-
-@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
-
-@noindent
-The functions @code{yylex}, @code{yyerror} and @code{main} can be the
-same as before.
-
-There are two important new features shown in this code.
-
-In the second section (Bison declarations), @code{%left} declares token
-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
-ordinarily don't need to be declared. We declare them here to specify
-the associativity.)
-
-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}.
-
-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
-$ @kbd{calc}
-@kbd{4 + 4.5 - (34/(8*3+-3))}
-6.880952381
-@kbd{-56 + 2}
--54
-@kbd{3 ^ 2}
-9
-@end example
-
-@node Simple Error Recovery
-@section Simple Error Recovery
-@cindex error recovery, simple
-
-Up to this point, this manual has not addressed the issue of @dfn{error
-recovery}---how to continue parsing after the parser detects a syntax
-error. All we have handled is error reporting with @code{yyerror}.
-Recall that by default @code{yyparse} returns after calling
-@code{yyerror}. This means that an erroneous input line causes the
-calculator program to exit. Now we show how to rectify this deficiency.
-
-The Bison language itself includes the reserved word @code{error}, which
-may be included in the grammar rules. In the example below it has
-been added to one of the alternatives for @code{line}:
-
-@example
-@group
-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 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.
-
-This form of error recovery deals with syntax errors. There are other
-kinds of errors; for example, division by zero, which raises an exception
-signal that is normally fatal. A real calculator program must handle this
-signal and use @code{longjmp} to return to @code{main} and resume parsing
-input lines; it would also have to discard the rest of the current line of
-input. We won't discuss this issue further because it is not specific to
-Bison programs.
-
-@node Location Tracking Calc
-@section Location Tracking Calculator: @code{ltcalc}
-@cindex location tracking calculator
-@cindex @code{ltcalc}
-@cindex calculator, location tracking
-
-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
-* 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 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.
-
-@example
-/* Location tracking calculator. */
-
-%@{
- #define YYSTYPE int
- #include <math.h>
- int yylex (void);
- void yyerror (char const *);
-%@}
-
-/* Bison declarations. */
-%token NUM
-
-%left '-' '+'
-%left '*' '/'
-%left NEG
-%right '^'
-
-%% /* The grammar follows. */
-@end example
-
-@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 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.
-
-@example
-@group
-input:
- /* empty */
-| input line
-;
-@end group
-
-@group
-line:
- '\n'
-| exp '\n' @{ printf ("%d\n", $1); @}
-;
-@end group
-
-@group
-exp:
- NUM @{ $$ = $1; @}
-| exp '+' exp @{ $$ = $1 + $3; @}
-| exp '-' exp @{ $$ = $1 - $3; @}
-| exp '*' exp @{ $$ = $1 * $3; @}
-@end group
-@group
-| exp '/' exp
- @{
- if ($3)
- $$ = $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 @{ $$ = pow ($1, $3); @}
-| '(' exp ')' @{ $$ = $2; @}
-@end group
-@end example
-
-This code shows how to reach locations inside of semantic actions, by
-using the pseudo-variables @code{@@@var{n}} for rule components, and the
-pseudo-variable @code{@@$} for groupings.
-
-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 analyzer, and make it
-able to feed the parser with the token locations, as it already does for
-semantic values.
-
-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
-int
-yylex (void)
-@{
- int c;
-@end group
-
-@group
- /* Skip white space. */
- while ((c = getchar ()) == ' ' || c == '\t')
- ++yylloc.last_column;
-@end group
-
-@group
- /* Step. */
- yylloc.first_line = yylloc.last_line;
- yylloc.first_column = yylloc.last_column;
-@end group
-
-@group
- /* Process numbers. */
- if (isdigit (c))
- @{
- yylval = c - '0';
- ++yylloc.last_column;
- while (isdigit (c = getchar ()))
- @{
- ++yylloc.last_column;
- yylval = yylval * 10 + c - '0';
- @}
- ungetc (c, stdin);
- return NUM;
- @}
-@end group
-
- /* Return end-of-input. */
- if (c == EOF)
- return 0;
-
-@group
- /* Return a single char, and update location. */
- if (c == '\n')
- @{
- ++yylloc.last_line;
- yylloc.last_column = 0;
- @}
- else
- ++yylloc.last_column;
- return c;
-@}
-@end group
-@end example
-
-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 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_line = yylloc.last_line = 1;
- 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.
-
-@node Multi-function Calc
-@section Multi-Function Calculator: @code{mfcalc}
-@cindex multi-function calculator
-@cindex @code{mfcalc}
-@cindex calculator, multi-function
-
-Now that the basics of Bison have been discussed, it is time to move on to
-a more advanced problem. The above calculators provided only five
-functions, @samp{+}, @samp{-}, @samp{*}, @samp{/} and @samp{^}. It would
-be nice to have a calculator that provides other mathematical functions such
-as @code{sin}, @code{cos}, etc.
-
-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 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:
-
-@example
-@var{function_name} (@var{argument})
-@end example
-
-@noindent
-At the same time, we will add memory to the calculator, by allowing you
-to create named variables, store values in them, and use them later.
-Here is a sample session with the multi-function calculator:
-
-@example
-$ @kbd{mfcalc}
-@kbd{pi = 3.141592653589}
-3.1415926536
-@kbd{sin(pi)}
-0.0000000000
-@kbd{alpha = beta1 = 2.3}
-2.3000000000
-@kbd{alpha}
-2.3000000000
-@kbd{ln(alpha)}
-0.8329091229
-@kbd{exp(ln(beta1))}
-2.3000000000
-$
-@end example
-
-Note that multiple assignment and nested function calls are permitted.
-
-@menu
-* Mfcalc Declarations:: Bison declarations for multi-function calculator.
-* Mfcalc Rules:: Grammar rules for the calculator.
-* Mfcalc Symbol Table:: Symbol table management subroutines.
-@end menu
-
-@node Mfcalc Declarations
-@subsection Declarations for @code{mfcalc}
-
-Here are the C and Bison declarations for the multi-function calculator.
-
-@comment file: mfcalc.y: 1
-@example
-@group
-%@{
- #include <math.h> /* For math functions, cos(), sin(), etc. */
- #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. */
-@}
-@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 */
-@end group
-@end example
-
-The above grammar introduces only two new features of the Bison language.
-These features allow semantic values to have various data types
-(@pxref{Multiple Types, ,More Than One Value Type}).
-
-The @code{%union} declaration specifies the entire list of possible types;
-this is instead of defining @code{YYSTYPE}. The allowable types are now
-double-floats (for @code{exp} and @code{NUM}) and pointers to entries in
-the symbol table. @xref{Union Decl, ,The Collection of Value Types}.
-
-Since values can now have various types, it is necessary to associate a
-type with each grammar symbol whose semantic value is used. These symbols
-are @code{NUM}, @code{VAR}, @code{FNCT}, and @code{exp}. Their
-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}.
-
-@node Mfcalc Rules
-@subsection Grammar Rules for @code{mfcalc}
-
-Here are the grammar rules for the multi-function calculator.
-Most of them are copied directly from @code{calc}; three rules,
-those which mention @code{VAR} or @code{FNCT}, are new.
-
-@comment file: mfcalc.y: 3
-@example
-%% /* The grammar follows. */
-@group
-input:
- /* empty */
-| input line
-;
-@end group
-
-@group
-line:
- '\n'
-| exp '\n' @{ printf ("%.10g\n", $1); @}
-| error '\n' @{ yyerrok; @}
-;
-@end group
-
-@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 group
-/* End of grammar. */
-%%
-@end example
-
-@node Mfcalc Symbol Table
-@subsection The @code{mfcalc} Symbol Table
-@cindex symbol table example
-
-The multi-function calculator requires a symbol table to keep track of the
-names and meanings of variables and functions. This doesn't affect the
-grammar rules (except for the actions) or the Bison declarations, but it
-requires some additional C functions for support.
-
-The symbol table itself consists of a linked list of records. Its
-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.
-
-@comment file: calc.h
-@example
-@group
-/* Function type. */
-typedef double (*func_t) (double);
-@end group
-
-@group
-/* Data type for links in the chain of symbols. */
-struct symrec
-@{
- 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 */
- @} value;
- struct symrec *next; /* link field */
-@};
-@end group
-
-@group
-typedef struct symrec symrec;
-
-/* The symbol table: a chain of `struct symrec'. */
-extern symrec *sym_table;
-
-symrec *putsym (char const *, int);
-symrec *getsym (char const *);
-@end group
-@end example
-
-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:
-
-@comment file: mfcalc.y: 3
-@example
-#include <stdio.h>
-
-@group
-/* Called by yyparse on error. */
-void
-yyerror (char const *s)
-@{
- printf ("%s\n", s);
-@}
-@end group
-
-@group
-struct init
-@{
- char const *fname;
- double (*fnct) (double);
-@};
-@end group
-
-@group
-struct init const arith_fncts[] =
-@{
- "sin", sin,
- "cos", cos,
- "atan", atan,
- "ln", log,
- "exp", exp,
- "sqrt", sqrt,
- 0, 0
-@};
-@end group
-
-@group
-/* The symbol table: a chain of `struct symrec'. */
-symrec *sym_table;
-@end group
-
-@group
-/* Put arithmetic functions in table. */
-void
-init_table (void)
-@{
- int i;
- for (i = 0; arith_fncts[i].fname != 0; i++)
- @{
- symrec *ptr = putsym (arith_fncts[i].fname, FNCT);
- ptr->value.fnctptr = arith_fncts[i].fnct;
- @}
-@}
-@end group
-
-@group
-int
-main (void)
-@{
- init_table ();
- return yyparse ();
-@}
-@end group
-@end example
-
-By simply editing the initialization list and adding the necessary include
-files, you can add additional functions to the calculator.
-
-Two important functions allow look-up and installation of symbols in the
-symbol table. The function @code{putsym} is passed a name and the type
-(@code{VAR} or @code{FNCT}) of the object to be installed. The object is
-linked to the front of the list, and a pointer to the object is returned.
-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.
-
-@comment file: mfcalc.y: 3
-@example
-#include <stdlib.h> /* malloc. */
-#include <string.h> /* strlen. */
-
-@group
-symrec *
-putsym (char const *sym_name, int sym_type)
-@{
- 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->next = (struct symrec *)sym_table;
- sym_table = ptr;
- return ptr;
-@}
-@end group
-
-@group
-symrec *
-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)
- return ptr;
- return 0;
-@}
-@end group
-@end example
-
-The function @code{yylex} must now recognize variables, numeric values, and
-the single-character arithmetic operators. Strings of alphanumeric
-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
-the name appears in the table, a pointer to its location and its type
-(@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}.
-
-No change is needed in the handling of numeric values and arithmetic
-operators in @code{yylex}.
-
-@comment file: mfcalc.y: 3
-@example
-@group
-#include <ctype.h>
-@end group
-
-@group
-int
-yylex (void)
-@{
- int c;
-
- /* Ignore white space, get first nonwhite character. */
- while ((c = getchar ()) == ' ' || c == '\t')
- continue;
-
- if (c == EOF)
- return 0;
-@end group
-
-@group
- /* Char starts a number => parse the number. */
- if (c == '.' || isdigit (c))
- @{
- ungetc (c, stdin);
- scanf ("%lf", &yylval.val);
- return NUM;
- @}
-@end group
-
-@group
- /* Char starts an identifier => read the name. */
- if (isalpha (c))
- @{
- /* Initially make the buffer long enough
- for a 40-character symbol name. */
- static size_t length = 40;
- static char *symbuf = 0;
- symrec *s;
- int i;
-@end group
-
- if (!symbuf)
- symbuf = (char *) malloc (length + 1);
-
- i = 0;
- do
-@group
- @{
- /* If buffer is full, make it bigger. */
- if (i == length)
- @{
- length *= 2;
- symbuf = (char *) realloc (symbuf, length + 1);
- @}
- /* Add this character to the buffer. */
- symbuf[i++] = c;
- /* Get another character. */
- c = getchar ();
- @}
-@end group
-@group
- while (isalnum (c));
-
- ungetc (c, stdin);
- symbuf[i] = '\0';
-@end group
-
-@group
- s = getsym (symbuf);
- if (s == 0)
- s = putsym (symbuf, VAR);
- yylval.tptr = s;
- return s->type;
- @}
-
- /* Any other character is a token by itself. */
- return c;
-@}
-@end group
-@end example
-
-The error reporting function is unchanged, and the new version of
-@code{main} includes a call to @code{init_table} and sets the @code{yydebug}
-on user demand (@xref{Tracing, , Tracing Your Parser}, for details):
-
-@comment file: mfcalc.y: 3
-@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[])
-@{
- int i;
- /* Enable parse traces on option -p. */
- for (i = 1; i < argc; ++i)
- if (!strcmp(argv[i], "-p"))
- yydebug = 1;
- 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.
-
-@node Exercises
-@section Exercises
-@cindex exercises
-
-@enumerate
-@item
-Add some new functions from @file{math.h} to the initialization list.
-
-@item
-Add another array that contains constants and their values. Then
-modify @code{init_table} to add these constants to the symbol table.
-It will be easiest to give the constants type @code{VAR}.
-
-@item
-Make the program report an error if the user refers to an
-uninitialized variable in any way except to store a value in it.
-@end enumerate
-
-@node Grammar File
-@chapter Bison Grammar Files
-
-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 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.
-* 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
-@section Outline of a Bison Grammar
-
-A Bison grammar file has four main sections, shown here with the
-appropriate delimiters:
-
-@example
-%@{
- @var{Prologue}
-%@}
-
-@var{Bison declarations}
-
-%%
-@var{Grammar rules}
-%%
-
-@var{Epilogue}
-@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.
-* 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
-@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 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 (yyoutput, $$); @} <field1>
-@end group
-
-@group
-%code requires @{ #include "type2.h" @}
-%union @{ type2 field2; @}
-%destructor @{ type2_free ($$); @} <field2>
-%printer @{ type2_print (yyoutput, $$); @} <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
-@cindex Bison declarations (introduction)
-@cindex declarations, Bison (introduction)
-
-The @var{Bison declarations} section contains declarations that define
-terminal and nonterminal symbols, specify precedence, and so on.
-In some simple grammars you may not need any declarations.
-@xref{Declarations, ,Bison Declarations}.
-
-@node Grammar Rules
-@subsection The Grammar Rules Section
-@cindex grammar rules section
-@cindex rules section for grammar
-
-The @dfn{grammar rules} section contains one or more Bison grammar
-rules, and nothing else. @xref{Rules, ,Syntax of Grammar Rules}.
-
-There must always be at least one grammar rule, and the first
-@samp{%%} (which precedes the grammar rules) may never be omitted even
-if it is the first thing in the file.
-
-@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
-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 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
-@cindex nonterminal symbol
-@cindex terminal symbol
-@cindex token type
-@cindex symbol
-
-@dfn{Symbols} in Bison grammars represent the grammatical classifications
-of the language.
-
-A @dfn{terminal symbol} (also known as a @dfn{token type}) represents a
-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, 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
-such name must be defined with a Bison declaration such as
-@code{%token}. @xref{Token Decl, ,Token Type Names}.
-
-@item
-@cindex character token
-@cindex literal token
-@cindex single-character literal
-A @dfn{character token type} (or @dfn{literal character token}) is
-written in the grammar using the same syntax used in C for character
-constants; for example, @code{'+'} is a character token type. A
-character token type doesn't need to be declared unless you need to
-specify its semantic value data type (@pxref{Value Type, ,Data Types of
-Semantic Values}), associativity, or precedence (@pxref{Precedence,
-,Operator Precedence}).
-
-By convention, a character token type is used only to represent a
-token that consists of that particular character. Thus, the token
-type @code{'+'} is used to represent the character @samp{+} as a
-token. Nothing enforces this convention, but if you depart from it,
-your program will confuse other readers.
-
-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 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
-@cindex literal string token
-@cindex multicharacter literal
-A @dfn{literal string token} is written like a C string constant; for
-example, @code{"<="} is a literal string token. A literal string token
-doesn't need to be declared unless you need to specify its semantic
-value data type (@pxref{Value Type}), associativity, or precedence
-(@pxref{Precedence}).
-
-You can associate the literal string token with a symbolic name as an
-alias, using the @code{%token} declaration (@pxref{Token Decl, ,Token
-Declarations}). If you don't do that, the lexical analyzer has to
-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.
-
-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
-type @code{"<="} to represent the string @samp{<=} as a token. Bison
-does not enforce this convention, but if you depart from it, people who
-read your program will be confused.
-
-All the escape sequences used in string literals in C can be used in
-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, 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}
-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}.
-
-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. 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
-@cindex rule syntax
-@cindex grammar rule syntax
-@cindex syntax of grammar rules
-
-A Bison grammar rule has the following general form:
-
-@example
-@group
-@var{result}: @var{components}@dots{};
-@end group
-@end example
-
-@noindent
-where @var{result} is the nonterminal symbol that this rule describes,
-and @var{components} are various terminal and nonterminal symbols that
-are put together by this rule (@pxref{Symbols}).
-
-For example,
-
-@example
-@group
-exp: exp '+' exp;
-@end group
-@end example
-
-@noindent
-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}.
-
-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:
-
-@example
-@{@var{C statements}@}
-@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}.
-
-@findex |
-Multiple rules for the same @var{result} can be written separately or can
-be joined with the vertical-bar character @samp{|} as follows:
-
-@example
-@group
-@var{result}:
- @var{rule1-components}@dots{}
-| @var{rule2-components}@dots{}
-@dots{}
-;
-@end group
-@end example
-
-@noindent
-They are still considered distinct rules even when joined in this way.
-
-If @var{components} in a rule is empty, it means that @var{result} can
-match the empty string. For example, here is how to define a
-comma-separated sequence of zero or more @code{exp} groupings:
-
-@example
-@group
-expseq:
- /* empty */
-| expseq1
-;
-@end group
-
-@group
-expseq1:
- exp
-| expseq1 ',' exp
-;
-@end group
-@end example
-
-@noindent
-It is customary to write a comment @samp{/* empty */} in each rule
-with no components.
-
-@node Recursion
-@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
-comma-separated sequence of one or more expressions:
-
-@example
-@group
-expseq1:
- exp
-| expseq1 ',' exp
-;
-@end group
-@end example
-
-@cindex left recursion
-@cindex right recursion
-@noindent
-Since the recursive use of @code{expseq1} is the leftmost symbol in the
-right hand side, we call this @dfn{left recursion}. By contrast, here
-the same construct is defined using @dfn{right recursion}:
-
-@example
-@group
-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.
-
-@cindex mutual recursion
-@dfn{Indirect} or @dfn{mutual} recursion occurs when the result of the
-rule does not appear directly on its right hand side, but does appear
-in rules for other nonterminals which do appear on its right hand
-side.
-
-For example:
-
-@example
-@group
-expr:
- primary
-| primary '+' primary
-;
-@end group
-
-@group
-primary:
- constant
-| '(' expr ')'
-;
-@end group
-@end example
-
-@noindent
-defines two mutually-recursive nonterminals, since each refers to the
-other.
-
-@node Semantics
-@section Defining Language Semantics
-@cindex defining language semantics
-@cindex language semantics, defining
-
-The grammar rules for a language determine only the syntax. The semantics
-are determined by the semantic values associated with various tokens and
-groupings, and by the actions taken when various groupings are recognized.
-
-For example, the calculator calculates properly because the value
-associated with each expression is the proper number; it adds properly
-because the action for the grouping @w{@samp{@var{x} + @var{y}}} is to add
-the numbers associated with @var{x} and @var{y}.
-
-@menu
-* Value Type:: Specifying one data type for all semantic values.
-* Multiple Types:: Specifying several alternative data types.
-* Actions:: An action is the semantic definition of a grammar rule.
-* Action Types:: Specifying data types for actions to operate on.
-* Mid-Rule Actions:: Most actions go at the end of a rule.
- This says when, why and how to use the exceptional
- action in the middle of a rule.
-@end menu
-
-@node Value Type
-@subsection Data Types of Semantic Values
-@cindex semantic value type
-@cindex value type, semantic
-@cindex data types of semantic values
-@cindex default data type
-
-In a simple program it may be sufficient to use the same data type for
-the semantic values of all language constructs. This was true in the
-RPN and infix calculator examples (@pxref{RPN Calc, ,Reverse Polish
-Notation Calculator}).
-
-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
-#define YYSTYPE double
-@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}).
-
-@node Multiple Types
-@subsection More Than One Value Type
-
-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 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, 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
-which semantic values are used. This is done for tokens with the
-@code{%token} Bison declaration (@pxref{Token Decl, ,Token Type Names})
-and for groupings with the @code{%type} Bison declaration (@pxref{Type
-Decl, ,Nonterminal Symbols}).
-@end itemize
-
-@node Actions
-@subsection Actions
-@cindex action
-@vindex $$
-@vindex $@var{n}
-@vindex $@var{name}
-@vindex $[@var{name}]
-
-An action accompanies a syntactic rule and contains C code to be executed
-each time an instance of that rule is recognized. The task of most actions
-is to compute a semantic value for the grouping built by the rule from the
-semantic values associated with tokens or smaller groupings.
-
-An action consists of braced code containing C statements, and can be
-placed at any position in the rule;
-it is executed at that position. Most rules have just one action at the
-end of the rule, following all the components. Actions in the middle of
-a rule are tricky and used only for special purposes (@pxref{Mid-Rule
-Actions, ,Actions in Mid-Rule}).
-
-The C code in an action can refer to the semantic values of the
-components matched by the rule with the construct @code{$@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 @{ $$ = $1 + $3; @}
-@end group
-@end example
-
-Or, in terms of named references:
-
-@example
-@group
-exp[result]:
-@dots{}
-| exp[left] '+' exp[right] @{ $result = $left + $right; @}
-@end group
-@end example
-
-@noindent
-This rule constructs an @code{exp} from two smaller @code{exp} groupings
-connected by a plus-sign token. In the action, @code{$1} and @code{$3}
-(@code{$left} and @code{$right})
-refer to the semantic values of the two component @code{exp} groupings,
-which are the first and third symbols on the right hand side of the rule.
-The sum is stored into @code{$$} (@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}.
-
-@xref{Named References}, for more information about using the named
-references construct.
-
-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{$$ = $1}.} Thus, the value of the first symbol in the rule
-becomes the value of the whole rule. Of course, the default action is
-valid only if the two data types match. There is no meaningful default
-action for an empty rule; every empty rule must have an explicit action
-unless the rule's value does not matter.
-
-@code{$@var{n}} with @var{n} zero or negative is allowed for reference
-to tokens and groupings on the stack @emph{before} those that match the
-current rule. This is a very risky practice, and to use it reliably
-you must be certain of the context in which the rule is applied. Here
-is a case in which you can use this reliably:
-
-@example
-@group
-foo:
- expr bar '+' expr @{ @dots{} @}
-| expr bar '-' expr @{ @dots{} @}
-;
-@end group
-
-@group
-bar:
- /* empty */ @{ previous_expr = $0; @}
-;
-@end group
-@end example
-
-As long as @code{bar} is used only in the fashion shown here, @code{$0}
-always refers to the @code{expr} which precedes @code{bar} in the
-definition of @code{foo}.
-
-@vindex yylval
-It is also possible to access the semantic value of the lookahead token, if
-any, from a semantic action.
-This semantic value is stored in @code{yylval}.
-@xref{Action Features, ,Special Features for Use in Actions}.
-
-@node Action Types
-@subsection Data Types of Values in Actions
-@cindex action data types
-@cindex data types in actions
-
-If you have chosen a single data type for semantic values, the @code{$$}
-and @code{$@var{n}} constructs always have that data type.
-
-If you have used @code{%union} to specify a variety of data types, then you
-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{$$} or
-@code{$@var{n}}, its data type is determined by which symbol it refers to
-in the rule. In this example,
-
-@example
-@group
-exp:
- @dots{}
-| exp '+' exp @{ $$ = $1 + $3; @}
-@end group
-@end example
-
-@noindent
-@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{$2} were used, it would have the data type declared for the
-terminal symbol @code{'+'}, whatever that might be.
-
-Alternatively, you can specify the data type when you refer to the value,
-by inserting @samp{<@var{type}>} after the @samp{$} at the beginning of the
-reference. For example, if you have defined types as shown here:
-
-@example
-@group
-%union @{
- int itype;
- double dtype;
-@}
-@end group
-@end example
-
-@noindent
-then you can write @code{$<itype>1} to refer to the first subunit of the
-rule as an integer, or @code{$<dtype>1} to refer to it as a double.
-
-@node Mid-Rule Actions
-@subsection Actions in Mid-Rule
-@cindex actions in mid-rule
-@cindex mid-rule actions
-
-Occasionally it is useful to put an action in the middle of a rule.
-These actions are written just like usual end-of-rule actions, but they
-are executed before the parser even recognizes the following components.
-
-A mid-rule action may refer to the components preceding it using
-@code{$@var{n}}, but it may not refer to subsequent components because
-it is run before they are parsed.
-
-The mid-rule action itself counts as one of the components of the rule.
-This makes a difference when there is another action later in the same rule
-(and usually there is another at the end): you have to count the actions
-along with the symbols when working out which number @var{n} to use in
-@code{$@var{n}}.
-
-The mid-rule action can also have a semantic value. The action can set
-its value with an assignment to @code{$$}, and actions later in the rule
-can refer to the value using @code{$@var{n}}. Since there is no symbol
-to name the action, there is no way to declare a data type for the value
-in advance, so you must use the @samp{$<@dots{}>@var{n}} construct to
-specify a data type each time you refer to this value.
-
-There is no way to set the value of the entire rule with a mid-rule
-action, because assignments to @code{$$} do not have that effect. The
-only way to set the value for the entire rule is with an ordinary action
-at the end of the rule.
-
-Here is an example from a hypothetical compiler, handling a @code{let}
-statement that looks like @samp{let (@var{variable}) @var{statement}} and
-serves to create a variable named @var{variable} temporarily for the
-duration of @var{statement}. To parse this construct, we must put
-@var{variable} into the symbol table while @var{statement} is parsed, then
-remove it afterward. Here is how it is done:
-
-@example
-@group
-stmt:
- LET '(' var ')'
- @{ $<context>$ = push_context (); declare_variable ($3); @}
- stmt
- @{ $$ = $6; pop_context ($<context>5); @}
-@end group
-@end example
-
-@noindent
-As soon as @samp{let (@var{variable})} has been recognized, the first
-action is run. It saves a copy of the current semantic context (the
-list of accessible variables) as its semantic value, using alternative
-@code{context} in the data-type union. Then it calls
-@code{declare_variable} to add the new variable to that list. Once the
-first action is finished, the embedded statement @code{stmt} can be
-parsed. Note that the mid-rule action is component number 5, so the
-@samp{stmt} is component number 6.
-
-After the embedded statement is parsed, its semantic value becomes the
-value of the entire @code{let}-statement. Then the semantic value from the
-earlier action is used to restore the prior list of variables. This
-removes the temporary @code{let}-variable from the list so that it won't
-appear to exist while the rest of the program is parsed.
-
-@findex %destructor
-@cindex discarded symbols, mid-rule actions
-@cindex error recovery, mid-rule actions
-In the above example, if the parser initiates error recovery (@pxref{Error
-Recovery}) while parsing the tokens in the embedded statement @code{stmt},
-it might discard the previous semantic context @code{$<context>5} without
-restoring it.
-Thus, @code{$<context>5} needs a destructor (@pxref{Destructor Decl, , Freeing
-Discarded Symbols}).
-However, Bison currently provides no means to declare a destructor specific to
-a particular mid-rule action's semantic value.
-
-One solution is to bury the mid-rule action inside a nonterminal symbol and to
-declare a destructor for that symbol:
-
-@example
-@group
-%type <context> let
-%destructor @{ pop_context ($$); @} 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,
-can coexist in a working parser because the parser can shift the open-brace
-token and look at what follows before deciding whether there is a
-declaration or not:
-
-@example
-@group
-compound:
- '@{' declarations statements '@}'
-| '@{' statements '@}'
-;
-@end group
-@end example
-
-@noindent
-But when we add a mid-rule action as follows, the rules become nonfunctional:
-
-@example
-@group
-compound:
- @{ prepare_for_local_variables (); @}
- '@{' declarations statements '@}'
-@end group
-@group
-| '@{' statements '@}'
-;
-@end group
-@end example
-
-@noindent
-Now the parser is forced to decide whether to run the mid-rule action
-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{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 '@}'
-;
-@end group
-@end example
-
-@noindent
-But this does not help, because Bison does not realize that the two actions
-are identical. (Bison never tries to understand the C code in an action.)
-
-If the grammar is such that a declaration can be distinguished from a
-statement by the first token (which is true in C), then one solution which
-does work is to put the action after the open-brace, like this:
-
-@example
-@group
-compound:
- '@{' @{ prepare_for_local_variables (); @}
- declarations statements '@}'
-| '@{' statements '@}'
-;
-@end group
-@end example
-
-@noindent
-Now the first token of the following declaration or statement,
-which would in any case tell Bison which rule to use, can still do so.
-
-Another solution is to bury the action inside a nonterminal symbol which
-serves as a subroutine:
-
-@example
-@group
-subroutine:
- /* empty */ @{ prepare_for_local_variables (); @}
-;
-@end group
-
-@group
-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.
-
-@node Tracking Locations
-@section Tracking Locations
-@cindex location
-@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 additional information,
-especially symbol locations.
-
-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.
-* Actions and Locations:: Using locations in actions.
-* Location Default Action:: Defining a general way to compute locations.
-@end menu
-
-@node Location Type
-@subsection Data Type of Locations
-@cindex data type of locations
-@cindex default location type
-
-Defining a data type for locations is much simpler than for semantic values,
-since all tokens and groupings always use the same type.
-
-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
-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
-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;
- 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{@@$} to the
-beginning of the first symbol, and the end of @code{@@$} to the end of the
-last symbol.
-
-With this default action, the location tracking can be fully automatic. The
-example above simply rewrites this way:
-
-@example
-@group
-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
-@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). 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, @code{YYLLOC_DEFAULT} is defined this way:
-
-@example
-@group
-# 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
-result) should be modified by @code{YYLLOC_DEFAULT}.
-
-@item
-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
-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
-@cindex Bison declarations
-
-The @dfn{Bison declarations} section of a Bison grammar defines the symbols
-used in formulating the grammar and the data types of semantic values.
-@xref{Symbols}.
-
-All token type names (but not single-character literal tokens such as
-@code{'+'} and @code{'*'}) must be declared. Nonterminal symbols must be
-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 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.
-* Initial Action Decl:: Code run before parsing starts.
-* Destructor Decl:: Declaring how symbols are freed.
-* Printer Decl:: Declaring how symbol values are displayed.
-* 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
-@cindex token type names, declaring
-@cindex declaring literal string tokens
-@findex %token
-
-The basic way to declare a token type name (terminal symbol) is as follows:
-
-@example
-%token @var{name}
-@end example
-
-Bison will convert this into a @code{#define} directive in
-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
-@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
-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 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}).
-
-For example:
-
-@example
-@group
-%union @{ /* define stack type */
- double val;
- symrec *tptr;
-@}
-%token <val> NUM /* define token NUM and its type */
-@end group
-@end example
-
-You can associate a literal string token with a token type name by
-writing the literal string at the end of a @code{%token}
-declaration which declares the name. For example:
-
-@example
-%token arrow "=>"
-@end example
-
-@noindent
-For example, a grammar for the C language might specify these names with
-equivalent literal string tokens:
-
-@example
-%token <operator> OR "||"
-%token <operator> LE 134 "<="
-%left OR "<="
-@end example
-
-@noindent
-Once you equate the literal string and the token name, you can use them
-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 precedence declarations
-@cindex declaring operator precedence
-@cindex operator precedence, declaring
-
-Use the @code{%left}, @code{%right} or @code{%nonassoc} 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.
-
-The syntax of a precedence declaration is nearly the same as that of
-@code{%token}: either
-
-@example
-%left @var{symbols}@dots{}
-@end example
-
-@noindent
-or
-
-@example
-%left <@var{type}> @var{symbols}@dots{}
-@end example
-
-And indeed any of these declarations serves the purposes of @code{%token}.
-But in addition, they specify the associativity and relative precedence for
-all the @var{symbols}:
-
-@itemize @bullet
-@item
-The associativity of an operator @var{op} determines how repeated uses
-of the operator nest: whether @samp{@var{x} @var{op} @var{y} @var{op}
-@var{z}} is parsed by grouping @var{x} with @var{y} first or by
-grouping @var{y} with @var{z} first. @code{%left} specifies
-left-associativity (grouping @var{x} with @var{y} first) and
-@code{%right} specifies right-associativity (grouping @var{y} with
-@var{z} first). @code{%nonassoc} specifies no associativity, which
-means that @samp{@var{x} @var{op} @var{y} @var{op} @var{z}} is
-considered a syntax error.
-
-@item
-The precedence of an operator determines how it nests with other operators.
-All the tokens declared in a single precedence declaration have equal
-precedence and nest together according to their associativity.
-When two tokens declared in different precedence declarations associate,
-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 braced code containing the same thing that goes inside a
-@code{union} in C@.
-
-For example:
-
-@example
-@group
-%union @{
- double val;
- symrec *tptr;
-@}
-@end group
-@end example
-
-@noindent
-This says that the two alternative types are @code{double} and @code{symrec
-*}. They are given names @code{val} and @code{tptr}; these names are used
-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}).
-
-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
-@cindex value types, nonterminals, declaring
-@findex %type
-
-@noindent
-When you use @code{%union} to specify multiple value types, you must
-declare the value type of each nonterminal symbol for which values are
-used. This is done with a @code{%type} declaration, like this:
-
-@example
-%type <@var{type}> @var{nonterminal}@dots{}
-@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.
-
-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 Printer Decl
-@subsection Printing Semantic Values
-@cindex printing semantic values
-@findex %printer
-@findex <*>
-@findex <>
-When run-time traces are enabled (@pxref{Tracing, ,Tracing Your Parser}),
-the parser reports its actions, such as reductions. When a symbol involved
-in an action is reported, only its kind is displayed, as the parser cannot
-know how semantic values should be formatted.
-
-The @code{%printer} directive defines code that is called when a symbol is
-reported. Its syntax is the same as @code{%destructor} (@pxref{Destructor
-Decl, , Freeing Discarded Symbols}).
-
-@deffn {Directive} %printer @{ @var{code} @} @var{symbols}
-@findex %printer
-@vindex yyoutput
-@c This is the same text as for %destructor.
-Invoke the braced @var{code} whenever the parser displays one of the
-@var{symbols}. Within @var{code}, @code{yyoutput} denotes the output stream
-(a @code{FILE*} in C, and an @code{std::ostream&} in C++),
-@code{$$} designates the semantic value associated with the symbol, and
-@code{@@$} its location. The additional parser parameters are also
-available (@pxref{Parser Function, , The Parser Function @code{yyparse}}).
-
-The @var{symbols} are defined as for @code{%destructor} (@pxref{Destructor
-Decl, , Freeing Discarded Symbols}.): they can be per-type (e.g.,
-@samp{<ival>}), per-symbol (e.g., @samp{exp}, @samp{NUM}, @samp{"float"}),
-typed per-default (i.e., @samp{<*>}, or untyped per-default (i.e.,
-@samp{<>}).
-@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
-
-%printer @{ fprintf (yyoutput, "'%c'", $$); @} <character>
-%printer @{ fprintf (yyoutput, "&%p", $$); @} <*>
-%printer @{ fprintf (yyoutput, "\"%s\"", $$); @} STRING1 string1
-%printer @{ fprintf (yyoutput, "<>"); @} <>
-@end example
-
-@noindent
-guarantees that, when the parser print any symbol that has a semantic type
-tag other than @code{<character>}, it display the address of the semantic
-value by default. However, when the parser displays a @code{STRING1} or a
-@code{string1}, it formats it as a string in double quotes. It performs
-only the second @code{%printer} in this case, so it prints only once.
-Finally, the parser print @samp{<>} for any symbol, such as @code{TAGLESS},
-that has no semantic type tag. See also
-
-
-@node Expect Decl
-@subsection Suppressing Conflict Warnings
-@cindex suppressing conflict warnings
-@cindex preventing warnings about conflicts
-@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
-have harmless shift/reduce conflicts which are resolved in a predictable
-way and would be difficult to eliminate. It is desirable to suppress
-the warning about these conflicts unless the number of conflicts
-changes. You can do this with the @code{%expect} declaration.
-
-The declaration looks like this:
-
-@example
-%expect @var{n}
-@end example
-
-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:
-
-@itemize @bullet
-@item
-Compile your grammar without @code{%expect}. Use the @samp{-v} option
-to get a verbose list of where the conflicts occur. Bison will also
-print the number of conflicts.
-
-@item
-Check each of the conflicts to make sure that Bison's default
-resolution is what you really want. If not, rewrite the grammar and
-go back to the beginning.
-
-@item
-Add an @code{%expect} declaration, copying the number @var{n} from the
-number which Bison printed. With GLR parsers, add an
-@code{%expect-rr} declaration as well.
-@end itemize
-
-Now Bison will report an error if you introduce an unexpected conflict,
-but will keep silent otherwise.
-
-@node Start Decl
-@subsection The Start-Symbol
-@cindex declaring the start symbol
-@cindex start symbol, declaring
-@cindex default start symbol
-@findex %start
-
-Bison assumes by default that the start symbol for the grammar is the first
-nonterminal specified in the grammar specification section. The programmer
-may override this restriction with the @code{%start} declaration as follows:
-
-@example
-%start @var{symbol}
-@end example
-
-@node Pure Decl
-@subsection A Pure (Reentrant) Parser
-@cindex reentrant parser
-@cindex 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 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
-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{%define api.pure} says that you want the parser to be
-reentrant. It looks like this:
-
-@example
-%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}
-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.
-
-Whether the parser is pure has nothing to do with the grammar rules.
-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 @code{%define api.push-pull push} declaration with the
-@code{%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
-@code{%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 @code{%define api.pure} declaration does exactly the same thing to
-the generated parser with @code{%define api.push-pull both} as it did for
-@code{%define api.push-pull push}.
-
-@node Decl Summary
-@subsection Bison Declaration Summary
-@cindex Bison declaration summary
-@cindex declaration summary
-@cindex summary, Bison declaration
-
-Here is a summary of the declarations used to define a grammar:
-
-@deffn {Directive} %union
-Declare the collection of data types that semantic values may have
-(@pxref{Union Decl, ,The Collection of Value Types}).
-@end deffn
-
-@deffn {Directive} %token
-Declare a terminal symbol (token type name) with no precedence
-or associativity specified (@pxref{Token Decl, ,Token Type Names}).
-@end deffn
-
-@deffn {Directive} %right
-Declare a terminal symbol (token type name) that is right-associative
-(@pxref{Precedence Decl, ,Operator Precedence}).
-@end deffn
-
-@deffn {Directive} %left
-Declare a terminal symbol (token type name) that is left-associative
-(@pxref{Precedence Decl, ,Operator Precedence}).
-@end deffn
-
-@deffn {Directive} %nonassoc
-Declare a terminal symbol (token type name) that is nonassociative
-(@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
-
-@deffn {Directive} %type
-Declare the type of semantic values for a nonterminal symbol
-(@pxref{Type Decl, ,Nonterminal Symbols}).
-@end deffn
-
-@deffn {Directive} %start
-Specify the grammar's start symbol (@pxref{Start Decl, ,The
-Start-Symbol}).
-@end deffn
-
-@deffn {Directive} %expect
-Declare the expected number of shift-reduce conflicts
-(@pxref{Expect Decl, ,Suppressing Conflict Warnings}).
-@end deffn
-
-
-@sp 1
-@noindent
-In order to change the behavior of @command{bison}, use the following
-directives:
-
-@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
-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}.
-@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 syntax error messages.
-@end deffn
-
-@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}, @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 @code{%define 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
-
-@deffn {Directive} %no-lines
-Don't generate any @code{#line} preprocessor commands in the parser
-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 @code{%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}:
-
-@table @code
-@item YYNTOKENS
-The highest token number, plus one.
-@item YYNNTS
-The number of nonterminal symbols.
-@item YYNRULES
-The number of grammar rules,
-@item YYNSTATES
-The number of parser states (@pxref{Parser States}).
-@end table
-@end deffn
-
-@deffn {Directive} %verbose
-Write an extra output file containing verbose descriptions of the
-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
-
-@deffn {Directive} %yacc
-Pretend the option @option{--yacc} was given, i.e., imitate Yacc,
-including its naming conventions. @xref{Bison Options}, for more.
-@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:
-
-@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}.
-
-@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{""}.
-
-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 rest of this section summarizes variables and values that
-@code{%define} accepts.
-
-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:
-
-@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:
-
-@itemize @bullet
-@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
-
-@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 ================================================== 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.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
-
-@item namespace
-@findex %define namespace
-
-@itemize
-@item Languages(s): C++
-
-@item Purpose: Specify the namespace for the parser class.
-For example, if you specify:
-
-@smallexample
-%define namespace "foo::bar"
-@end smallexample
-
-Bison uses @code{foo::bar} verbatim in references such as:
-
-@smallexample
-foo::bar::parser::semantic_type
-@end smallexample
-
-However, to open a namespace, Bison removes any leading @code{::} and then
-splits on any remaining occurrences:
-
-@smallexample
-namespace foo @{ namespace bar @{
- class position;
- class location;
-@} @}
-@end smallexample
-
-@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
-@code{%define namespace} so that @code{%name-prefix} @emph{only} affects the
-lexical analyzer function.
-For example, if you specify:
-
-@smallexample
-%define namespace "foo"
-%name-prefix "bar::"
-@end smallexample
-
-The parser namespace is @code{foo} and @code{yylex} is referenced as
-@code{bar::lex}.
-@end itemize
-
-@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
-@end itemize
-
-
-@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:
-
-@itemize @bullet
-@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 itemize
-
-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
-@cindex C-language interface
-@cindex interface
-
-The Bison parser is actually a C function named @code{yyparse}. Here we
-describe the interface conventions of @code{yyparse} and the other
-functions that it needs to use.
-
-Keep in mind that the parser uses many C identifiers starting with
-@samp{yy} and @samp{YY} for internal purposes. If you use such an
-identifier (aside from those in this manual) in an action or in epilogue
-in the grammar file, you are likely to run into trouble.
-
-@menu
-* 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
-@section The Parser Function @code{yyparse}
-@findex yyparse
-
-You call the function @code{yyparse} to cause parsing to occur. This
-function reads tokens, executes actions, and ultimately returns when it
-encounters end-of-input or an unrecoverable syntax error. You can also
-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 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:
-
-@defmac YYACCEPT
-@findex YYACCEPT
-Return immediately with value 0 (to report success).
-@end defmac
-
-@defmac YYABORT
-@findex YYABORT
-Return immediately with value 1 (to report failure).
-@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}@}
-@findex %parse-param
-Declare that an argument declared by the braced-code
-@var{argument-declaration} is an additional @code{yyparse} argument.
-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@}
-%parse-param @{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 @code{%define api.push-pull push} or
-@code{%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 @code{%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 @code{%define api.push-pull push} or
-@code{%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 @code{%define api.push-pull push} or
-@code{%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}
-@findex yylex
-@cindex lexical analyzer
-
-The @dfn{lexical analyzer} function, @code{yylex}, recognizes tokens from
-the input stream and returns them to the parser. Bison does not create
-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 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 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 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 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, 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:
-
-@example
-int
-yylex (void)
-@{
- @dots{}
- if (c == EOF) /* Detect end-of-input. */
- return 0;
- @dots{}
- if (c == '+' || c == '-')
- return c; /* Assume token type for `+' is '+'. */
- @dots{}
- return INT; /* Return the type of the token. */
- @dots{}
-@}
-@end example
-
-@noindent
-This interface has been designed so that the output from the @code{lex}
-utility can be used without change as the definition of @code{yylex}.
-
-If the grammar uses literal string tokens, there are two ways that
-@code{yylex} can determine the token type codes for them:
-
-@itemize @bullet
-@item
-If the grammar defines symbolic token names as aliases for the
-literal string tokens, @code{yylex} can use these symbolic names like
-all others. In this case, the use of the literal string tokens in
-the grammar file has no effect on @code{yylex}.
-
-@item
-@code{yylex} can find the multicharacter token in the @code{yytname}
-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 escaped as necessary to be suitable as input
-to Bison.
-
-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.
-
-@example
-for (i = 0; i < YYNTOKENS; i++)
- @{
- if (yytname[i] != 0
- && yytname[i][0] == '"'
- && ! strncmp (yytname[i] + 1, token_buffer,
- strlen (token_buffer))
- && yytname[i][strlen (token_buffer) + 1] == '"'
- && yytname[i][strlen (token_buffer) + 2] == 0)
- break;
- @}
-@end example
-
-The @code{yytname} table is generated only if you use the
-@code{%token-table} declaration. @xref{Decl Summary}.
-@end itemize
-
-@node Token Values
-@subsection Semantic Values of Tokens
-
-@vindex yylval
-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
-@code{yylex}:
-
-@example
-@group
- @dots{}
- 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:
-
-@example
-@group
-%union @{
- int intval;
- double val;
- symrec *tptr;
-@}
-@end group
-@end example
-
-@noindent
-then the code in @code{yylex} might look like this:
-
-@example
-@group
- @dots{}
- yylval.intval = value; /* Put value onto Bison stack. */
- return INT; /* Return the type of the token. */
- @dots{}
-@end group
-@end example
-
-@node Token Locations
-@subsection Textual Locations of Tokens
-
-@vindex yylloc
-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
-four members are called @code{first_line}, @code{first_column},
-@code{last_line} and @code{last_column}. Note that the use of this
-feature makes the parser noticeably slower.
-
-@tindex YYLTYPE
-The data type of @code{yylloc} has the name @code{YYLTYPE}.
-
-@node Pure Calling
-@subsection Calling Conventions for Pure Parsers
-
-When you use the Bison declaration @code{%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
-pointers passed as arguments to @code{yylex}. You must declare them as
-shown here, and pass the information back by storing it through those
-pointers.
-
-@example
-int
-yylex (YYSTYPE *lvalp, YYLTYPE *llocp)
-@{
- @dots{}
- *lvalp = value; /* Put value onto Bison stack. */
- return INT; /* Return the type of the token. */
- @dots{}
-@}
-@end example
-
-If the grammar file does not use the @samp{@@} constructs to refer to
-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.
-
-
-If you wish to pass the additional parameter data to @code{yylex}, use
-@code{%lex-param} just like @code{%parse-param} (@pxref{Parser
-Function}).
-
-@deffn {Directive} lex-param @{@var{argument-declaration}@}
-@findex %lex-param
-Declare that the braced-code @var{argument-declaration} is an
-additional @code{yylex} argument declaration.
-@end deffn
-
-For instance:
-
-@example
-%parse-param @{int *nastiness@}
-%lex-param @{int *nastiness@}
-%parse-param @{int *randomness@}
-@end example
-
-@noindent
-results in the following signatures:
-
-@example
-int yylex (int *nastiness);
-int yyparse (int *nastiness, int *randomness);
-@end example
-
-If @code{%define api.pure} is added:
-
-@example
-int yylex (YYSTYPE *lvalp, int *nastiness);
-int yyparse (int *nastiness, int *randomness);
-@end example
-
-@noindent
-and finally, if both @code{%define api.pure} and @code{%locations} are used:
-
-@example
-int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
-int yyparse (int *nastiness, int *randomness);
-@end example
-
-@node Error Reporting
-@section The Error Reporting Function @code{yyerror}
-@cindex error reporting function
-@findex yyerror
-@cindex parse error
-@cindex 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
-in Actions}).
-
-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 syntax error, the string is normally
-@w{@code{"syntax error"}}.
-
-@findex %error-verbose
-If you invoke the directive @code{%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 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 const *s)
-@{
-@end group
-@group
- fprintf (stderr, "%s\n", s);
-@}
-@end group
-@end example
-
-After @code{yyerror} returns to @code{yyparse}, the latter will attempt
-error recovery if you have written suitable error recovery grammar rules
-(@pxref{Error Recovery}). If recovery is impossible, @code{yyparse} will
-immediately return 1.
-
-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:
-
-@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
-@code{%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.
-
-@deffn {Variable} $$
-Acts like a variable that contains the semantic value for the
-grouping made by the current rule. @xref{Actions}.
-@end deffn
-
-@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
-
-@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}.
-@end deffn
-
-@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}.
-@end deffn
-
-@deffn {Macro} YYABORT @code{;}
-Return immediately from @code{yyparse}, indicating failure.
-@xref{Parser Function, ,The Parser Function @code{yyparse}}.
-@end deffn
-
-@deffn {Macro} YYACCEPT @code{;}
-Return immediately from @code{yyparse}, indicating success.
-@xref{Parser Function, ,The Parser Function @code{yyparse}}.
-@end deffn
-
-@deffn {Macro} YYBACKUP (@var{token}, @var{value})@code{;}
-@findex YYBACKUP
-Unshift a token. This macro is allowed only for rules that reduce
-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 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
-
-@deffn {Macro} YYEMPTY
-Value stored in @code{yychar} when there is no lookahead token.
-@end deffn
-
-@deffn {Macro} YYEOF
-Value stored in @code{yychar} when the lookahead is the end of the input
-stream.
-@end deffn
-
-@deffn {Macro} YYERROR @code{;}
-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
-
-@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 @code{;}
-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
-
-@deffn {Macro} yyerrok @code{;}
-Resume generating error messages immediately for subsequent syntax
-errors. This is useful primarily in error rules.
-@xref{Error Recovery}.
-@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
-location of the grouping made by the current rule. @xref{Tracking
-Locations}.
-
-@c Check if those paragraphs are still useful or not.
-
-@c @example
-@c struct @{
-@c int first_line, last_line;
-@c int first_column, last_column;
-@c @};
-@c @end example
-
-@c Thus, to get the starting line number of the third component, you would
-@c use @samp{@@3.first_line}.
-
-@c In order for the members of this structure to contain valid information,
-@c you must make @code{yylex} supply this information about each token.
-@c If you need only certain members, then @code{yylex} need only fill in
-@c those members.
-
-@c The use of this feature makes the parser noticeably slower.
-@end deffn
-
-@deffn {Value} @@@var{n}
-@findex @@@var{n}
-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
-
-
-@node Algorithm
-@chapter The Bison Parser Algorithm
-@cindex Bison parser algorithm
-@cindex algorithm of parser
-@cindex shifting
-@cindex reduction
-@cindex parser stack
-@cindex stack, parser
-
-As Bison reads tokens, it pushes them onto a stack along with their
-semantic values. The stack is called the @dfn{parser stack}. Pushing a
-token is traditionally called @dfn{shifting}.
-
-For example, suppose the infix calculator has read @samp{1 + 5 *}, with a
-@samp{3} to come. The stack will have four elements, one for each token
-that was shifted.
-
-But the stack does not always have an element for each token read. When
-the last @var{n} tokens and groupings shifted match the components of a
-grammar rule, they can be combined according to that rule. This is called
-@dfn{reduction}. Those tokens and groupings are replaced on the stack by a
-single grouping whose symbol is the result (left hand side) of that rule.
-Running the rule's action is part of the process of reduction, because this
-is what computes the semantic value of the resulting grouping.
-
-For example, if the infix calculator's parser stack contains this:
-
-@example
-1 + 5 * 3
-@end example
-
-@noindent
-and the next input token is a newline character, then the last three
-elements can be reduced to 15 via the rule:
-
-@example
-expr: expr '*' expr;
-@end example
-
-@noindent
-Then the stack contains just these three elements:
-
-@example
-1 + 15
-@end example
-
-@noindent
-At this point, another reduction can be made, resulting in the single value
-16. Then the newline token can be shifted.
-
-The parser tries, by shifts and reductions, to reduce the entire input down
-to a single grouping whose symbol is the grammar's start-symbol
-(@pxref{Language and Grammar, ,Languages and Context-Free Grammars}).
-
-This kind of parser is known in the literature as a bottom-up parser.
-
-@menu
-* 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.
-* 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 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
-simple strategy is inadequate to handle most languages. Instead, when a
-reduction is possible, the parser sometimes ``looks ahead'' at the next
-token in order to decide what to do.
-
-When a token is read, it is not immediately shifted; first it becomes the
-@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 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 lookahead token, some rules may choose to delay their
-application.
-
-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
-;
-@end group
-
-@group
-term:
- '(' expr ')'
-| term '!'
-| NUMBER
-;
-@end group
-@end example
-
-Suppose that the tokens @w{@samp{1 + 2}} have been read and shifted; what
-should be done? If the following token is @samp{)}, then the first three
-tokens must be reduced to form an @code{expr}. This is the only valid
-course, because shifting the @samp{)} would produce a sequence of symbols
-@w{@code{term ')'}}, and no rule allows this.
-
-If the following token is @samp{!}, then it must be shifted immediately so
-that @w{@samp{2 !}} can be reduced to make a @code{term}. If instead the
-parser were to reduce before shifting, @w{@samp{1 + 2}} would become an
-@code{expr}. It would then be impossible to shift the @samp{!} because
-doing so would produce on the stack the sequence of symbols @code{expr
-'!'}. No rule allows that sequence.
-
-@vindex 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
-@section Shift/Reduce Conflicts
-@cindex conflicts
-@cindex shift/reduce conflicts
-@cindex dangling @code{else}
-@cindex @code{else}, dangling
-
-Suppose we are parsing a language which has if-then and if-then-else
-statements, with a pair of rules like this:
-
-@example
-@group
-if_stmt:
- IF expr THEN stmt
-| IF expr THEN stmt ELSE stmt
-;
-@end group
-@end example
-
-@noindent
-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 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
-rule.
-
-This situation, where either a shift or a reduction would be valid, is
-called a @dfn{shift/reduce conflict}. Bison is designed to resolve
-these conflicts by choosing to shift, unless otherwise directed by
-operator precedence declarations. To see the reason for this, let's
-contrast it with the other alternative.
-
-Since the parser prefers to shift the @code{ELSE}, the result is to attach
-the else-clause to the innermost if-statement, making these two inputs
-equivalent:
-
-@example
-if x then if y then win (); else lose;
-
-if x then do; if y then win (); else lose; end;
-@end example
-
-But if the parser chose to reduce when possible rather than shift, the
-result would be to attach the else-clause to the outermost if-statement,
-making these two inputs equivalent:
-
-@example
-if x then if y then win (); else lose;
-
-if x then do; if y then win (); end; else lose;
-@end example
-
-The conflict exists because the grammar as written is ambiguous: either
-parsing of the simple nested if-statement is legitimate. The established
-convention is that these ambiguities are resolved by attaching the
-else-clause to the innermost if-statement; this is what Bison accomplishes
-by choosing to shift rather than reduce. (It would ideally be cleaner to
-write an unambiguous grammar, but that is very hard to do in this case.)
-This particular ambiguity was first encountered in the specifications of
-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}, 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 grammar file that actually manifests
-the conflict:
-
-@example
-@group
-%token IF THEN ELSE variable
-%%
-@end group
-@group
-stmt:
- expr
-| if_stmt
-;
-@end group
-
-@group
-if_stmt:
- IF expr THEN stmt
-| IF expr THEN stmt ELSE stmt
-;
-@end group
-
-expr:
- variable
-;
-@end example
-
-@node Precedence
-@section Operator Precedence
-@cindex operator precedence
-@cindex precedence of operators
-
-Another situation where shift/reduce conflicts appear is in arithmetic
-expressions. Here shifting is not always the preferred resolution; the
-Bison declarations for operator precedence allow you to specify when to
-shift and when to reduce.
-
-@menu
-* Why Precedence:: An example showing why precedence is needed.
-* Using Precedence:: How to specify precedence in Bison grammars.
-* Precedence Examples:: How these features are used in the previous example.
-* How Precedence:: How they work.
-@end menu
-
-@node Why Precedence
-@subsection When Precedence is Needed
-
-Consider the following ambiguous grammar fragment (ambiguous because the
-input @w{@samp{1 - 2 * 3}} can be parsed in two different ways):
-
-@example
-@group
-expr:
- expr '-' expr
-| expr '*' expr
-| expr '<' expr
-| '(' expr ')'
-@dots{}
-;
-@end group
-@end example
-
-@noindent
-Suppose the parser has seen the tokens @samp{1}, @samp{-} and @samp{2};
-should it reduce them via the rule for the subtraction operator? It
-depends on the next token. Of course, if the next token is @samp{)}, we
-must reduce; shifting is invalid because no single rule can reduce the
-token sequence @w{@samp{- 2 )}} or anything starting with that. But if
-the next token is @samp{*} or @samp{<}, we have a choice: either
-shifting or reduction would allow the parse to complete, but with
-different results.
-
-To decide which one Bison should do, we must consider the results. If
-the next operator token @var{op} is shifted, then it must be reduced
-first in order to permit another opportunity to reduce the difference.
-The result is (in effect) @w{@samp{1 - (2 @var{op} 3)}}. On the other
-hand, if the subtraction is reduced before shifting @var{op}, the result
-is @w{@samp{(1 - 2) @var{op} 3}}. Clearly, then, the choice of shift or
-reduce should depend on the relative precedence of the operators
-@samp{-} and @var{op}: @samp{*} should be shifted first, but not
-@samp{<}.
-
-@cindex associativity
-What about input such as @w{@samp{1 - 2 - 5}}; should this be
-@w{@samp{(1 - 2) - 5}} or should it be @w{@samp{1 - (2 - 5)}}? For most
-operators we prefer the former, which is called @dfn{left association}.
-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 lookahead token is @samp{-}: shifting
-makes right-associativity.
-
-@node Using Precedence
-@subsection Specifying Operator Precedence
-@findex %left
-@findex %right
-@findex %nonassoc
-
-Bison allows you to specify these choices with the operator precedence
-declarations @code{%left} and @code{%right}. Each such declaration
-contains a list of tokens, which are operators whose precedence and
-associativity is being declared. The @code{%left} declaration makes all
-those operators left-associative and the @code{%right} declaration makes
-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 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
-precedence is lowest, the next such declaration declares the operators
-whose precedence is a little higher, and so on.
-
-@node Precedence Examples
-@subsection Precedence Examples
-
-In our example, we would want the following declarations:
-
-@example
-%left '<'
-%left '-'
-%left '*'
-@end example
-
-In a more complete example, which supports other operators as well, we
-would declare them in groups of equal precedence. For example, @code{'+'} is
-declared with @code{'-'}:
-
-@example
-%left '<' '>' '=' NE LE GE
-%left '+' '-'
-%left '*' '/'
-@end example
-
-@noindent
-(Here @code{NE} and so on stand for the operators for ``not equal''
-and so on. We assume that these tokens are more than one character long
-and therefore are represented by names, not character literals.)
-
-@node How Precedence
-@subsection How Precedence Works
-
-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 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 lookahead token has no precedence, then the default is to shift.
-
-@node Contextual Precedence
-@section Context-Dependent Precedence
-@cindex context-dependent precedence
-@cindex unary operator precedence
-@cindex precedence, context-dependent
-@cindex precedence, unary operator
-@findex %prec
-
-Often the precedence of an operator depends on the context. This sounds
-outlandish at first, but it is really very common. For example, a minus
-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
-only one precedence declared in this way. For context-dependent
-precedence, you need to use an additional mechanism: the @code{%prec}
-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.
-It's not necessary for that symbol to appear otherwise in the rule. The
-modifier's syntax is:
-
-@example
-%prec @var{terminal-symbol}
-@end example
-
-@noindent
-and it is written after the components of the rule. Its effect is to
-assign the rule the precedence of @var{terminal-symbol}, overriding
-the precedence that would be deduced for it in the ordinary way. The
-altered rule precedence then affects how conflicts involving that rule
-are resolved (@pxref{Precedence, ,Operator Precedence}).
-
-Here is how @code{%prec} solves the problem of unary minus. First, declare
-a precedence for a fictitious terminal symbol named @code{UMINUS}. There
-are no tokens of this type, but the symbol serves to stand for its
-precedence:
-
-@example
-@dots{}
-%left '+' '-'
-%left '*'
-%left UMINUS
-@end example
-
-Now the precedence of @code{UMINUS} can be used in specific rules:
-
-@example
-@group
-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
-@cindex parser state
-@cindex state (of parser)
-
-The function @code{yyparse} is implemented using a finite-state machine.
-The values pushed on the parser stack are not simply token type codes; they
-represent the entire sequence of terminal and nonterminal symbols at or
-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 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
-the top of the stack, and replaced by one grouping. In other words,
-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 lookahead token
-is erroneous in the current state. This causes error processing to begin
-(@pxref{Error Recovery}).
-
-@node Reduce/Reduce
-@section Reduce/Reduce Conflicts
-@cindex reduce/reduce conflict
-@cindex conflicts, reduce/reduce
-
-A reduce/reduce conflict occurs if there are two or more rules that apply
-to the same sequence of input. This usually indicates a serious error
-in the grammar.
-
-For example, here is an erroneous attempt to define a sequence
-of zero or more @code{word} groupings.
-
-@example
-@group
-sequence:
- /* empty */ @{ printf ("empty sequence\n"); @}
-| maybeword
-| sequence word @{ printf ("added word %s\n", $2); @}
-;
-@end group
-
-@group
-maybeword:
- /* empty */ @{ printf ("empty maybeword\n"); @}
-| word @{ printf ("single word %s\n", $1); @}
-;
-@end group
-@end example
-
-@noindent
-The error is an ambiguity: there is more than one way to parse a single
-@code{word} into a @code{sequence}. It could be reduced to a
-@code{maybeword} and then into a @code{sequence} via the second rule.
-Alternatively, nothing-at-all could be reduced into a @code{sequence}
-via the first rule, and this could be combined with the @code{word}
-using the third rule for @code{sequence}.
-
-There is also more than one way to reduce nothing-at-all into a
-@code{sequence}. This can be done directly via the first rule,
-or indirectly via @code{maybeword} and then the second rule.
-
-You might think that this is a distinction without a difference, because it
-does not change whether any particular input is valid or not. But it does
-affect which actions are run. One parsing order runs the second rule's
-action; the other runs the first rule's action and the third rule's action.
-In this example, the output of the program changes.
-
-Bison resolves a reduce/reduce conflict by choosing to use the rule that
-appears first in the grammar, but it is very risky to rely on this. Every
-reduce/reduce conflict must be studied and usually eliminated. Here is the
-proper way to define @code{sequence}:
-
-@example
-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
-;
-
-words:
- /* empty */
-| words word
-;
-
-redirects:
- /* empty */
-| redirects redirect
-;
-@end example
-
-@noindent
-The intention here is to define a sequence which can contain either
-@code{word} or @code{redirect} groupings. The individual definitions of
-@code{sequence}, @code{words} and @code{redirects} are error-free, but the
-three together make a subtle ambiguity: even an empty input can be parsed
-in infinitely many ways!
-
-Consider: nothing-at-all could be a @code{words}. Or it could be two
-@code{words} in a row, or three, or any number. It could equally well be a
-@code{redirects}, or two, or any number. Or it could be a @code{words}
-followed by three @code{redirects} and another @code{words}. And so on.
-
-Here are two ways to correct these rules. First, to make it a single level
-of sequence:
-
-@example
-sequence:
- /* empty */
-| sequence word
-| sequence redirect
-;
-@end example
-
-Second, to prevent either a @code{words} or a @code{redirects}
-from being empty:
-
-@example
-@group
-sequence:
- /* empty */
-| sequence words
-| sequence redirects
-;
-@end group
-
-@group
-words:
- word
-| words word
-;
-@end group
-
-@group
-redirects:
- redirect
-| redirects redirect
-;
-@end group
-@end example
-
-@node Mysterious Conflicts
-@section Mysterious Conflicts
-@cindex Mysterious Conflicts
-
-Sometimes reduce/reduce conflicts can occur that don't look warranted.
-Here is an example:
-
-@example
-@group
-%token ID
-
-%%
-def: param_spec return_spec ',';
-param_spec:
- type
-| name_list ':' type
-;
-@end group
-@group
-return_spec:
- type
-| name ':' type
-;
-@end group
-@group
-type: ID;
-@end group
-@group
-name: ID;
-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 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
-@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 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).
-
-@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
-@group
-%token BOGUS
-@dots{}
-%%
-@dots{}
-return_spec:
- type
-| name ':' type
-| ID BOGUS /* This rule is never used. */
-;
-@end group
-@end example
-
-This corrects the problem because it introduces the possibility of an
-additional active rule in the context after the @code{ID} at the beginning of
-@code{return_spec}. This rule is not active in the corresponding context
-in a @code{param_spec}, so the two contexts receive distinct parser states.
-As long as the token @code{BOGUS} is never generated by @code{yylex},
-the added rule cannot alter the way actual input is parsed.
-
-In this particular example, there is another way to solve the problem:
-rewrite the rule for @code{return_spec} to use @code{ID} directly
-instead of via @code{name}. This also causes the two confusing
-contexts to have different sets of active rules, because the one for
-@code{return_spec} activates the altered rule for @code{return_spec}
-rather than the one for @code{name}.
-
-@example
-param_spec:
- type
-| name_list ':' type
-;
-return_spec:
- 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{%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
-
-@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 run out of memory if too many tokens are shifted and
-not reduced. When this happens, the parser function @code{yyparse}
-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, see @ref{Recursion, ,Recursive Rules}.
-
-@vindex YYMAXDEPTH
-By defining the macro @code{YYMAXDEPTH}, you can control how deep 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.
-
-The stack space allowed is not necessarily allocated. If you specify a
-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} 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}.
-
-@c FIXME: C++ output.
-Because of semantic differences between C and C++, the deterministic
-parsers in C produced by Bison cannot grow when compiled
-by C++ compilers. In this precise case (compiling a C parser as C++) you are
-suggested to grow @code{YYINITDEPTH}. The Bison maintainers hope to fix
-this deficiency in a future release.
-
-@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 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.
-
-In a simple interactive command parser where each input is one line, it may
-be sufficient to allow @code{yyparse} to return 1 on error and have the
-caller ignore the rest of the input line when that happens (and then call
-@code{yyparse} again). But this is inadequate for a compiler, because it
-forgets all the syntactic context leading up to the error. A syntax error
-deep within a function in the compiler input should not cause the compiler
-to treat the following line like the beginning of a source file.
-
-@findex error
-You can define how to recover from a syntax error by writing rules to
-recognize the special token @code{error}. This is a terminal symbol that
-is always defined (you need not declare it) and reserved for error
-handling. The Bison parser generates an @code{error} token whenever a
-syntax error happens; if you have provided a rule to recognize this token
-in the current context, the parse can continue.
-
-For example:
-
-@example
-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{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{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{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
-@code{error} token is acceptable. (This means that the subexpressions
-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. 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
-stmt: error ';' /* On error, skip until ';' is read. */
-@end example
-
-It is also useful to recover to the matching close-delimiter of an
-opening-delimiter that has already been parsed. Otherwise the
-close-delimiter will probably appear to be unmatched, and generate another,
-spurious error message:
-
-@example
-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{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{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
-after three consecutive input tokens have been successfully shifted will
-error messages resume.
-
-Note that rules which accept the @code{error} token may have actions, just
-as any other rules can.
-
-@findex yyerrok
-You can make error messages resume immediately by using the macro
-@code{yyerrok} in an action. If you do this in the error rule's action, no
-error messages will be suppressed. This macro requires no arguments;
-@samp{yyerrok;} is a valid C statement.
-
-@findex yyclearin
-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 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 lookahead token ought to be discarded
-with @samp{yyclearin;}.
-
-@vindex YYRECOVERING
-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
-
-The Bison paradigm is to parse tokens first, then group them into larger
-syntactic units. In many languages, the meaning of a token is affected by
-its context. Although this violates the Bison paradigm, certain techniques
-(known as @dfn{kludges}) may enable you to write Bison parsers for such
-languages.
-
-@menu
-* Semantic Tokens:: Token parsing can depend on the semantic context.
-* Lexical Tie-ins:: Token parsing can depend on the syntactic context.
-* Tie-in Recovery:: Lexical tie-ins have implications for how
- error recovery rules must be written.
-@end menu
-
-(Actually, ``kludge'' means any technique that gets its job done but is
-neither clean nor robust.)
-
-@node Semantic Tokens
-@section Semantic Info in Token Types
-
-The C language has a context dependency: the way an identifier is used
-depends on what its current meaning is. For example, consider this:
-
-@example
-foo (x);
-@end example
-
-This looks like a function call statement, but if @code{foo} is a typedef
-name, then this is actually a declaration of @code{x}. How can a Bison
-parser for C decide how to parse this input?
-
-The method used in GNU C is to have two different token types,
-@code{IDENTIFIER} and @code{TYPENAME}. When @code{yylex} finds an
-identifier, it looks up the current declaration of the identifier in order
-to decide which token type to return: @code{TYPENAME} if the identifier is
-declared as a typedef, @code{IDENTIFIER} otherwise.
-
-The grammar rules can then express the context dependency by the choice of
-token type to recognize. @code{IDENTIFIER} is accepted as an expression,
-but @code{TYPENAME} is not. @code{TYPENAME} can start a declaration, but
-@code{IDENTIFIER} cannot. In contexts where the meaning of the identifier
-is @emph{not} significant, such as in declarations that can shadow a
-typedef name, either @code{TYPENAME} or @code{IDENTIFIER} is
-accepted---there is one rule for each of the two token types.
-
-This technique is simple to use if the decision of which kinds of
-identifiers to allow is made at a place close to where the identifier is
-parsed. But in C this is not always so: C allows a declaration to
-redeclare a typedef name provided an explicit type has been specified
-earlier:
-
-@example
-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
-construct itself by a complicated syntactic structure---the ``declarator''.
-
-As a result, part of the Bison parser for C needs to be duplicated, with
-all the nonterminal names changed: once for parsing a declaration in
-which a typedef name can be redefined, and once for parsing a
-declaration in which that can't be done. Here is a part of the
-duplication, with actions omitted for brevity:
-
-@example
-@group
-initdcl:
- declarator maybeasm '=' init
-| declarator maybeasm
-;
-@end group
-
-@group
-notype_initdcl:
- notype_declarator maybeasm '=' init
-| notype_declarator maybeasm
-;
-@end group
-@end example
-
-@noindent
-Here @code{initdcl} can redeclare a typedef name, but @code{notype_initdcl}
-cannot. The distinction between @code{declarator} and
-@code{notype_declarator} is the same sort of thing.
-
-There is some similarity between this technique and a lexical tie-in
-(described next), in that information which alters the lexical analysis is
-changed during parsing by other parts of the program. The difference is
-here the information is global, and is used for other purposes in the
-program. A true lexical tie-in has a special-purpose flag controlled by
-the syntactic context.
-
-@node Lexical Tie-ins
-@section Lexical Tie-ins
-@cindex lexical tie-in
-
-One way to handle context-dependency is the @dfn{lexical tie-in}: a flag
-which is set by Bison actions, whose purpose is to alter the way tokens are
-parsed.
-
-For example, suppose we have a language vaguely like C, but with a special
-construct @samp{hex (@var{hex-expr})}. After the keyword @code{hex} comes
-an expression in parentheses in which all integers are hexadecimal. In
-particular, the token @samp{a1b} must be treated as an integer rather than
-as an identifier if it appears in that context. Here is how you can do it:
-
-@example
-@group
-%@{
- 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 @{ $$ = make_sum ($1, $3); @}
-@dots{}
-;
-@end group
-
-@group
-constant:
- INTEGER
-| STRING
-;
-@end group
-@end example
-
-@noindent
-Here we assume that @code{yylex} looks at the value of @code{hexflag}; when
-it is nonzero, all integers are parsed in hexadecimal, and tokens starting
-with letters are parsed as integers if possible.
-
-The declaration of @code{hexflag} shown in the prologue of the grammar
-file is needed to make it accessible to the actions (@pxref{Prologue,
-,The Prologue}). You must also write the code in @code{yylex} to obey
-the flag.
-
-@node Tie-in Recovery
-@section Lexical Tie-ins and Error Recovery
-
-Lexical tie-ins make strict demands on any error recovery rules you have.
-@xref{Error Recovery}.
-
-The reason for this is that the purpose of an error recovery rule is to
-abort the parsing of one construct and resume in some larger construct.
-For example, in C-like languages, a typical error recovery rule is to skip
-tokens until the next semicolon, and then start a new statement, like this:
-
-@example
-stmt:
- expr ';'
-| IF '(' expr ')' stmt @{ @dots{} @}
-@dots{}
-| error ';' @{ hexflag = 0; @}
-;
-@end example
-
-If there is a syntax error in the middle of a @samp{hex (@var{expr})}
-construct, this error rule will apply, and then the action for the
-completed @samp{hex (@var{expr})} will never run. So @code{hexflag} would
-remain set for the entire rest of the input, or until the next @code{hex}
-keyword, causing identifiers to be misinterpreted as integers.
-
-To avoid this problem the error recovery rule itself clears @code{hexflag}.
-
-There may also be an error recovery rule that works within expressions.
-For example, there could be a rule which applies within parentheses
-and skips to the close-parenthesis:
-
-@example
-@group
-expr:
- @dots{}
-| '(' expr ')' @{ $$ = $2; @}
-| '(' error ')'
-@dots{}
-@end group
-@end example
-
-If this rule acts within the @code{hex} construct, it is not going to abort
-that construct (since it applies to an inner level of parentheses within
-the construct). Therefore, it should not clear the flag: the rest of
-the @code{hex} construct should be parsed with the flag still in effect.
-
-What if there is an error recovery rule which might abort out of the
-@code{hex} construct or might not, depending on circumstances? There is no
-way you can write the action to determine whether a @code{hex} construct is
-being aborted or not. So if you are using a lexical tie-in, you had better
-make sure your error recovery rules are not of this kind. Each rule must
-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
-
-Developing a parser can be a challenge, especially if you don't understand
-the algorithm (@pxref{Algorithm, ,The Bison Parser Algorithm}). This
-chapter explains how to generate and read the detailed description of the
-automaton, and how to enable and understand the parser run-time traces.
-
-@menu
-* Understanding:: Understanding the structure of your parser.
-* Tracing:: Tracing the execution of your parser.
-@end menu
-
-@node Understanding
-@section Understanding Your Parser
-
-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).
-
-The textual file is generated when the options @option{--report} or
-@option{--verbose} are specified, see @ref{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}.
-
-The following grammar file, @file{calc.y}, will be used in the sequel:
-
-@example
-%token NUM STR
-%left '+' '-'
-%left '*'
-%%
-exp:
- exp '+' exp
-| exp '-' exp
-| exp '*' exp
-| exp '/' exp
-| NUM
-;
-useless: STR;
-%%
-@end example
-
-@command{bison} reports:
-
-@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
-
-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.
-
-@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):
-
-@example
-Nonterminals useless in grammar
- useless
-
-Terminals unused in grammar
- STR
-
-Rules useless in grammar
- 6 useless: STR
-@end example
-
-@noindent
-The next section lists states that still have conflicts.
-
-@example
-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
-
-@noindent
-Then Bison reproduces the exact grammar it used:
-
-@example
-Grammar
-
- 0 $accept: exp $end
-
- 1 exp: exp '+' exp
- 2 | exp '-' exp
- 3 | exp '*' exp
- 4 | exp '/' exp
- 5 | NUM
-@end example
-
-@noindent
-and reports the uses of the symbols:
-
-@example
-@group
-Terminals, with rules where they appear
-
-$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
-
-$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
-
-When a Bison grammar compiles properly but parses ``incorrectly'', the
-@code{yydebug} parser-trace feature helps figuring out why.
-
-@menu
-* Enabling Traces:: Activating run-time trace support
-* Mfcalc Traces:: Extending @code{mfcalc} to support traces
-* The YYPRINT Macro:: Obsolete interface for semantic value reports
-@end menu
-
-@node Enabling Traces
-@subsection Enabling Traces
-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 is a Bison extension, which will prove
-useful when Bison will output parsers 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 debug option so that debugging is
-always possible.
-
-@findex YYFPRINTF
-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 automaton
-description file (@pxref{Understanding, ,Understanding Your Parser}).
-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/C++/Java program and you can use
-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.
-
-@node Mfcalc Traces
-@subsection Enabling Debug Traces for @code{mfcalc}
-
-The debugging information normally gives the token type of each token read,
-but not its semantic value. The @code{%printer} directive allows specify
-how semantic values are reported, see @ref{Printer Decl, , Printing
-Semantic Values}. For backward compatibility, Yacc like C parsers may also
-use the @code{YYPRINT} (@pxref{The YYPRINT Macro, , The @code{YYPRINT}
-Macro}), but its use is discouraged.
-
-As a demonstration of @code{%printer}, consider the multi-function
-calculator, @code{mfcalc} (@pxref{Multi-function Calc}). To enable run-time
-traces, and semantic value reports, insert the following directives in its
-prologue:
-
-@comment file: mfcalc.y: 2
-@example
-/* Generate the parser description file. */
-%verbose
-/* Enable run-time traces (yydebug). */
-%define parse.trace
-
-/* Formatting semantic values. */
-%printer @{ fprintf (yyoutput, "%s", $$->name); @} VAR;
-%printer @{ fprintf (yyoutput, "%s()", $$->name); @} FNCT;
-%printer @{ fprintf (yyoutput, "%g", $$); @} <val>;
-@end example
-
-The @code{%define} directive instructs Bison to generate run-time trace
-support. Then, activation of these traces is controlled at run-time by the
-@code{yydebug} variable, which is disabled by default. Because these traces
-will refer to the ``states'' of the parser, it is helpful to ask for the
-creation of a description of that parser; this is the purpose of (admittedly
-ill-named) @code{%verbose} directive.
-
-The set of @code{%printer} directives demonstrates how to format the
-semantic value in the traces. Note that the specification can be done
-either on the symbol type (e.g., @code{VAR} or @code{FNCT}), or on the type
-tag: since @code{<val>} is the type for both @code{NUM} and @code{exp}, this
-printer will be used for them.
-
-Here is a sample of the information provided by run-time traces. The traces
-are sent onto standard error.
-
-@example
-$ @kbd{echo 'sin(1-1)' | ./mfcalc -p}
-Starting parse
-Entering state 0
-Reducing stack by rule 1 (line 34):
--> $$ = nterm input ()
-Stack now 0
-Entering state 1
-@end example
-
-@noindent
-This first batch shows a specific feature of this grammar: the first rule
-(which is in line 34 of @file{mfcalc.y} can be reduced without even having
-to look for the first token. The resulting left-hand symbol (@code{$$}) is
-a valueless (@samp{()}) @code{input} non terminal (@code{nterm}).
-
-Then the parser calls the scanner.
-@example
-Reading a token: Next token is token FNCT (sin())
-Shifting token FNCT (sin())
-Entering state 6
-@end example
-
-@noindent
-That token (@code{token}) is a function (@code{FNCT}) whose value is
-@samp{sin} as formatted per our @code{%printer} specification: @samp{sin()}.
-The parser stores (@code{Shifting}) that token, and others, until it can do
-something about it.
-
-@example
-Reading a token: Next token is token '(' ()
-Shifting token '(' ()
-Entering state 14
-Reading a token: Next token is token NUM (1.000000)
-Shifting token NUM (1.000000)
-Entering state 4
-Reducing stack by rule 6 (line 44):
- $1 = token NUM (1.000000)
--> $$ = nterm exp (1.000000)
-Stack now 0 1 6 14
-Entering state 24
-@end example
-
-@noindent
-The previous reduction demonstrates the @code{%printer} directive for
-@code{<val>}: both the token @code{NUM} and the resulting non-terminal
-@code{exp} have @samp{1} as value.
-
-@example
-Reading a token: Next token is token '-' ()
-Shifting token '-' ()
-Entering state 17
-Reading a token: Next token is token NUM (1.000000)
-Shifting token NUM (1.000000)
-Entering state 4
-Reducing stack by rule 6 (line 44):
- $1 = token NUM (1.000000)
--> $$ = nterm exp (1.000000)
-Stack now 0 1 6 14 24 17
-Entering state 26
-Reading a token: Next token is token ')' ()
-Reducing stack by rule 11 (line 49):
- $1 = nterm exp (1.000000)
- $2 = token '-' ()
- $3 = nterm exp (1.000000)
--> $$ = nterm exp (0.000000)
-Stack now 0 1 6 14
-Entering state 24
-@end example
-
-@noindent
-The rule for the subtraction was just reduced. The parser is about to
-discover the end of the call to @code{sin}.
-
-@example
-Next token is token ')' ()
-Shifting token ')' ()
-Entering state 31
-Reducing stack by rule 9 (line 47):
- $1 = token FNCT (sin())
- $2 = token '(' ()
- $3 = nterm exp (0.000000)
- $4 = token ')' ()
--> $$ = nterm exp (0.000000)
-Stack now 0 1
-Entering state 11
-@end example
-
-@noindent
-Finally, the end-of-line allow the parser to complete the computation, and
-display its result.
-
-@example
-Reading a token: Next token is token '\n' ()
-Shifting token '\n' ()
-Entering state 22
-Reducing stack by rule 4 (line 40):
- $1 = nterm exp (0.000000)
- $2 = token '\n' ()
-@result{} 0
--> $$ = nterm line ()
-Stack now 0 1
-Entering state 10
-Reducing stack by rule 2 (line 35):
- $1 = nterm input ()
- $2 = nterm line ()
--> $$ = nterm input ()
-Stack now 0
-Entering state 1
-@end example
-
-The parser has returned into state 1, in which it is waiting for the next
-expression to evaluate, or for the end-of-file token, which causes the
-completion of the parsing.
-
-@example
-Reading a token: Now at end of input.
-Shifting token $end ()
-Entering state 2
-Stack now 0 1 2
-Cleanup: popping token $end ()
-Cleanup: popping nterm input ()
-@end example
-
-
-@node The YYPRINT Macro
-@subsection The @code{YYPRINT} Macro
-
-@findex YYPRINT
-Before @code{%printer} support, semantic values could be displayed using the
-@code{YYPRINT} macro, which works only for terminal symbols and only with
-the @file{yacc.c} skeleton.
-
-@deffn {Macro} YYPRINT (@var{stream}, @var{token}, @var{value});
-@findex YYPRINT
-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}).
-
-For @file{yacc.c} only. Obsoleted by @code{%printer}.
-@end deffn
-
-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.
-* Option Cross Key:: Alphabetical list of long options.
-* Yacc Library:: Yacc-compatible @code{yylex} and @code{main}.
-@end menu
-
-@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 namespace
-Use the @samp{%define namespace} directive to change the namespace
-name, see @ref{%define Summary,,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. @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
-
-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}@footnote{In the future techniques to allow complex types
-within pseudo-unions (similar to Boost variants) might be implemented to
-alleviate these issues.}, 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++ 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 semantics value and locations.
-@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
-
-@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 {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})
-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}.
-@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
-@code{%define api.pure} directive. Therefore the interface is as follows.
-
-@deftypemethod {parser} {int} yylex (semantic_type* @var{yylval}, location_type* @var{yylloc}, @var{type1} @var{arg1}, ...)
-Return the next token. Its type is the return value, its semantic
-value and location being @var{yylval} and @var{yylloc}. Invocations of
-@samp{%lex-param @{@var{type1} @var{arg1}@}} yield additional arguments.
-@end deftypemethod
-
-
-@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 interaction. 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::token_type \
- yylex (yy::calcxx_parser::semantic_type* yylval, \
- yy::calcxx_parser::location_type* yylloc, \
- 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 two members function 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. Return 0 on success.
- int parse (const std::string& f);
- std::string file;
- 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 %code requires
-Then come the declarations/inclusions needed to define the
-@code{%union}. Because the parser uses the parsing driver and
-reciprocally, both cannot include the header of the other. 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 simply
-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.
-%parse-param @{ calcxx_driver& driver @}
-%lex-param @{ calcxx_driver& driver @}
-@end example
-
-@noindent
-Then we request the location tracking feature, and initialize the
-first location's file name. Afterward new locations are computed
-relatively to the previous locations: the file name will be
-automatically 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 two following 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
-%debug
-%error-verbose
-@end example
-
-@noindent
-Semantic values cannot use ``real'' objects, but only pointers to
-them.
-
-@comment file: calc++-parser.yy
-@example
-// Symbols.
-%union
-@{
- int ival;
- std::string *sval;
-@};
-@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 named are provided for each
-symbol. Note that the tokens names are prefixed by @code{TOKEN_} to
-avoid name clashes.
-
-@comment file: calc++-parser.yy
-@example
-%token END 0 "end of file"
-%token ASSIGN ":="
-%token <sval> IDENTIFIER "identifier"
-%token <ival> NUMBER "number"
-%type <ival> exp
-@end example
-
-@noindent
-To enable memory deallocation during error recovery, use
-@code{%destructor}.
-
-@c FIXME: Document %printer, and mention that it takes a braced-code operand.
-@comment file: calc++-parser.yy
-@example
-%printer @{ yyoutput << *$$; @} "identifier"
-%destructor @{ delete $$; @} "identifier"
-
-%printer @{ yyoutput << $$; @} <ival>
-@end example
-
-@noindent
-The grammar itself is straightforward.
-
-@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; delete $1; @};
-
-%left '+' '-';
-%left '*' '/';
-exp: exp '+' exp @{ $$ = $1 + $3; @}
- | exp '-' exp @{ $$ = $1 - $3; @}
- | exp '*' exp @{ $$ = $1 * $3; @}
- | exp '/' exp @{ $$ = $1 / $3; @}
- | "identifier" @{ $$ = driver.variables[*$1]; delete $1; @}
- | "number" @{ $$ = $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 yy::calcxx_parser::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 <cstdlib>
-# include <cerrno>
-# include <climits>
-# 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
-
-/* By default yylex returns int, we use token_type.
- Unfortunately yyterminate by default returns 0, which is
- not of token_type. */
-#define yyterminate() return token::END
-%@}
-@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 the scanner tracing features.
-
-@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, the end position is
-advanced of its width. In case it matched 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
-%@{
-# define YY_USER_ACTION yylloc->columns (yyleng);
-%@}
-@end group
-%%
-%@{
- yylloc->step ();
-%@}
-@{blank@}+ yylloc->step ();
-[\n]+ yylloc->lines (yyleng); yylloc->step ();
-@end example
-
-@noindent
-The rules are simple, just note the use of the driver to report errors.
-It is convenient to use a typedef to shorten
-@code{yy::calcxx_parser::token::identifier} into
-@code{token::identifier} for instance.
-
-@comment file: calc++-scanner.ll
-@example
-%@{
- typedef yy::calcxx_parser::token token;
-%@}
- /* Convert ints to the actual type of tokens. */
-[-+*/] return yy::calcxx_parser::token_type (yytext[0]);
-":=" return token::ASSIGN;
-@{int@} @{
- errno = 0;
- long n = strtol (yytext, NULL, 10);
- if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE))
- driver.error (*yylloc, "integer is out of range");
- yylval->ival = n;
- return token::NUMBER;
-@}
-@{id@} yylval->sval = new std::string (yytext); return token::IDENTIFIER;
-. driver.error (*yylloc, "invalid character");
-%%
-@end example
-
-@noindent
-Finally, because the scanner related driver's member function 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.empty () || 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[])
-@{
- calcxx_driver driver;
- for (int i = 1; i < argc; ++i)
- if (argv[i] == std::string ("-p"))
- driver.trace_parsing = true;
- else if (argv[i] == std::string ("-s"))
- driver.trace_scanning = true;
- else if (!driver.parse (argv[i]))
- std::cout << driver.result << std::endl;
-@}
-@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 @code{%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 debugging and verbose errors are always compiled
-in. Thus the @code{%debug} and @code{%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 @code{%debug} and
-@code{%verbose-error} explicitly if needed. Also, in the future the
-@code{%token-table} directive might enable a public interface to
-access the token names and codes.
-
-@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 @code{%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 @code{%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 @code{%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
-@code{%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
-@code{%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.
-
-The Java package name of the parser class can be specified using the
-@code{%define package} directive. The superclass and the implemented
-interfaces of the parser class can be specified with the @code{%define
-extends} and @code{%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.
-
-@c FIXME: The following constants and variables are still undocumented:
-@c @code{bisonVersion}, @code{bisonSkeleton} and @code{errorVerbose}.
-
-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.
-
-Token names defined by @code{%token} and the predefined @code{EOF} token
-name are added as constant fields to the parser class.
-
-@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{%parse-param}s and/or @code{%lex-param}s are
-used.
-@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{%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{%lex-param}s.
-@end deftypeop
-
-@deftypemethod {YYParser} {boolean} parse ()
-Run the syntactic analysis, and return @code{true} on success,
-@code{false} otherwise.
-@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
-
-
-@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.
-
-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 @code{%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 @code{%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 @code{%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 @code{%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 @code{%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 $$
-The semantic value for the grouping made by the current rule. As a
-value, this is in the base type (@code{Object} or as specified by
-@code{%define stype}) as in not cast to the declared subtype because
-casts are not allowed on the left-hand side of Java assignments.
-Use an explicit Java cast if the correct subtype is needed.
-@xref{Java Semantic Values}.
-@end defvar
-
-@defvar $<@var{typealt}>$
-Same as @code{$$} 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
-
-@deftypefn {Statement} return YYABORT @code{;}
-Return immediately from the parser, indicating failure.
-@xref{Java Parser Interface}.
-@end deftypefn
-
-@deftypefn {Statement} return YYACCEPT @code{;}
-Return immediately from the parser, indicating success.
-@xref{Java Parser Interface}.
-@end deftypefn
-
-@deftypefn {Statement} {return} YYERROR @code{;}
-Start error recovery (without printing an error message).
-@xref{Error Recovery}.
-@end deftypefn
-
-@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} {protected void} yyerror (String msg)
-@deftypefnx {Function} {protected void} yyerror (Position pos, String msg)
-@deftypefnx {Function} {protected void} yyerror (Location loc, String msg)
-Print an error message using the @code{yyerror} method of the scanner
-instance in use.
-@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.
-@xref{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. @xref{Java Semantic Values}, and
-@ref{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 @code{%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 (@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
-@code{%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 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 import} 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 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 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, see @ref{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
-
-@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
-
-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:
-
-@example
-%token START_FOO START_BAR;
-%start start;
-start:
- START_FOO foo
-| START_BAR bar;
-@end example
-
-These tokens prevents the introduction of new conflicts. As far as the
-parser goes, that is all that is needed.
-
-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
- /* @r{Prologue.} */
-%%
-%@{
- if (start_token)
- @{
- int t = start_token;
- start_token = 0;
- return t;
- @}
-%@}
- /* @r{The rules.} */
-@end example
-
-
-@node Secure? Conform?
-@section Secure? Conform?
-
-@quotation
-Is Bison secure? Does it conform to POSIX?
-@end quotation
-
-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.
-
-@node I can't build Bison
-@section I can't build Bison
-
-@quotation
-I can't build Bison because @command{make} complains that
-@code{msgfmt} is not found.
-What should I do?
-@end quotation
-
-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.
-
-
-@node Where can I find help?
-@section Where can I find help?
-
-@quotation
-I'm having trouble using Bison. Where can I find help?
-@end quotation
-
-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.
-
-@node Bug Reports
-@section Bug Reports
-
-@quotation
-I found a bug. What should I include in the bug report?
-@end quotation
-
-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.
-
-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.
-
-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').
-
-Patches are most welcome, but not required. That is, do not hesitate to
-send a bug report just because you cannot provide a fix.
-
-Send bug reports to @email{bug-bison@@gnu.org}.
-
-@node More Languages
-@section More Languages
-
-@quotation
-Will Bison ever have C++ and Java support? How about @var{insert your
-favorite language here}?
-@end quotation
-
-C++ and Java support is there now, and is documented. We'd love to add other
-languages; contributions are welcome.
-
-@node Beta Testing
-@section Beta Testing
-
-@quotation
-What is involved in being a beta tester?
-@end quotation
-
-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.
-
-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.
-
-@node Mailing Lists
-@section Mailing Lists
-
-@quotation
-How do I join the help-bison and bug-bison mailing lists?
-@end quotation
-
-See @url{http://lists.gnu.org/}.
-
-@c ================================================= Table of Symbols
-
-@node Table of Symbols
-@appendix Bison Symbols
-@cindex Bison symbols, table of
-@cindex symbols in Bison, table of
-
-@deffn {Variable} @@$
-In an action, the location of the left-hand side of the rule.
-@xref{Tracking Locations}.
-@end deffn
-
-@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
-
-@deffn {Variable} @@@var{name}
-In an action, the location of a symbol addressed by name. @xref{Tracking
-Locations}.
-@end deffn
-
-@deffn {Variable} @@[@var{name}]
-In an action, the location of a symbol addressed by name. @xref{Tracking
-Locations}.
-@end deffn
-
-@deffn {Variable} $$
-In an action, the semantic value of the left-hand side of the rule.
-@xref{Actions}.
-@end deffn
-
-@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
-
-@deffn {Variable} $@var{name}
-In an action, the semantic value of a symbol addressed by name.
-@xref{Actions}.
-@end deffn
-
-@deffn {Variable} $[@var{name}]
-In an action, the semantic value of a symbol addressed by name.
-@xref{Actions}.
-@end deffn
-
-@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
-
-@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 {Construct} /*@dots{}*/
-Comment delimiters, as in C.
-@end deffn
-
-@deffn {Delimiter} :
-Separates a rule's result from its components. @xref{Rules, ,Syntax of
-Grammar Rules}.
-@end deffn
-
-@deffn {Delimiter} ;
-Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}.
-@end deffn
-
-@deffn {Delimiter} |
-Separates alternate rules for the same result nonterminal.
-@xref{Rules, ,Syntax of Grammar Rules}.
-@end deffn
-
-@deffn {Directive} <*>
-Used to define a default tagged @code{%destructor} or default tagged
-@code{%printer}.
-
-This feature is experimental.
-More user feedback will help to determine whether it should become a permanent
-feature.
-
-@xref{Destructor Decl, , Freeing Discarded Symbols}.
-@end deffn
-
-@deffn {Directive} <>
-Used to define a default tagless @code{%destructor} or default tagless
-@code{%printer}.
-
-This feature is experimental.
-More user feedback will help to determine whether it should become a permanent
-feature.
-
-@xref{Destructor Decl, , Freeing Discarded Symbols}.
-@end deffn
-
-@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
-
-@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
-
-@deffn {Directive} %debug
-Equip the parser for debugging. @xref{Decl Summary}.
-@end deffn
-
-@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
-
-@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
-Bison declaration to create a parser header file, which is usually
-meant for the scanner. @xref{Decl Summary}.
-@end deffn
-
-@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
-
-@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
-Bison declaration to request verbose, specific error message strings
-when @code{yyerror} is called. @xref{Error Reporting}.
-@end deffn
-
-@deffn {Directive} %file-prefix "@var{prefix}"
-Bison declaration to set the prefix of the output files. @xref{Decl
-Summary}.
-@end deffn
-
-@deffn {Directive} %glr-parser
-Bison declaration to produce a GLR parser. @xref{GLR
-Parsers, ,Writing GLR Parsers}.
-@end deffn
-
-@deffn {Directive} %initial-action
-Run user code before parsing. @xref{Initial Action Decl, , Performing Actions before Parsing}.
-@end deffn
-
-@deffn {Directive} %language
-Specify the programming language for the generated parser.
-@xref{Decl Summary}.
-@end deffn
-
-@deffn {Directive} %left
-Bison declaration to assign left associativity to token(s).
-@xref{Precedence Decl, ,Operator Precedence}.
-@end deffn
-
-@deffn {Directive} %lex-param @{@var{argument-declaration}@}
-Bison declaration to specifying an additional parameter 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
-
-@deffn {Directive} %no-lines
-Bison declaration to avoid generating @code{#line} directives in the
-parser implementation file. @xref{Decl Summary}.
-@end deffn
-
-@deffn {Directive} %nonassoc
-Bison declaration to assign nonassociativity to token(s).
-@xref{Precedence Decl, ,Operator Precedence}.
-@end deffn
-
-@deffn {Directive} %output "@var{file}"
-Bison declaration to set the name of the parser implementation file.
-@xref{Decl Summary}.
-@end deffn
-
-@deffn {Directive} %parse-param @{@var{argument-declaration}@}
-Bison declaration to specifying an additional parameter that
-@code{yyparse} should accept. @xref{Parser Function,, The Parser
-Function @code{yyparse}}.
-@end deffn
-
-@deffn {Directive} %prec
-Bison declaration to assign a precedence to a specific rule.
-@xref{Contextual Precedence, ,Context-Dependent Precedence}.
-@end deffn
-
-@deffn {Directive} %pure-parser
-Deprecated version of @code{%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 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
-
-@deffn {Directive} %start
-Bison declaration to specify the start symbol. @xref{Start Decl, ,The
-Start-Symbol}.
-@end deffn
-
-@deffn {Directive} %token
-Bison declaration to declare token(s) without specifying precedence.
-@xref{Token Decl, ,Token Type Names}.
-@end deffn
-
-@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
-
-@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
-
-@deffn {Directive} %union
-Bison declaration to specify several possible data types for semantic
-values. @xref{Union Decl, ,The Collection of Value Types}.
-@end deffn
-
-@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}}.
-
-For Java parsers, this functionality is invoked using @code{return YYABORT;}
-instead.
-@end deffn
-
-@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}}.
-
-For Java parsers, this functionality is invoked using @code{return YYACCEPT;}
-instead.
-@end deffn
-
-@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
-
-@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
-
-@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
-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}.
-
-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 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
-@code{%error-verbose} is preferred. @xref{Error Reporting}.
-@end deffn
-
-@deffn {Macro} YYFPRINTF
-Macro used to output run-time traces.
-@xref{Enabling Traces}.
-@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 {Macro} YYPRINT
-Macro used to output token semantic values. For @file{yacc.c} only.
-Obsoleted by @code{%printer}.
-@xref{The YYPRINT Macro, , The @code{YYPRINT} Macro}.
-@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 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.
-Thus, if there is a rule which says that an integer can be used as an
-expression, integers are allowed @emph{anywhere} an expression is
-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.
-
-@item Empty string
-Analogous to the empty set in set theory, the empty string is a
-character string of length zero.
-
-@item Finite-state stack machine
-A ``machine'' that has discrete states in which it is said to exist at
-each instant in time. As input to the machine is processed, the
-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}.
-
-@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@.
-@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.
-@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
-
-@item Left associativity
-Operators having left associativity are analyzed from left to right:
-@samp{a+b+c} first computes @samp{a+b} and then combines with
-@samp{c}. @xref{Precedence, ,Operator Precedence}.
-
-@item Left recursion
-A rule whose result symbol is also its first component symbol; for
-example, @samp{expseq1 : expseq1 ',' exp;}. @xref{Recursion, ,Recursive
-Rules}.
-
-@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}.
-
-@item Lexical analyzer (scanner)
-A function that reads an input stream and returns tokens one by one.
-@xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
-
-@item Lexical tie-in
-A flag, set by actions in the grammar rules, which alters the way
-tokens are parsed. @xref{Lexical Tie-ins}.
-
-@item Literal string token
-A token which consists of two or more fixed characters. @xref{Symbols}.
-
-@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 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
-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 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
-analyzer.
-
-@item Postfix operator
-An arithmetic operator that is placed after the operands upon which it
-performs some operation.
-
-@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}.
-
-@item Reentrant
-A reentrant subprogram is a subprogram which can be in invoked any
-number of times in parallel, without interference between the various
-invocations. @xref{Pure Decl, ,A Pure (Reentrant) Parser}.
-
-@item Reverse polish notation
-A language in which all operators are postfix operators.
-
-@item Right recursion
-A rule whose result symbol is also its last component symbol; for
-example, @samp{expseq1: exp ',' expseq1;}. @xref{Recursion, ,Recursive
-Rules}.
-
-@item Semantics
-In computer languages, the semantics are specified by the actions
-taken for each instance of the language, i.e., the meaning of
-each statement. @xref{Semantics, ,Defining Language Semantics}.
-
-@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}.
-
-@item Single-character literal
-A single character that is recognized and interpreted as is.
-@xref{Grammar in Bison, ,From Formal Rules to Bison Input}.
-
-@item Start symbol
-The nonterminal symbol that stands for a complete valid utterance in
-the language being parsed. The start symbol is usually listed as the
-first nonterminal symbol in a language specification.
-@xref{Start Decl, ,The Start-Symbol}.
-
-@item Symbol table
-A data structure where symbol names and associated data are stored
-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.
-The input of the Bison parser is a stream of tokens which comes from
-the lexical analyzer. @xref{Symbols}.
-
-@item 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
-
-@node Bibliography
-@unnumbered Bibliography
-
-@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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