@copying
-This manual (@value{UPDATED}) is for @acronym{GNU} Bison (version
-@value{VERSION}), the @acronym{GNU} parser generator.
+This manual (@value{UPDATED}) is for GNU Bison (version
+@value{VERSION}), the GNU parser generator.
-Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998,
-1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 Free Software
+Copyright @copyright{} 1988-1993, 1995, 1998-2011 Free Software
Foundation, Inc.
@quotation
Permission is granted to copy, distribute and/or modify this document
-under the terms of the @acronym{GNU} Free Documentation License,
-Version 1.2 or any later version published by the Free Software
+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 @acronym{GNU} Manual,'' and with the Back-Cover Texts as in
+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
-``@acronym{GNU} Free Documentation License.''
+``GNU Free Documentation License.''
(a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
-modify this @acronym{GNU} manual. Buying copies from the @acronym{FSF}
-supports it in developing @acronym{GNU} and promoting software
+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). @acronym{GNU} parser generator (Yacc replacement).
+* bison: (bison). GNU parser generator (Yacc replacement).
@end direntry
@titlepage
51 Franklin Street, Fifth Floor @*
Boston, MA 02110-1301 USA @*
Printed copies are available from the Free Software Foundation.@*
-@acronym{ISBN} 1-882114-44-2
+ISBN 1-882114-44-2
@sp 2
Cover art by Etienne Suvasa.
@end titlepage
@menu
* Introduction::
* Conditions::
-* Copying:: The @acronym{GNU} General Public License says
- how you can copy and share Bison
+* Copying:: The GNU General Public License says
+ how you can copy and share Bison.
Tutorial sections:
-* Concepts:: Basic concepts for understanding Bison.
-* Examples:: Three simple explained examples of using Bison.
+* Concepts:: Basic concepts for understanding Bison.
+* Examples:: Three simple explained examples of using Bison.
Reference sections:
-* Grammar File:: Writing Bison declarations and rules.
-* Interface:: C-language interface to the parser function @code{yyparse}.
-* Algorithm:: How the Bison parser works at run-time.
-* Error Recovery:: Writing rules for error recovery.
+* Grammar File:: Writing Bison declarations and rules.
+* Interface:: C-language interface to the parser function @code{yyparse}.
+* Algorithm:: How the Bison parser works at run-time.
+* Error Recovery:: Writing rules for error recovery.
* Context Dependency:: What to do if your language syntax is too
- messy for Bison to handle straightforwardly.
-* Debugging:: Understanding or debugging Bison parsers.
-* Invocation:: How to run Bison (to produce the parser source file).
-* 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.
-* Index:: Cross-references to the text.
+ messy for Bison to handle straightforwardly.
+* Debugging:: Understanding or debugging Bison parsers.
+* Invocation:: How to run Bison (to produce the parser source file).
+* 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.
+* 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:: Tracking Locations.
-* Bison Parser:: What are Bison's input and output,
- how is the output used?
-* Stages:: Stages in writing and running Bison grammars.
-* Grammar Layout:: Overall structure of a Bison grammar file.
-
-Writing @acronym{GLR} Parsers
-
-* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars.
-* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities.
-* GLR Semantic Actions:: Deferred semantic actions have special concerns.
-* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler.
+* 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:: Tracking Locations.
+* Bison Parser:: What are Bison's input and output,
+ how is the output used?
+* Stages:: Stages in writing and running Bison grammars.
+* Grammar Layout:: Overall structure of a Bison grammar file.
+
+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.
+* RPN Calc:: Reverse polish notation calculator;
+ a first example with no operator precedence.
+* Infix Calc:: Infix (algebraic) notation calculator.
+ Operator precedence is introduced.
* Simple Error Recovery:: Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$.
-* Multi-function Calc:: Calculator with memory and trig functions.
- It uses multiple data-types for semantic values.
-* Exercises:: Ideas for improving the multi-function calculator.
+* Multi-function Calc:: Calculator with memory and trig functions.
+ It uses multiple data-types for semantic values.
+* Exercises:: Ideas for improving the multi-function calculator.
Reverse Polish Notation Calculator
-* Decls: Rpcalc Decls. Prologue (declarations) for rpcalc.
-* Rules: Rpcalc Rules. Grammar Rules for rpcalc, with explanation.
-* Lexer: Rpcalc Lexer. The lexical analyzer.
-* Main: Rpcalc Main. The controlling function.
-* Error: Rpcalc Error. The error reporting function.
-* Gen: Rpcalc Gen. Running Bison on the grammar file.
-* Comp: Rpcalc Compile. Run the C compiler on the output code.
+* Rpcalc Declarations:: Prologue (declarations) for rpcalc.
+* Rpcalc Rules:: Grammar Rules for rpcalc, with explanation.
+* Rpcalc Lexer:: The lexical analyzer.
+* Rpcalc Main:: The controlling function.
+* Rpcalc Error:: The error reporting function.
+* Rpcalc Generate:: Running Bison on the grammar file.
+* Rpcalc Compile:: Run the C compiler on the output code.
Grammar Rules for @code{rpcalc}
Location Tracking Calculator: @code{ltcalc}
-* Decls: Ltcalc Decls. Bison and C declarations for ltcalc.
-* Rules: Ltcalc Rules. Grammar rules for ltcalc, with explanations.
-* Lexer: Ltcalc Lexer. The lexical analyzer.
+* Ltcalc Declarations:: Bison and C declarations for ltcalc.
+* Ltcalc Rules:: Grammar rules for ltcalc, with explanations.
+* Ltcalc Lexer:: The lexical analyzer.
Multi-Function Calculator: @code{mfcalc}
-* Decl: Mfcalc Decl. Bison declarations for multi-function calculator.
-* Rules: Mfcalc Rules. Grammar rules for the calculator.
-* Symtab: Mfcalc Symtab. Symbol table management subroutines.
+* Mfcalc Declarations:: Bison declarations for multi-function calculator.
+* Mfcalc Rules:: Grammar rules for the calculator.
+* Mfcalc Symbol Table:: Symbol table management subroutines.
Bison Grammar Files
Outline of a Bison Grammar
-* Prologue:: Syntax and usage of the prologue.
+* 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.
+* 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
* 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.
+* Named References:: Using named references in actions.
Tracking Locations
Parser C-Language Interface
-* Parser Function:: How to call @code{yyparse} and what it returns.
-* Lexical:: You must supply a function @code{yylex}
- which reads tokens.
-* Error Reporting:: You must supply a function @code{yyerror}.
-* Action Features:: Special features for use in actions.
-* Internationalization:: How to let the parser speak in the user's
- native language.
+* 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}).
+* 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
* Contextual Precedence:: When an operator's precedence depends on context.
* Parser States:: The parser is a finite-state-machine with stack.
* Reduce/Reduce:: When two rules are applicable in the same situation.
-* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
+* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
* Generalized LR Parsing:: Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted. How to avoid it.
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
+* 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 @acronym{POSIX} safe?
-* I can't build Bison:: Troubleshooting
-* Where can I find help?:: Troubleshouting
-* Bug Reports:: Troublereporting
-* Other Languages:: Parsers in Java and others
-* Beta Testing:: Experimenting development versions
-* Mailing Lists:: Meeting other Bison users
+* 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.
+* Copying This Manual:: License for copying this manual.
@end detailmenu
@end menu
@cindex introduction
@dfn{Bison} is a general-purpose parser generator that converts an
-annotated context-free grammar into an @acronym{LALR}(1) or
-@acronym{GLR} parser for that grammar. 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.
+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.
+We begin with tutorial chapters that explain the basic concepts of
+using Bison and show three explained examples, each building on the
+last. If you don't know Bison or Yacc, start by reading these
+chapters. Reference chapters follow, which describe specific aspects
+of Bison in detail.
Bison was written primarily by Robert Corbett; Richard Stallman made it
Yacc-compatible. Wilfred Hansen of Carnegie Mellon University added
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 @acronym{LALR}(1)
+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 @acronym{GNU} programming tools, such as the @acronym{GNU} C
+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
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 @acronym{GPL}
+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.
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 @acronym{GNU} tools.
+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
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:: Tracking Locations.
-* Bison Parser:: What are Bison's input and output,
- how is the output used?
-* Stages:: Stages in writing and running Bison grammars.
-* Grammar Layout:: Overall structure of a Bison grammar file.
+* Language and Grammar:: Languages and context-free grammars,
+ as mathematical ideas.
+* Grammar in Bison:: How we represent grammars for Bison's sake.
+* Semantic Values:: Each token or syntactic grouping can have
+ a semantic value (the value of an integer,
+ the name of an identifier, etc.).
+* Semantic Actions:: Each rule can have an action containing C code.
+* GLR Parsers:: Writing parsers for general context-free languages.
+* Locations Overview:: Tracking Locations.
+* Bison Parser:: What are Bison's input and output,
+ how is the output used?
+* Stages:: Stages in writing and running Bison grammars.
+* Grammar Layout:: Overall structure of a Bison grammar file.
@end menu
@node Language and Grammar
recursive, but there must be at least one rule which leads out of the
recursion.
-@cindex @acronym{BNF}
+@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 ``@acronym{BNF}'', which was developed in
+is @dfn{Backus-Naur Form} or ``BNF'', which was developed in
order to specify the language Algol 60. Any grammar expressed in
-@acronym{BNF} is a context-free grammar. The input to Bison is
-essentially machine-readable @acronym{BNF}.
-
-@cindex @acronym{LALR}(1) grammars
-@cindex @acronym{LR}(1) grammars
-There are various important subclasses of context-free grammar. Although it
-can handle almost all context-free grammars, Bison is optimized for what
-are called @acronym{LALR}(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. Strictly speaking, that is a description of an
-@acronym{LR}(1) grammar, and @acronym{LALR}(1) involves additional
-restrictions that are
-hard to explain simply; but it is rare in actual practice to find an
-@acronym{LR}(1) grammar that fails to be @acronym{LALR}(1).
+BNF is a context-free grammar. The input to Bison is
+essentially machine-readable BNF.
+
+@cindex LALR(1) grammars
+@cindex IELR(1) grammars
+@cindex LR(1) 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{Mystery Conflicts, ,Mysterious Reduce/Reduce 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{Decl Summary,,lr.type}, to learn how.
-@cindex @acronym{GLR} parsing
-@cindex generalized @acronym{LR} (@acronym{GLR}) parsing
+@cindex GLR parsing
+@cindex generalized LR (GLR) parsing
@cindex ambiguous grammars
@cindex nondeterministic parsing
-Parsers for @acronym{LALR}(1) grammars are @dfn{deterministic}, meaning
+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
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 @acronym{GLR}
-parsing (for Generalized @acronym{LR}). Bison's @acronym{GLR} parsers
+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.
from the values of the two subexpressions.
@node GLR Parsers
-@section Writing @acronym{GLR} Parsers
-@cindex @acronym{GLR} parsing
-@cindex generalized @acronym{LR} (@acronym{GLR}) parsing
+@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 standard
-@acronym{LALR}(1) parsing algorithm cannot decide whether to apply a
+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
(@pxref{Reduce/Reduce}), and @dfn{shift/reduce} conflicts
(@pxref{Shift/Reduce}).
-To use a grammar that is not easily modified to be @acronym{LALR}(1), a
+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 @acronym{LR}
-(@acronym{GLR}) parser. These parsers handle Bison grammars that
+(@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 @acronym{LALR}(1) parsers. However, when
+declarations) identically to deterministic parsers. However, when
faced with unresolved shift/reduce and reduce/reduce conflicts,
-@acronym{GLR} parsers use the simple expedient of doing both,
+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
merged result.
@menu
-* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars.
-* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities.
-* GLR Semantic Actions:: Deferred semantic actions have special concerns.
-* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler.
+* 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 @acronym{GLR} on Unambiguous Grammars
-@cindex @acronym{GLR} parsing, unambiguous grammars
-@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, unambiguous grammars
+@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 @acronym{GLR} algorithm
-to parse grammars that are unambiguous, but fail to be @acronym{LALR}(1).
-Such grammars typically require more than one symbol of lookahead,
-or (in rare cases) fall into the category of grammars in which the
-@acronym{LALR}(1) algorithm throws away too much information (they are in
-@acronym{LR}(1), but not @acronym{LALR}(1), @ref{Mystery 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
@noindent
The original language standard allows only numeric
literals and constant identifiers for the subrange bounds (@samp{lo}
-and @samp{hi}), but Extended Pascal (@acronym{ISO}/@acronym{IEC}
+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
valid, and more-complicated cases can come up in practical programs.)
These two declarations look identical until the @samp{..} token.
-With normal @acronym{LALR}(1) one-token lookahead it is not
+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
work.
A simple solution to this problem is to declare the parser to
-use the @acronym{GLR} algorithm.
-When the @acronym{GLR} parser reaches the critical state, it
+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
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 @acronym{LALR}(1) algorithm actually allows
-for. In this example, @acronym{LALR}(2) would suffice, but also some cases
-that are not @acronym{LALR}(@math{k}) for any @math{k} can be handled this way.
+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 @acronym{GLR} parser can take quadratic or cubic worst-case time,
+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.
@end group
@end example
-When used as a normal @acronym{LALR}(1) grammar, Bison correctly complains
+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
type t = (a) .. b;
@end example
-The parser can be turned into a @acronym{GLR} parser, while also telling Bison
+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 input file (before the first
@samp{%%}):
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 @acronym{GLR},
+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 @acronym{GLR}
-splitting is only done where it is intended. A @acronym{GLR} parser
+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
-@acronym{LALR} parser statically choosing the wrong alternative in a
+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 @acronym{GLR} to shift the complications from the
+eliminated by using GLR to shift the complications from the
lexer to the parser. You must check the remaining cases for
correctness.
they cannot be used within the same enumerated type declaration.
@node Merging GLR Parses
-@subsection Using @acronym{GLR} to Resolve Ambiguities
-@cindex @acronym{GLR} parsing, ambiguous grammars
-@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, ambiguous grammars
+@subsection Using GLR to Resolve Ambiguities
+@cindex GLR parsing, ambiguous grammars
+@cindex generalized LR (GLR) parsing, ambiguous grammars
@findex %dprec
@findex %merge
@cindex conflicts
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
-@acronym{GLR} parser, it therefore splits the problem into two parses, one for
+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
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 @acronym{GLR} parser requires a specification in the
+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
@end example
@noindent
-This is another example of using @acronym{GLR} to parse an unambiguous
+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
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 @acronym{GLR} parser.
+action in a GLR parser.
@vindex yychar
-@cindex @acronym{GLR} parsers and @code{yychar}
+@cindex GLR parsers and @code{yychar}
@vindex yylval
-@cindex @acronym{GLR} parsers and @code{yylval}
+@cindex GLR parsers and @code{yylval}
@vindex yylloc
-@cindex @acronym{GLR} parsers and @code{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},
@xref{Lookahead, ,Lookahead Tokens}.
@findex yyclearin
-@cindex @acronym{GLR} parsers and @code{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.
memory referenced by @code{yylval}.
@findex YYERROR
-@cindex @acronym{GLR} parsers and @code{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 @acronym{GLR} operation, the effect of @code{YYERROR} is
-the same as its effect in an @acronym{LALR}(1) parser.
+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 @acronym{GLR} parsers.
+describes a special usage of @code{YYLLOC_DEFAULT} in GLR parsers.
@node Compiler Requirements
-@subsection Considerations when Compiling @acronym{GLR} Parsers
+@subsection Considerations when Compiling GLR Parsers
@cindex @code{inline}
-@cindex @acronym{GLR} parsers and @code{inline}
+@cindex GLR parsers and @code{inline}
-The @acronym{GLR} parsers require a compiler for @acronym{ISO} C89 or
+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
In some cases the Bison parser file includes system headers, and in
those cases your code should respect the identifiers reserved by those
-headers. On some non-
@acronym{GNU} hosts, @code{<alloca.h>}, @code{<malloc.h>},
+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
source file to try them.
@menu
-* RPN Calc:: Reverse polish notation calculator;
- a first example with no operator precedence.
-* Infix Calc:: Infix (algebraic) notation calculator.
- Operator precedence is introduced.
+* RPN Calc:: Reverse polish notation calculator;
+ a first example with no operator precedence.
+* Infix Calc:: Infix (algebraic) notation calculator.
+ Operator precedence is introduced.
* Simple Error Recovery:: Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$.
-* Multi-function Calc:: Calculator with memory and trig functions.
- It uses multiple data-types for semantic values.
-* Exercises:: Ideas for improving the multi-function calculator.
+* Multi-function Calc:: Calculator with memory and trig functions.
+ It uses multiple data-types for semantic values.
+* Exercises:: Ideas for improving the multi-function calculator.
@end menu
@node RPN Calc
@samp{.y} extension is a convention used for Bison input files.
@menu
-* Decls: Rpcalc Decls. Prologue (declarations) for rpcalc.
-* Rules: Rpcalc Rules. Grammar Rules for rpcalc, with explanation.
-* Lexer: Rpcalc Lexer. The lexical analyzer.
-* Main: Rpcalc Main. The controlling function.
-* Error: Rpcalc Error. The error reporting function.
-* Gen: Rpcalc Gen. Running Bison on the grammar file.
-* Comp: Rpcalc Compile. Run the C compiler on the output code.
+* Rpcalc Declarations:: Prologue (declarations) for rpcalc.
+* Rpcalc Rules:: Grammar Rules for rpcalc, with explanation.
+* Rpcalc Lexer:: The lexical analyzer.
+* Rpcalc Main:: The controlling function.
+* Rpcalc Error:: The error reporting function.
+* Rpcalc Generate:: Running Bison on the grammar file.
+* Rpcalc Compile:: Run the C compiler on the output code.
@end menu
-@node Rpcalc Decls
+@node Rpcalc Declarations
@subsection Declarations for @code{rpcalc}
Here are the C and Bison declarations for the reverse polish notation
tokens by calling the lexical analyzer. @xref{Lexical, ,The Lexical
Analyzer Function @code{yylex}}.
-Only a simple lexical analyzer is needed for the @acronym{RPN}
+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
The semantic value of the token (if it has one) is stored into the
global variable @code{yylval}, which is where the Bison parser will look
for it. (The C data type of @code{yylval} is @code{YYSTYPE}, which was
-defined at the beginning of the grammar; @pxref{Rpcalc Decls,
+defined at the beginning of the grammar; @pxref{Rpcalc Declarations,
,Declarations for @code{rpcalc}}.)
A token type code of zero is returned if the end-of-input is encountered.
cause the calculator program to exit. This is not clean behavior for a
real calculator, but it is adequate for the first example.
-@node Rpcalc Gen
+@node Rpcalc Generate
@subsection Running Bison to Make the Parser
@cindex running Bison (introduction)
analyzer.
@menu
-* Decls: Ltcalc Decls. Bison and C declarations for ltcalc.
-* Rules: Ltcalc Rules. Grammar rules for ltcalc, with explanations.
-* Lexer: Ltcalc Lexer. The lexical analyzer.
+* Ltcalc Declarations:: Bison and C declarations for ltcalc.
+* Ltcalc Rules:: Grammar rules for ltcalc, with explanations.
+* Ltcalc Lexer:: The lexical analyzer.
@end menu
-@node Ltcalc Decls
+@node Ltcalc Declarations
@subsection Declarations for @code{ltcalc}
The C and Bison declarations for the location tracking calculator are
Note that multiple assignment and nested function calls are permitted.
@menu
-* Decl: Mfcalc Decl. Bison declarations for multi-function calculator.
-* Rules: Mfcalc Rules. Grammar rules for the calculator.
-* Symtab: Mfcalc Symtab. Symbol table management subroutines.
+* Mfcalc Declarations:: Bison declarations for multi-function calculator.
+* Mfcalc Rules:: Grammar rules for the calculator.
+* Mfcalc Symbol Table:: Symbol table management subroutines.
@end menu
-@node Mfcalc Decl
+@node Mfcalc Declarations
@subsection Declarations for @code{mfcalc}
Here are the C and Bison declarations for the multi-function calculator.
%%
@end smallexample
-@node Mfcalc Symtab
+@node Mfcalc Symbol Table
@subsection The @code{mfcalc} Symbol Table
@cindex symbol table example
@end example
Comments enclosed in @samp{/* @dots{} */} may appear in any of the sections.
-As a @acronym{GNU} extension, @samp{//} introduces a comment that
+As a GNU extension, @samp{//} introduces a comment that
continues until end of line.
@menu
-* Prologue:: Syntax and usage of the prologue.
+* 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.
+* 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
@findex %code requires
@findex %code provides
@findex %code top
-(The prologue alternatives described here are experimental.
-More user feedback will help to determine whether they should become permanent
-features.)
The functionality of @var{Prologue} sections can often be subtle and
inflexible.
equivalent groupings. The symbol name is used in writing grammar rules.
By convention, it should be all lower case.
-Symbol names can contain letters, digits (not at the beginning),
-underscores and periods. Periods make sense only in nonterminals.
+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:
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
-@acronym{ASCII} environment, but then compile and run the resulting
+ASCII environment, but then compile and run the resulting
program in an environment that uses an incompatible character set like
-@acronym{EBCDIC}, the resulting program may not work because the tables
-generated by Bison will assume @acronym{ASCII} numeric values for
+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
-@acronym{ASCII} environment, so installers on platforms that are
-incompatible with @acronym{ASCII} must rebuild those files before
+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
* 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.
+* Named References:: Using named references in actions.
@end menu
@node Value 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
-@acronym{RPN} and infix calculator examples (@pxref{RPN Calc, ,Reverse Polish
+RPN and infix calculator examples (@pxref{RPN Calc, ,Reverse Polish
Notation Calculator}).
Bison normally uses the type @code{int} for semantic values if your
@cindex action
@vindex $$
@vindex $@var{n}
+@vindex $@var{name}
+@vindex $[@var{name}]
An action accompanies a syntactic rule and contains C code to be executed
each time an instance of that rule is recognized. The task of most actions
The C code in an action can refer to the semantic values of the components
matched by the rule with the construct @code{$@var{n}}, which stands for
the value of the @var{n}th component. The semantic value for the grouping
-being constructed is @code{$$}. Bison translates both of these
+being constructed is @code{$$}. In addition, the semantic values of
+symbols can be accessed with the named references construct
+@code{$@var{name}} or @code{$[@var{name}]}. Bison translates both of these
constructs into expressions of the appropriate type when it copies the
-actions into the parser file. @code{$$} is translated to a modifiable
+actions into the parser file. @code{$$} (or @code{$@var{name}}, when it
+stands for the current grouping) is translated to a modifiable
lvalue, so it can be assigned to.
Here is a typical example:
@end group
@end example
+Or, in terms of named references:
+
+@example
+@group
+exp[result]: @dots{}
+ | exp[left] '+' exp[right]
+ @{ $result = $left + $right; @}
+@end group
+@end example
+
@noindent
This rule constructs an @code{exp} from two smaller @code{exp} groupings
connected by a plus-sign token. In the action, @code{$1} and @code{$3}
+(@code{$left} and @code{$right})
refer to the semantic values of the two component @code{exp} groupings,
which are the first and third symbols on the right hand side of the rule.
-The sum is stored into @code{$$} so that it becomes the semantic value of
+The sum is stored into @code{$$} (@code{$result}) so that it becomes the
+semantic value of
the addition-expression just recognized by the rule. If there were a
useful semantic value associated with the @samp{+} token, it could be
referred to as @code{$2}.
+@xref{Named References,,Using Named References}, for more information
+about using the named references construct.
+
Note that the vertical-bar character @samp{|} is really a rule
separator, and actions are attached to a single rule. This is a
difference with tools like Flex, for which @samp{|} stands for either
Now Bison can execute the action in the rule for @code{subroutine} without
deciding which rule for @code{compound} it will eventually use.
+@node Named References
+@subsection Using Named References
+@cindex named references
+
+While every semantic value can be accessed with positional references
+@code{$@var{n}} and @code{$$}, it's often much more convenient to refer to
+them by name. 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
+The positional @code{$$}, @code{@@$}, @code{$n}, and @code{@@n} can be
+mixed with @code{$name} and @code{@@name} arbitrarily. For example:
+
+@example
+@group
+invocation: op '(' args ')'
+ @{ $$ = 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 / $right; @}
+@end group
+@end example
+
+@noindent
+Explicit names may be declared for RHS and for LHS symbols as well. In order
+to access a semantic value generated by a mid-rule action, an explicit name
+may also be declared by putting a bracketed name after the closing brace of
+the mid-rule action code:
+@example
+@group
+exp[res]: exp[x] '+' @{$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, $[then.stmt]); @}
+@end group
+@end example
+
+It often happens that named references are followed by a dot, dash or other
+C punctuation marks and operators. By default, Bison will read
+@code{$name.suffix} as a reference to symbol value @code{$name} followed by
+@samp{.suffix}, i.e., an access to the @samp{suffix} field of the semantic
+value. In order to force Bison to recognize @code{name.suffix} in its entirety
+as the name of a semantic value, bracketed syntax @code{$[name.suffix]}
+must be used.
+
+
@node Locations
@section Tracking Locations
@cindex location
@} YYLTYPE;
@end example
-At the beginning of the parsing, Bison initializes all these fields to 1
-for @code{yylloc}.
+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 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.
@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,,Using Named References}, for more information
+about using the named references construct.
+
Here is a basic example using the default data type for locations:
@example
@node Location Default Action
@subsection Default Action for Locations
@vindex YYLLOC_DEFAULT
-@cindex @acronym{GLR} parsers and @code{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
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 @acronym{GLR}
+Before reporting an unresolvable syntactic ambiguity, a GLR
parser invokes @code{YYLLOC_DEFAULT} recursively to compute the location
of that ambiguity.
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 @acronym{GLR} parser reports an ambiguity, which of multiple candidate
+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
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 @acronym{POSIX}, a tag is allowed after the
+As an extension to POSIX, a tag is allowed after the
@code{union}. For example:
@example
@code{union value}. If you do not specify a tag, it defaults to
@code{YYSTYPE}.
-As another extension to @acronym{POSIX}, you may specify multiple
+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.
Bison reports an error if the number of shift/reduce conflicts differs
from @var{n}, or if there are any reduce/reduce conflicts.
-For normal @acronym{LALR}(1) parsers, reduce/reduce conflicts are more
+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 @acronym{GLR}
+reduce/reduce conflicts for these parsers. With GLR
parsers, however, both kinds of conflicts are routine; otherwise,
-there would be no need to use @acronym{GLR} parsing. Therefore, it is
+there would be no need to use GLR parsing. Therefore, it is
also possible to specify an expected number of reduce/reduce conflicts
-in @acronym{GLR} parsers, using the declaration:
+in GLR parsers, using the declaration:
@example
%expect-rr @var{n}
@item
Add an @code{%expect} declaration, copying the number @var{n} from the
-number which Bison printed. With @acronym{GLR} parsers, add an
+number which Bison printed. With GLR parsers, add an
@code{%expect-rr} declaration as well.
@end itemize
-Now Bison will warn you if you introduce an unexpected conflict, but
-will keep silent otherwise.
+Now Bison will report an error if you introduce an unexpected conflict,
+but will keep silent otherwise.
@node Start Decl
@subsection The Start-Symbol
@subsection A Push Parser
@cindex push parser
@cindex push parser
-@findex %define api.push_pull
+@findex %define api.push-pull
(The current push parsing interface is experimental and may evolve.
More user feedback will help to stabilize it.)
Normally, Bison generates a pull parser.
The following Bison declaration says that you want the parser to be a push
-parser (@pxref{Decl Summary,,%define api.push_pull}):
+parser (@pxref{Decl Summary,,%define api.push-pull}):
@example
-%define api.push_pull "push"
+%define api.push-pull push
@end example
In almost all cases, you want to ensure that your push parser is also
@example
%define api.pure
-%define api.push_pull "push"
+%define api.push-pull push
@end example
There is a major notable functional difference between the pure push parser
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
+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{%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
@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"}.
+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
For a detailed discussion, see @ref{Prologue Alternatives}.
For Java, the default location is inside the parser class.
-
-(Like all the Yacc prologue alternatives, this directive is experimental.
-More user feedback will help to determine whether it should become a permanent
-feature.)
@end deffn
@deffn {Directive} %code @var{qualifier} @{@var{code}@}
@var{qualifier} identifies the purpose of @var{code} and thus the location(s)
where Bison should generate it.
-Not all values of @var{qualifier} are available for all target languages:
+Not all @var{qualifier}s are accepted for all target languages.
+Unaccepted @var{qualifier}s produce an error.
+Some of the accepted @var{qualifier}s are:
@itemize @bullet
@item requires
@end itemize
@end itemize
-(Like all the Yacc prologue alternatives, this directive is experimental.
-More user feedback will help to determine whether it should become a permanent
-feature.)
-
@cindex Prologue
For a detailed discussion of how to use @code{%code} in place of the
traditional Yacc prologue for C/C++, see @ref{Prologue Alternatives}.
@deffn {Directive} %debug
In the parser file, define the macro @code{YYDEBUG} to 1 if it is not
already defined, so that the debugging facilities are compiled.
-@end deffn
@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.
-The possible choices for @var{variable}, as well as their meanings, depend on
-the selected target language and/or the parser skeleton (@pxref{Decl
-Summary,,%language}).
-Bison will warn if a @var{variable} is defined multiple times.
+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}]}.
+
+@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}"} is always equivalent to specifying it as
+Omitting @code{"@var{value}"} entirely is always equivalent to specifying
@code{""}.
-Some @var{variable}s may be used as Booleans.
+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 @code{true}
-@item @code{"@var{value}"} is omitted (or is @code{""}).
-This is equivalent to @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 @code{@var{value}} is @code{false}.
@item @var{variable} is never defined.
-In this case, Bison selects a default value, which may depend on the selected
-target language and/or parser skeleton.
+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
@item Accepted Values: Boolean
-@item Default Value: @code{"false"}
+@item Default Value: @code{false}
@end itemize
-@item api.push_pull
-@findex %define api.push_pull
+@item api.push-pull
+@findex %define api.push-pull
@itemize @bullet
-@item Language(s): C (LALR(1) only)
+@item Language(s): C (deterministic parsers only)
-@item Purpose: Requests a pull parser, a push parser, or both.
+@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 Accepted Values: @code{pull}, @code{push}, @code{both}
-@item Default Value: @code{"pull"}
+@item Default Value: @code{pull}
@end itemize
-@item lr.keep_unreachable_states
-@findex %define lr.keep_unreachable_states
+@c ================================================== lr.default-reductions
+
+@item lr.default-reductions
+@cindex default reductions
+@findex %define lr.default-reductions
+@cindex delayed syntax errors
+@cindex syntax errors delayed
+@cindex LAC
+@findex %nonassoc
@itemize @bullet
@item Language(s): all
-@item Purpose: Requests that Bison allow unreachable parser states to remain in
-the parser tables.
+@item Purpose: Specify the kind of states that are permitted to
+contain default reductions.
+That is, in such a state, Bison selects the reduction with the largest
+lookahead set to be the default parser action and then removes that
+lookahead set.
+(The ability to specify where default reductions should be used is
+experimental.
+More user feedback will help to stabilize it.)
+
+@item Accepted Values:
+@itemize
+@item @code{all}.
+This is the traditional Bison behavior.
+The main advantage is a significant decrease in the size of the parser
+tables.
+The disadvantage is that, when the generated parser encounters a
+syntactically unacceptable token, the parser might then perform
+unnecessary default reductions before it can detect the syntax error.
+Such delayed syntax error detection is usually inherent in
+LALR and IELR parser tables anyway due to
+LR state merging (@pxref{Decl Summary,,lr.type}).
+Furthermore, the use of @code{%nonassoc} can contribute to delayed
+syntax error detection even in the case of canonical LR.
+As an experimental feature, delayed syntax error detection can be
+overcome in all cases by enabling LAC (@pxref{Decl
+Summary,,parse.lac}, for details, including a discussion of the effects
+of delayed syntax error detection).
+
+@item @code{consistent}.
+@cindex consistent states
+A consistent state is a state that has only one possible action.
+If that action is a reduction, then the parser does not need to request
+a lookahead token from the scanner before performing that action.
+However, the parser recognizes the ability to ignore the lookahead token
+in this way only when such a reduction is encoded as a default
+reduction.
+Thus, if default reductions are permitted only in consistent states,
+then a canonical LR parser that does not employ
+@code{%nonassoc} detects a syntax error as soon as it @emph{needs} the
+syntactically unacceptable token from the scanner.
+
+@item @code{accepting}.
+@cindex accepting state
+In the accepting state, the default reduction is actually the accept
+action.
+In this case, a canonical LR parser that does not employ
+@code{%nonassoc} detects a syntax error as soon as it @emph{reaches} the
+syntactically unacceptable token in the input.
+That is, it does not perform any extra reductions.
+@end itemize
+
+@item Default Value:
+@itemize
+@item @code{accepting} if @code{lr.type} is @code{canonical-lr}.
+@item @code{all} 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.
Bison considers a state to be unreachable if there exists no sequence of
transitions from the start state to that state.
A state can become unreachable during conflict resolution if Bison disables a
@item Accepted Values: Boolean
-@item Default Value: @code{"false"}
+@item Default Value: @code{false}
@item Caveats:
@end itemize
@end itemize
+@c ================================================== lr.type
+
+@item lr.type
+@findex %define lr.type
+@cindex LALR
+@cindex IELR
+@cindex LR
+
+@itemize @bullet
+@item Language(s): all
+
+@item Purpose: Specify the type of parser tables within the
+LR(1) family.
+(This feature is experimental.
+More user feedback will help to stabilize it.)
+
+@item Accepted Values:
+@itemize
+@item @code{lalr}.
+While Bison generates LALR parser tables by default for
+historical reasons, IELR or canonical LR is almost
+always preferable for deterministic parsers.
+The trouble is that LALR parser tables can suffer from
+mysterious conflicts and thus may not accept the full set of sentences
+that IELR and canonical LR accept.
+@xref{Mystery Conflicts}, for details.
+However, there are at least two scenarios where LALR may be
+worthwhile:
+@itemize
+@cindex GLR with LALR
+@item 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 use of LALR parser tables is guaranteed not
+to alter the language accepted by the parser.
+LALR parser tables are the smallest parser tables Bison can
+currently generate, so they may be preferable.
+Nevertheless, once you begin to resolve conflicts statically,
+GLR begins to behave more like a deterministic parser, and so
+IELR and canonical LR can be helpful to avoid
+LALR's mysterious behavior.
+
+@item Occasionally during development, an especially malformed grammar
+with a major recurring flaw may severely impede the IELR or
+canonical LR parser table generation algorithm.
+LALR can be a quick way to generate parser tables in order to
+investigate such problems while ignoring the more subtle differences
+from IELR and canonical LR.
+@end itemize
+
+@item @code{ielr}.
+IELR is a minimal LR algorithm.
+That is, given any grammar (LR or non-LR),
+IELR and canonical LR always accept exactly the same
+set of sentences.
+However, as for LALR, the number of parser states is often an
+order of magnitude less for IELR 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 can significantly reduce the complexity of developing of a grammar.
+
+@item @code{canonical-lr}.
+@cindex delayed syntax errors
+@cindex syntax errors delayed
+@cindex LAC
+@findex %nonassoc
+While inefficient, canonical LR parser tables can be an
+interesting means to explore a grammar because they have a property that
+IELR and LALR tables do not.
+That is, if @code{%nonassoc} is not used and default reductions are left
+disabled (@pxref{Decl Summary,,lr.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 using LAC (@pxref{Decl
+Summary,,parse.lac}) are also able to achieve this behavior without
+sacrificing @code{%nonassoc} or default reductions.
+@end itemize
+
+@item Default Value: @code{lalr}
+@end itemize
+
@item namespace
@findex %define namespace
@itemize
@item Languages(s): C++
-@item Purpose: Specifies the namespace for the parser class.
+@item Purpose: Specify the namespace for the parser class.
For example, if you specify:
@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
+@cindex LAC
+@cindex lookahead correction
+
+@itemize
+@item Languages(s): C
+
+@item Purpose: Enable LAC (lookahead correction) to improve
+syntax error handling.
+
+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 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 (with @code{%error-verbose} or
+@code{#define YYERROR_VERBOSE}), 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, and parser state merging. Thus,
+IELR and LALR suffer the most. Canonical
+LR can suffer only if @code{%nonassoc} is used or if default
+reductions are enabled for inconsistent states.
+
+LAC is a new mechanism within the parsing algorithm that
+completely solves these problems for canonical LR,
+IELR, and LALR without sacrificing @code{%nonassoc},
+default reductions, or state mering. Conceptually, the 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{Decl
+Summary,,lr.default-reductions}) affects how soon the parser detects a
+syntax error: when it @emph{reaches} an erroneous token or when it
+eventually @emph{needs} that token as a lookahead. The latter behavior
+is probably more intuitive, so Bison currently provides no way to
+achieve the former behavior while default reductions are fully enabled.
+
+Thus, when LAC is in use, for some fixed decision of whether
+to enable default reductions in consistent states, canonical
+LR and IELR behave 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.
+
+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 proven
+insignificant for practical grammars.
+
+@item Accepted Values: @code{none}, @code{full}
+
+@item Default Value: @code{none}
+@end itemize
@end itemize
@end deffn
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
@deffn {Directive} %skeleton "@var{file}"
Specify the skeleton to use.
-You probably don't need this option unless you are developing Bison.
-You should use @code{%language} if you want to specify the skeleton for a
-different language, because it is clearer and because it will always choose the
-correct skeleton for non-deterministic or push parsers.
+@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.
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.
+* 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
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.
+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)
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"}
+stream. This function is available if the @code{%define api.push-pull both}
declaration is used.
@xref{Push Decl, ,A Push Parser}.
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.
+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 fuction will return a valid parser instance if there was memory available
+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.
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.
+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)
@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}).
+* 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
Obviously, in location tracking pure parsers, @code{yyerror} should have
an access to the current location.
-This is indeed the case for the @acronym{GLR}
+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:
void yyerror (int *nastiness, char const *msg); /* GLR parsers. */
@end example
-Finally, @acronym{GLR} and Yacc parsers share the same @code{yyerror} calling
+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.
@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 @acronym{GLR} parsers.
+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.
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 @acronym{UTF}-8
+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 @acronym{GNU} Autoconf and
-@acronym{GNU} Automake.
+steps. Here we assume a package that uses GNU Autoconf and
+GNU Automake.
@enumerate
@item
@cindex bison-i18n.m4
-Into the directory containing the @acronym{GNU} Autoconf macros used
+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.
* Contextual Precedence:: When an operator's precedence depends on context.
* Parser States:: The parser is a finite-state-machine with stack.
* Reduce/Reduce:: When two rules are applicable in the same situation.
-* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
+* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
* Generalized LR Parsing:: Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted. How to avoid it.
@end menu
Algol 60 and is called the ``dangling @code{else}'' ambiguity.
To avoid warnings from Bison about predictable, legitimate shift/reduce
-conflicts, use the @code{%expect @var{n}} declaration. There will be no
-warning as long as the number of shift/reduce conflicts is exactly @var{n}.
+conflicts, use the @code{%expect @var{n}} declaration.
+There will be no warning as long as the number of shift/reduce conflicts
+is exactly @var{n}, and Bison will report an error if there is a
+different number.
@xref{Expect Decl, ,Suppressing Conflict Warnings}.
The definition of @code{if_stmt} above is solely to blame for the
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 @acronym{LR}(1).
-
-@cindex @acronym{LR}(1)
-@cindex @acronym{LALR}(1)
-However, Bison, like most parser generators, cannot actually handle all
-@acronym{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
+@code{ID} follows. In other words, this grammar is LR(1).
+
+@cindex LR(1)
+@cindex LALR(1)
+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 @acronym{LALR}(1).
-
-In general, it is better to fix deficiencies than to document them. But
-this particular deficiency is intrinsically hard to fix; parser
-generators that can handle @acronym{LR}(1) grammars are hard to write
-and tend to
-produce parsers that are very large. In practice, Bison is more useful
-as it is now.
-
-When the problem arises, you can often fix it by identifying the two
-parser states that are being confused, and adding something to make them
-look distinct. In the above example, adding one rule to
+occurrence means that the grammar is not LALR(1).
+
+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 generation algorithm.
+Either IELR(1) or canonical LR(1) would suffice, but
+the former is more efficient and easier to debug during development.
+@xref{Decl Summary,,lr.type}, 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
;
@end example
-For a more detailed exposition of @acronym{LALR}(1) parsers and parser
+For a more detailed exposition of LALR(1) parsers and parser
generators, please see:
Frank DeRemer and Thomas Pennello, Efficient Computation of
-@acronym{LALR}(1) Look-Ahead Sets, @cite{@acronym{ACM} Transactions on
+LALR(1) Look-Ahead Sets, @cite{ACM Transactions on
Programming Languages and Systems}, Vol.@: 4, No.@: 4 (October 1982),
pp.@: 615--649 @uref{http://doi.acm.org/10.1145/69622.357187}.
@node Generalized LR Parsing
-@section Generalized @acronym{LR} (@acronym{GLR}) Parsing
-@cindex @acronym{GLR} parsing
-@cindex generalized @acronym{LR} (@acronym{GLR}) parsing
+@section Generalized LR (GLR) Parsing
+@cindex GLR parsing
+@cindex generalized LR (GLR) parsing
@cindex ambiguous grammars
@cindex nondeterministic parsing
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{Mystery Conflicts}),
-there are languages where Bison's particular choice of how to
+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 @acronym{LR} (or @acronym{GLR}). A Bison @acronym{GLR}
+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 @acronym{GLR} parser encounters such 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 @acronym{GLR} parsing stack is what is in effect a tree 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
stream.
Whenever the parser makes a transition from having multiple
-states to having one, it reverts to the normal @acronym{LALR}(1) parsing
+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
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 @acronym{GLR} parsing tree that
-permits the processing of any @acronym{LALR}(1) grammar in linear time (in the
+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
-@acronym{LALR}(1)) grammar in
+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
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 @acronym{LALR}(1) portions of a
-grammar, in particular, it is only slightly slower than with the default
-Bison parser.
+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 @acronym{GLR} parsers, please see: Elizabeth
+For a more detailed exposition of GLR parsers, please see: Elizabeth
Scott, Adrian Johnstone and Shamsa Sadaf Hussain, Tomita-Style
-Generalised @acronym{LR} Parsers, Royal Holloway, University of
+Generalised LR Parsers, Royal Holloway, University of
London, Department of Computer Science, TR-00-12,
@uref{http://www.cs.rhul.ac.uk/research/languages/publications/tomita_style_1.ps},
(2000-12-24).
@vindex YYINITDEPTH
You can control how much stack is allocated initially by defining the
-macro @code{YYINITDEPTH} to a positive integer. For the C
-@acronym{LALR}(1) parser, this value must be a compile-time constant
+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 semantical differences between C and C++, the
-@acronym{LALR}(1) parsers in C produced by Bison cannot grow when compiled
+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.
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 @acronym{GNU} C is to have two different token types,
+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
@command{bison} reports:
@example
-calc.y: warning: 1 nonterminal and 1 rule useless in grammar
+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
'+' . exp}. Since there is no default action, any other token than
those listed above will trigger a syntax error.
+@cindex accepting state
The state 3 is named the @dfn{final state}, or the @dfn{accepting
state}:
NUM)}, which corresponds to shifting @samp{/}, or as @samp{(NUM + NUM) /
NUM}, which corresponds to reducing rule 1.
-Because in @acronym{LALR}(1) parsing a single decision can be made, Bison
+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 in between
square brackets.
@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 @acronym{POSIX} Yacc. You could use
+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 @acronym{POSIX} compliant too.
+,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 @acronym{POSIX} and Yacc portability matter to
+preprocessor. Unless POSIX and Yacc portability matter to
you, this is
the preferred solution.
@end table
value (from @code{yylval}).
Here is an example of @code{YYPRINT} suitable for the multi-function
-calculator (@pxref{Mfcalc Decl, ,Declarations for @code{mfcalc}}):
+calculator (@pxref{Mfcalc Declarations, ,Declarations for @code{mfcalc}}):
@smallexample
%@{
@noindent
will produce @file{output.c++} and @file{outfile.h++}.
-For compatibility with @acronym{POSIX}, the standard Bison
+For compatibility with POSIX, the standard Bison
distribution also contains a shell script called @command{yacc} that
invokes Bison with the @option{-y} option.
file name conventions, so that the parser output file is called
@file{y.tab.c}, and the other outputs are called @file{y.output} and
@file{y.tab.h}.
-Also, if generating an @acronym{LALR}(1) parser in C, generate @code{#define}
+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 @acronym{POSIX}:
+distribution contains such a script for compatibility with POSIX:
@example
#! /bin/sh
like @samp{%glr-parser}, Bison might not be Yacc-compatible even if
this option is specified.
-@item -W
-@itemx --warnings
+@item -W [@var{category}]
+@itemx --warnings[=@var{category}]
Output warnings falling in @var{category}. @var{category} can be one
of:
@table @code
@item yacc
-Incompatibilities with @acronym{POSIX} Yacc.
+Incompatibilities with POSIX Yacc.
@item all
All the warnings.
@end table
A category can be turned off by prefixing its name with @samp{no-}. For
-instance, @option{-Wno-syntax} will hide the warnings about unused
-variables.
+instance, @option{-Wno-yacc} will hide the warnings about
+POSIX Yacc incompatibilities.
@end table
@noindent
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{Decl Summary, ,%define}) 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
+makefiles 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
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}.
Specify the skeleton to use, similar to @code{%skeleton}
(@pxref{Decl Summary, , Bison Declaration Summary}).
-You probably don't need this option unless you are developing Bison.
-You should use @option{--language} if you want to specify the skeleton for a
-different language, because it is clearer and because it will always
-choose the correct skeleton for non-deterministic or push parsers.
+@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.
@table @code
@item state
Description of the grammar, conflicts (resolved and unresolved), and
-@acronym{LALR} automaton.
+parser's automaton.
@item lookahead
Implies @code{state} and augments the description of the automaton with
The other output files' names are constructed from @var{file} as
described under the @samp{-v} and @samp{-d} options.
-@item -g[@var{file}]
+@item -g [@var{file}]
@itemx --graph[=@var{file}]
-Output a graphical representation of the @acronym{LALR}(1) grammar
+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, @acronym{DOT}} format.
+@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}]
+@item -x [@var{file}]
@itemx --xml[=@var{file}]
-Output an XML report of the @acronym{LALR}(1) automaton computed by Bison.
+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}.
@node Option Cross Key
@section Option Cross Key
-@c FIXME: How about putting the directives too?
Here is a list of options, alphabetized by long option, to help you find
-the corresponding short option.
+the corresponding short option and directive.
-@multitable {@option{--defines=@var{defines-file}}} {@option{-b @var{file-prefix}XXX}}
-@headitem Long Option @tab Short Option
+@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
The Yacc library contains default implementations of the
@code{yyerror} and @code{main} functions. These default
-implementations are normally not useful, but @acronym{POSIX} requires
+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 @acronym{GNU} General
+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
@node C++ Bison Interface
@subsection C++ Bison Interface
-@c - %language "C++"
+@c - %skeleton "lalr1.cc"
@c - Always pure
@c - initial action
-The C++ @acronym{LALR}(1) parser is selected using the language directive,
-@samp{%language "C++"}, or the synonymous command-line option
-@option{--language=c++}.
+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}
it describes an additional member of the parser class, and an
additional argument for its constructor.
-@defcv {Type} {parser} {semantic_value_type}
-@defcvx {Type} {parser} {location_value_type}
+@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 definition of the tokens as the
+@code{yytokentype} enumeration. To refer to the token @code{FOO}, the
+scanner should 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.
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_value_type& @var{yylval}, location_type& @var{yylloc}, @var{type1} @var{arg1}, ...)
+@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.
@subsubsection Calc++ Parser
The parser definition file @file{calc++-parser.yy} starts by asking for
-the C++ LALR(1) 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.
+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
-%language "C++" /* -*- C++ -*- */
+%skeleton "lalr1.cc" /* -*- C++ -*- */
%require "@value{VERSION}"
%defines
%define parser_class_name "calcxx_parser"
@noindent
Then we request the location tracking feature, and initialize the
-first location's file name. Afterwards new locations are computed
+first location's file name. Afterward new locations are computed
relatively to the previous locations: the file name will be
automatically propagated.
@example
%@{ /* -*- C++ -*- */
# include <cstdlib>
-# include <errno.h>
-# include <limits.h>
+# include <cerrno>
+# include <climits>
# include <string>
# include "calc++-driver.hh"
# include "calc++-parser.hh"
@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
+* 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
Java.
Push parsers are currently unsupported in Java and @code{%define
-api.push_pull} have no effect.
+api.push-pull} have no effect.
-@acronym{GLR} parsers are currently unsupported in Java. Do not use the
+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
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}}.
+@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
@end deftypeivar
@deftypeop {Constructor} {Location} {} Location (Position @var{loc})
-Create a @code{Location} denoting an empty range located at a given point.
+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})
@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 ther methods in the
+value and location are saved and returned by the their methods in the
interface.
Use @code{%define lex_throws} to specify any uncaught exceptions.
@end deftypemethod
@deftypemethod {Lexer} {Object} getLVal ()
-Return the semantical value of the last token that yylex returned.
+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 deffn
@deffn {Statement} {return YYERROR;}
-Start error recovery without printing an error message.
-@xref{Error Recovery}.
-@end deffn
-
-@deffn {Statement} {return YYFAIL;}
-Print an error message and start error recovery.
+Start error recovery without printing an error message.
@xref{Error Recovery}.
@end deffn
@item
Java lacks unions, so @code{%union} has no effect. Instead, semantic
values have a common base type: @code{Object} or as specified by
-@code{%define stype}. Angle backets on @code{%token}, @code{type},
+@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.
@pxref{Java Action Features}.
@item
-The prolog declarations have a different meaning than in C/C++ code.
+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
* 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 @acronym{POSIX} safe?
+* Secure? Conform?:: Is Bison POSIX safe?
* I can't build Bison:: Troubleshooting
* Where can I find help?:: Troubleshouting
* Bug Reports:: Troublereporting
execute simple instructions one after the others.
@cindex abstract syntax tree
-@cindex @acronym{AST}
+@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{@acronym{AST}} for short. Then, walking through this 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.
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
-@acronym{POSIX} specification for Yacc. If you run into problems,
+POSIX specification for Yacc. If you run into problems,
please send us a bug report.
@node I can't build Bison
side of the rule. @xref{Locations, , Locations Overview}.
@end deffn
+@deffn {Variable} @@@var{name}
+In an action, the location of a symbol addressed by name.
+@xref{Locations, , Locations Overview}.
+@end deffn
+
+@deffn {Variable} @@[@var{name}]
+In an action, the location of a symbol addressed by name.
+@xref{Locations, , Locations Overview}.
+@end deffn
+
@deffn {Variable} $$
In an action, the semantic value of the left-hand side of the rule.
@xref{Actions}.
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.
Equip the parser for debugging. @xref{Decl 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}
@deffn {Directive} %define @var{define-variable}
@deffnx {Directive} %define @var{define-variable} @var{value}
+@deffnx {Directive} %define @var{define-variable} "@var{value}"
Define a variable to adjust Bison's behavior.
@xref{Decl Summary,,%define}.
@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
-@acronym{GLR} Parsers}.
+GLR Parsers}.
@end deffn
@deffn {Symbol} $end
@end deffn
@deffn {Directive} %glr-parser
-Bison declaration to produce a @acronym{GLR} parser. @xref{GLR
-Parsers, ,Writing @acronym{GLR} Parsers}.
+Bison declaration to produce a GLR parser. @xref{GLR
+Parsers, ,Writing GLR Parsers}.
@end deffn
@deffn {Directive} %initial-action
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 @acronym{GLR} Parsers}.
+@xref{GLR Parsers, ,Writing GLR Parsers}.
@end deffn
@deffn {Directive} %name-prefix "@var{prefix}"
@end deffn
@deffn {Macro} YYSTACK_USE_ALLOCA
-Macro used to control the use of @code{alloca} when the C
-@acronym{LALR}(1) parser needs to extend its stacks. If defined to 0,
+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,
@cindex glossary
@table @asis
-@item Backus-Naur Form (@acronym{BNF}; also called ``Backus Normal Form'')
+@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{Decl Summary,,lr.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
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{Decl Summary,,lr.default-reductions}.
+
@item Dynamic allocation
Allocation of memory that occurs during execution, rather than at
compile time or on entry to a function.
parsed, and the states correspond to various stages in the grammar
rules. @xref{Algorithm, ,The Bison Parser Algorithm}.
-@item Generalized @acronym{LR} (@acronym{GLR})
+@item Generalized LR (GLR)
A parsing algorithm that can handle all context-free grammars, including those
-that are not @acronym{LALR}(1). It resolves situations that Bison's
-usual @acronym{LALR}(1)
+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
-@acronym{LR} Parsing}.
+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)
+A minimal LR(1) parser table generation 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 of a grammar.
+@xref{Decl Summary,,lr.type}.
+
@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{Decl Summary,,parse.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.
A token already read but not yet shifted. @xref{Lookahead, ,Lookahead
Tokens}.
-@item @acronym{LALR}(1)
+@item LALR(1)
The class of context-free grammars that Bison (like most other parser
-generators) can handle; a subset of @acronym{LR}(1). @xref{Mystery
-Conflicts, ,Mysterious Reduce/Reduce Conflicts}.
+generators) can handle by default; a subset of LR(1).
+@xref{Mystery Conflicts, ,Mysterious Reduce/Reduce Conflicts}.
-@item @acronym{LR}(1)
+@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.
@bye
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