@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, 2009 Free
-Software Foundation, Inc.
+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
+* Copying:: The GNU General Public License says
how you can copy and share Bison.
Tutorial sections:
* 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).
+* Invocation:: How to run Bison (to produce the parser implementation).
* Other Languages:: Creating C++ and Java parsers.
* FAQ:: Frequently Asked Questions
* Table of Symbols:: All the keywords of the Bison language are explained.
* Stages:: Stages in writing and running Bison grammars.
* Grammar Layout:: Overall structure of a Bison grammar file.
-Writing @acronym{GLR} Parsers
+Writing GLR Parsers
-* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars.
-* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities.
+* 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:: @acronym{GLR} parsers require a modern C compiler.
+* Compiler Requirements:: GLR parsers require a modern C compiler.
Examples
* 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
* Pure Decl:: Requesting a reentrant parser.
* Push Decl:: Requesting a push parser.
* Decl Summary:: Table of all Bison declarations.
+* %code Summary:: Inserting code into the parser source.
Parser C-Language Interface
* 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
@cindex introduction
@dfn{Bison} is a general-purpose parser generator that converts an
-annotated context-free grammar into a deterministic or @acronym{GLR}
-parser employing @acronym{LALR}(1), @acronym{IELR}(1), or canonical
-@acronym{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.
-
-We begin with tutorial chapters that explain the basic concepts of using
-Bison and show three explained examples, each building on the last. If you
-don't know Bison or Yacc, start by reading these chapters. Reference
-chapters follow which describe specific aspects of Bison in detail.
-
-Bison was written primarily by Robert Corbett; Richard Stallman made it
-Yacc-compatible. Wilfred Hansen of Carnegie Mellon University added
-multi-character string literals and other features.
+annotated context-free grammar into a deterministic LR or generalized
+LR (GLR) parser employing LALR(1) parser tables. As an experimental
+feature, Bison can also generate IELR(1) or canonical LR(1) parser
+tables. Once you are proficient with Bison, you can use it to develop
+a wide range of language parsers, from those used in simple desk
+calculators to complex programming languages.
+
+Bison is upward compatible with Yacc: all properly-written Yacc
+grammars ought to work with Bison with no change. Anyone familiar
+with Yacc should be able to use Bison with little trouble. You need
+to be fluent in C or C++ programming in order to use Bison or to
+understand this manual. Java is also supported as an experimental
+feature.
+
+We begin with tutorial chapters that explain the basic concepts of
+using Bison and show three explained examples, each building on the
+last. If you don't know Bison or Yacc, start by reading these
+chapters. Reference chapters follow, which describe specific aspects
+of Bison in detail.
+
+Bison was written originally by Robert Corbett. Richard Stallman made
+it Yacc-compatible. Wilfred Hansen of Carnegie Mellon University
+added multi-character string literals and other features. Since then,
+Bison has grown more robust and evolved many other new features thanks
+to the hard work of a long list of volunteers. For details, see the
+@file{THANKS} and @file{ChangeLog} files included in the Bison
+distribution.
This edition corresponds to version @value{VERSION} of Bison.
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
policy decision; it resulted from applying the usual General Public
License to all of the Bison source code.
-The output of the Bison utility---the Bison parser file---contains a
-verbatim copy of a sizable piece of Bison, which is the code for the
-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}
-terms to the skeleton code for the parser's implementation,
+The main output of the Bison utility---the Bison parser implementation
+file---contains a verbatim copy of a sizable piece of Bison, which is
+the code for the parser's implementation. (The actions from your
+grammar are inserted into this implementation at one point, but most
+of the rest of the implementation is not changed.) When we applied
+the GPL terms to the skeleton code for the parser's implementation,
the effect was to restrict the use of Bison output to free software.
We didn't change the terms because of sympathy for people who want to
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
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}.
+BNF is a context-free grammar. The input to Bison is
+essentially machine-readable BNF.
-@cindex @acronym{LALR}(1) grammars
-@cindex @acronym{IELR}(1) grammars
-@cindex @acronym{LR}(1) grammars
+@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 @acronym{LR}(1) grammars.
+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 @acronym{LALR}(1), which is hard to explain simply.
+restrictions of LALR(1), which is hard to explain simply.
@xref{Mystery Conflicts, ,Mysterious Reduce/Reduce Conflicts}, for
more information on this.
-To escape these additional restrictions, you can request
-@acronym{IELR}(1) or canonical @acronym{LR}(1) parser tables.
+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{LR}(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 deterministic
-@acronym{LR}(1) parsing algorithm cannot decide whether to apply a
+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{LR}(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 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.
+* 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:: @acronym{GLR} parsers require a modern C compiler.
+* 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{LR}(1).
+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
@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{LR}(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{LR}(1) algorithm actually allows
-for. In this example, @acronym{LR}(2) would suffice, but also some cases
-that are not @acronym{LR}(@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{LR}(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
-to be silent about the one known reduce/reduce conflict, by
-adding these two declarations to the Bison input file (before the first
+The parser can be turned into a GLR parser, while also telling Bison
+to be silent about the one known reduce/reduce conflict, by adding
+these two declarations to the Bison grammar file (before the first
@samp{%%}):
@example
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{LR} 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
+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
of the first symbol, and the end of the last symbol.
@node Bison Parser
-@section Bison Output: the Parser File
+@section Bison Output: the Parser Implementation File
@cindex Bison parser
@cindex Bison utility
@cindex lexical analyzer, purpose
@cindex parser
-When you run Bison, you give it a Bison grammar file as input. The output
-is a C source file that parses the language described by the grammar.
-This file is called a @dfn{Bison parser}. Keep in mind that the Bison
-utility and the Bison parser are two distinct programs: the Bison utility
-is a program whose output is the Bison parser that becomes part of your
-program.
+When you run Bison, you give it a Bison grammar file as input. The
+most important output is a C source file that implements a parser for
+the language described by the grammar. This parser is called a
+@dfn{Bison parser}, and this file is called a @dfn{Bison parser
+implementation file}. Keep in mind that the Bison utility and the
+Bison parser are two distinct programs: the Bison utility is a program
+whose output is the Bison parser implementation file that becomes part
+of your program.
The job of the Bison parser is to group tokens into groupings according to
the grammar rules---for example, to build identifiers and operators into
parsing characters of text, but Bison does not depend on this.
@xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
-The Bison parser file is C code which defines a function named
-@code{yyparse} which implements that grammar. This function does not make
-a complete C program: you must supply some additional functions. One is
-the lexical analyzer. Another is an error-reporting function which the
-parser calls to report an error. In addition, a complete C program must
-start with a function called @code{main}; you have to provide this, and
-arrange for it to call @code{yyparse} or the parser will never run.
-@xref{Interface, ,Parser C-Language Interface}.
+The Bison parser implementation file is C code which defines a
+function named @code{yyparse} which implements that grammar. This
+function does not make a complete C program: you must supply some
+additional functions. One is the lexical analyzer. Another is an
+error-reporting function which the parser calls to report an error.
+In addition, a complete C program must start with a function called
+@code{main}; you have to provide this, and arrange for it to call
+@code{yyparse} or the parser will never run. @xref{Interface, ,Parser
+C-Language Interface}.
Aside from the token type names and the symbols in the actions you
-write, all symbols defined in the Bison parser file itself
-begin with @samp{yy} or @samp{YY}. This includes interface functions
-such as the lexical analyzer function @code{yylex}, the error reporting
-function @code{yyerror} and the parser function @code{yyparse} itself.
-This also includes numerous identifiers used for internal purposes.
-Therefore, you should avoid using C identifiers starting with @samp{yy}
-or @samp{YY} in the Bison grammar file except for the ones defined in
-this manual. Also, you should avoid using the C identifiers
-@samp{malloc} and @samp{free} for anything other than their usual
-meanings.
-
-In some cases the Bison parser 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>},
-@code{<stddef.h>}, and @code{<stdlib.h>} are included as needed to
-declare memory allocators and related types. @code{<libintl.h>} is
-included if message translation is in use
-(@pxref{Internationalization}). Other system headers may
-be included if you define @code{YYDEBUG} to a nonzero value
-(@pxref{Tracing, ,Tracing Your Parser}).
+write, all symbols defined in the Bison parser implementation file
+itself begin with @samp{yy} or @samp{YY}. This includes interface
+functions such as the lexical analyzer function @code{yylex}, the
+error reporting function @code{yyerror} and the parser function
+@code{yyparse} itself. This also includes numerous identifiers used
+for internal purposes. Therefore, you should avoid using C
+identifiers starting with @samp{yy} or @samp{YY} in the Bison grammar
+file except for the ones defined in this manual. Also, you should
+avoid using the C identifiers @samp{malloc} and @samp{free} for
+anything other than their usual meanings.
+
+In some cases the Bison parser implementation file includes system
+headers, and in those cases your code should respect the identifiers
+reserved by those headers. On some non-GNU hosts, @code{<alloca.h>},
+@code{<malloc.h>}, @code{<stddef.h>}, and @code{<stdlib.h>} are
+included as needed to declare memory allocators and related types.
+@code{<libintl.h>} is included if message translation is in use
+(@pxref{Internationalization}). Other system headers may be included
+if you define @code{YYDEBUG} to a nonzero value (@pxref{Tracing,
+,Tracing Your Parser}).
@node Stages
@section Stages in Using Bison
The second example will illustrate how operator precedence is handled.
The source code for this calculator is named @file{rpcalc.y}. The
-@samp{.y} extension is a convention used for Bison input files.
+@samp{.y} extension is a convention used for Bison grammar files.
@menu
* Rpcalc Declarations:: Prologue (declarations) for rpcalc.
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
Before running Bison to produce a parser, we need to decide how to
arrange all the source code in one or more source files. For such a
-simple example, the easiest thing is to put everything in one file. The
-definitions of @code{yylex}, @code{yyerror} and @code{main} go at the
-end, in the epilogue of the file
+simple example, the easiest thing is to put everything in one file,
+the grammar file. The definitions of @code{yylex}, @code{yyerror} and
+@code{main} go at the end, in the epilogue of the grammar file
(@pxref{Grammar Layout, ,The Overall Layout of a Bison Grammar}).
For a large project, you would probably have several source files, and use
@code{make} to arrange to recompile them.
-With all the source in a single file, you use the following command to
-convert it into a parser file:
+With all the source in the grammar file, you use the following command
+to convert it into a parser implementation file:
@example
bison @var{file}.y
@end example
@noindent
-In this example the file was called @file{rpcalc.y} (for ``Reverse Polish
-@sc{calc}ulator''). Bison produces a file named @file{@var{file}.tab.c},
-removing the @samp{.y} from the original file name. The file output by
-Bison contains the source code for @code{yyparse}. The additional
-functions in the input file (@code{yylex}, @code{yyerror} and @code{main})
-are copied verbatim to the output.
+In this example, the grammar file is called @file{rpcalc.y} (for
+``Reverse Polish @sc{calc}ulator''). Bison produces a parser
+implementation file named @file{@var{file}.tab.c}, removing the
+@samp{.y} from the grammar file name. The parser implementation file
+contains the source code for @code{yyparse}. The additional functions
+in the grammar file (@code{yylex}, @code{yyerror} and @code{main}) are
+copied verbatim to the parser implementation file.
@node Rpcalc Compile
-@subsection Compiling the Parser File
+@subsection Compiling the Parser Implementation File
@cindex compiling the parser
-Here is how to compile and run the parser file:
+Here is how to compile and run the parser implementation file:
@example
@group
Bison takes as input a context-free grammar specification and produces a
C-language function that recognizes correct instances of the grammar.
-The Bison grammar input file conventionally has a name ending in @samp{.y}.
+The Bison grammar file conventionally has a name ending in @samp{.y}.
@xref{Invocation, ,Invoking Bison}.
@menu
@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
The @var{Prologue} section contains macro definitions and declarations
of functions and variables that are used in the actions in the grammar
-rules. These are copied to the beginning of the parser file so that
-they precede the definition of @code{yyparse}. You can use
-@samp{#include} to get the declarations from a header file. If you
-don't need any C declarations, you may omit the @samp{%@{} and
+rules. These are copied to the beginning of the parser implementation
+file so that they precede the definition of @code{yyparse}. You can
+use @samp{#include} to get the declarations from a header file. If
+you don't need any C declarations, you may omit the @samp{%@{} and
@samp{%@}} delimiters that bracket this section.
The @var{Prologue} section is terminated by the first occurrence
@findex %code top
The functionality of @var{Prologue} sections can often be subtle and
-inflexible.
-As an alternative, Bison provides a %code directive with an explicit qualifier
-field, which identifies the purpose of the code and thus the location(s) where
-Bison should generate it.
-For C/C++, the qualifier can be omitted for the default location, or it can be
-one of @code{requires}, @code{provides}, @code{top}.
-@xref{Decl Summary,,%code}.
+inflexible. As an alternative, Bison provides a @code{%code}
+directive with an explicit qualifier field, which identifies the
+purpose of the code and thus the location(s) where Bison should
+generate it. For C/C++, the qualifier can be omitted for the default
+location, or it can be one of @code{requires}, @code{provides},
+@code{top}. @xref{%code Summary}.
Look again at the example of the previous section:
@end smallexample
@noindent
-Notice that there are two @var{Prologue} sections here, but there's a subtle
-distinction between their functionality.
-For example, if you decide to override Bison's default definition for
-@code{YYLTYPE}, in which @var{Prologue} section should you write your new
-definition?
-You should write it in the first since Bison will insert that code into the
-parser source code file @emph{before} the default @code{YYLTYPE} definition.
-In which @var{Prologue} section should you prototype an internal function,
-@code{trace_token}, that accepts @code{YYLTYPE} and @code{yytokentype} as
-arguments?
-You should prototype it in the second since Bison will insert that code
+Notice that there are two @var{Prologue} sections here, but there's a
+subtle distinction between their functionality. For example, if you
+decide to override Bison's default definition for @code{YYLTYPE}, in
+which @var{Prologue} section should you write your new definition?
+You should write it in the first since Bison will insert that code
+into the parser implementation file @emph{before} the default
+@code{YYLTYPE} definition. In which @var{Prologue} section should you
+prototype an internal function, @code{trace_token}, that accepts
+@code{YYLTYPE} and @code{yytokentype} as arguments? You should
+prototype it in the second since Bison will insert that code
@emph{after} the @code{YYLTYPE} and @code{yytokentype} definitions.
This distinction in functionality between the two @var{Prologue} sections is
explicit which kind you intend.
Moreover, both kinds are always available even in the absence of @code{%union}.
-The @code{%code top} block above logically contains two parts.
-The first two lines before the warning need to appear near the top of the
-parser source code file.
-The first line after the warning is required by @code{YYSTYPE} and thus also
-needs to appear in the parser source code file.
-However, if you've instructed Bison to generate a parser header file
-(@pxref{Decl Summary, ,%defines}), you probably want that line to appear before
-the @code{YYSTYPE} definition in that header file as well.
-The @code{YYLTYPE} definition should also appear in the parser header file to
-override the default @code{YYLTYPE} definition there.
+The @code{%code top} block above logically contains two parts. The
+first two lines before the warning need to appear near the top of the
+parser implementation file. The first line after the warning is
+required by @code{YYSTYPE} and thus also needs to appear in the parser
+implementation file. However, if you've instructed Bison to generate
+a parser header file (@pxref{Decl Summary, ,%defines}), you probably
+want that line to appear before the @code{YYSTYPE} definition in that
+header file as well. The @code{YYLTYPE} definition should also appear
+in the parser header file to override the default @code{YYLTYPE}
+definition there.
In other words, in the @code{%code top} block above, all but the first two
lines are dependency code required by the @code{YYSTYPE} and @code{YYLTYPE}
@end smallexample
@noindent
-Now Bison will insert @code{#include "ptypes.h"} and the new @code{YYLTYPE}
-definition before the Bison-generated @code{YYSTYPE} and @code{YYLTYPE}
-definitions in both the parser source code file and the parser header file.
-(By the same reasoning, @code{%code requires} would also be the appropriate
-place to write your own definition for @code{YYSTYPE}.)
-
-When you are writing dependency code for @code{YYSTYPE} and @code{YYLTYPE}, you
-should prefer @code{%code requires} over @code{%code top} regardless of whether
-you instruct Bison to generate a parser header file.
-When you are writing code that you need Bison to insert only into the parser
-source code file and that has no special need to appear at the top of that
-file, you should prefer the unqualified @code{%code} over @code{%code top}.
-These practices will make the purpose of each block of your code explicit to
-Bison and to other developers reading your grammar file.
-Following these practices, we expect the unqualified @code{%code} and
-@code{%code requires} to be the most important of the four @var{Prologue}
+Now Bison will insert @code{#include "ptypes.h"} and the new
+@code{YYLTYPE} definition before the Bison-generated @code{YYSTYPE}
+and @code{YYLTYPE} definitions in both the parser implementation file
+and the parser header file. (By the same reasoning, @code{%code
+requires} would also be the appropriate place to write your own
+definition for @code{YYSTYPE}.)
+
+When you are writing dependency code for @code{YYSTYPE} and
+@code{YYLTYPE}, you should prefer @code{%code requires} over
+@code{%code top} regardless of whether you instruct Bison to generate
+a parser header file. When you are writing code that you need Bison
+to insert only into the parser implementation file and that has no
+special need to appear at the top of that file, you should prefer the
+unqualified @code{%code} over @code{%code top}. These practices will
+make the purpose of each block of your code explicit to Bison and to
+other developers reading your grammar file. Following these
+practices, we expect the unqualified @code{%code} and @code{%code
+requires} to be the most important of the four @var{Prologue}
alternatives.
-At some point while developing your parser, you might decide to provide
-@code{trace_token} to modules that are external to your parser.
-Thus, you might wish for Bison to insert the prototype into both the parser
-header file and the parser source code file.
-Since this function is not a dependency required by @code{YYSTYPE} or
+At some point while developing your parser, you might decide to
+provide @code{trace_token} to modules that are external to your
+parser. Thus, you might wish for Bison to insert the prototype into
+both the parser header file and the parser implementation file. Since
+this function is not a dependency required by @code{YYSTYPE} or
@code{YYLTYPE}, it doesn't make sense to move its prototype to a
-@code{%code requires}.
-More importantly, since it depends upon @code{YYLTYPE} and @code{yytokentype},
-@code{%code requires} is not sufficient.
-Instead, move its prototype from the unqualified @code{%code} to a
-@code{%code provides}:
+@code{%code requires}. More importantly, since it depends upon
+@code{YYLTYPE} and @code{yytokentype}, @code{%code requires} is not
+sufficient. Instead, move its prototype from the unqualified
+@code{%code} to a @code{%code provides}:
@smallexample
%code top @{
@end smallexample
@noindent
-Bison will insert the @code{trace_token} prototype into both the parser header
-file and the parser source code file after the definitions for
-@code{yytokentype}, @code{YYLTYPE}, and @code{YYSTYPE}.
-
-The above examples are careful to write directives in an order that reflects
-the layout of the generated parser source code and header files:
-@code{%code top}, @code{%code requires}, @code{%code provides}, and then
-@code{%code}.
-While your grammar files may generally be easier to read if you also follow
-this order, Bison does not require it.
-Instead, Bison lets you choose an organization that makes sense to you.
+Bison will insert the @code{trace_token} prototype into both the
+parser header file and the parser implementation file after the
+definitions for @code{yytokentype}, @code{YYLTYPE}, and
+@code{YYSTYPE}.
+
+The above examples are careful to write directives in an order that
+reflects the layout of the generated parser implementation and header
+files: @code{%code top}, @code{%code requires}, @code{%code provides},
+and then @code{%code}. While your grammar files may generally be
+easier to read if you also follow this order, Bison does not require
+it. Instead, Bison lets you choose an organization that makes sense
+to you.
You may declare any of these directives multiple times in the grammar file.
In that case, Bison concatenates the contained code in declaration order.
@cindex epilogue
@cindex C code, section for additional
-The @var{Epilogue} is copied verbatim to the end of the parser file, just as
-the @var{Prologue} is copied to the beginning. This is the most convenient
-place to put anything that you want to have in the parser file but which need
-not come before the definition of @code{yyparse}. For example, the
-definitions of @code{yylex} and @code{yyerror} often go here. Because
-C requires functions to be declared before being used, you often need
-to declare functions like @code{yylex} and @code{yyerror} in the Prologue,
-even if you define them in the Epilogue.
-@xref{Interface, ,Parser C-Language Interface}.
+The @var{Epilogue} is copied verbatim to the end of the parser
+implementation file, just as the @var{Prologue} is copied to the
+beginning. This is the most convenient place to put anything that you
+want to have in the parser implementation file but which need not come
+before the definition of @code{yyparse}. For example, the definitions
+of @code{yylex} and @code{yyerror} often go here. Because C requires
+functions to be declared before being used, you often need to declare
+functions like @code{yylex} and @code{yyerror} in the Prologue, even
+if you define them in the Epilogue. @xref{Interface, ,Parser
+C-Language Interface}.
If the last section is empty, you may omit the @samp{%%} that separates it
from the grammar rules.
equivalent groupings. The symbol name is used in writing grammar rules.
By convention, it should be all lower case.
-Symbol names can contain letters, underscores, periods, dashes, and (not
-at the beginning) digits. Dashes in symbol names are a GNU
-extension, incompatible with @acronym{POSIX} Yacc. 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.
+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:
character, so @code{yylex} can use the identical value to generate the
requisite code, though you may need to convert it to @code{unsigned
char} to avoid sign-extension on hosts where @code{char} is signed.
-Each named token type becomes a C macro in
-the parser file, so @code{yylex} can use the name to stand for the code.
-(This is why periods don't make sense in terminal symbols.)
-@xref{Calling Convention, ,Calling Convention for @code{yylex}}.
+Each named token type becomes a C macro in the parser implementation
+file, so @code{yylex} can use the name to stand for the code. (This
+is why periods don't make sense in terminal symbols.) @xref{Calling
+Convention, ,Calling Convention for @code{yylex}}.
If @code{yylex} is defined in a separate file, you need to arrange for the
token-type macro definitions to be available there. Use the @samp{-d}
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
braces, much like a compound statement in C@. Braced code can contain
any sequence of C tokens, so long as its braces are balanced. Bison
does not check the braced code for correctness directly; it merely
-copies the code to the output file, where the C compiler can check it.
+copies the code to the parser implementation file, where the C
+compiler can check it.
Within braced code, the balanced-brace count is not affected by braces
within comments, string literals, or character constants, but it is
* 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 $$
@vindex $@var{n}
+@vindex $@var{name}
+@vindex $[@var{name}]
An action accompanies a syntactic rule and contains C code to be executed
each time an instance of that rule is recognized. The task of most actions
a rule are tricky and used only for special purposes (@pxref{Mid-Rule
Actions, ,Actions in Mid-Rule}).
-The C code in an action can refer to the semantic values of the components
-matched by the rule with the construct @code{$@var{n}}, which stands for
-the value of the @var{n}th component. The semantic value for the grouping
-being constructed is @code{$$}. Bison translates both of these
-constructs into expressions of the appropriate type when it copies the
-actions into the parser file. @code{$$} is translated to a modifiable
-lvalue, so it can be assigned to.
+The C code in an action can refer to the semantic values of the
+components matched by the rule with the construct @code{$@var{n}},
+which stands for the value of the @var{n}th component. The semantic
+value for the grouping being constructed is @code{$$}. In addition,
+the semantic values of symbols can be accessed with the named
+references construct @code{$@var{name}} or @code{$[@var{name}]}.
+Bison translates both of these constructs into expressions of the
+appropriate type when it copies the actions into the parser
+implementation file. @code{$$} (or @code{$@var{name}}, when it stands
+for the current grouping) is translated to a modifiable lvalue, so it
+can be assigned to.
Here is a typical example:
@end group
@end example
+Or, in terms of named references:
+
+@example
+@group
+exp[result]: @dots{}
+ | exp[left] '+' exp[right]
+ @{ $result = $left + $right; @}
+@end group
+@end example
+
@noindent
This rule constructs an @code{exp} from two smaller @code{exp} groupings
connected by a plus-sign token. In the action, @code{$1} and @code{$3}
+(@code{$left} and @code{$right})
refer to the semantic values of the two component @code{exp} groupings,
which are the first and third symbols on the right hand side of the rule.
-The sum is stored into @code{$$} 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
@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
declared if you need to specify which data type to use for the semantic
value (@pxref{Multiple Types, ,More Than One Value Type}).
-The first rule in the file also specifies the start symbol, by default.
-If you want some other symbol to be the start symbol, you must declare
-it explicitly (@pxref{Language and Grammar, ,Languages and Context-Free
-Grammars}).
+The first rule in the grammar file also specifies the start symbol, by
+default. If you want some other symbol to be the start symbol, you
+must declare it explicitly (@pxref{Language and Grammar, ,Languages
+and Context-Free Grammars}).
@menu
* Require Decl:: Requiring a Bison version.
* Pure Decl:: Requesting a reentrant parser.
* Push Decl:: Requesting a push parser.
* Decl Summary:: Table of all Bison declarations.
+* %code Summary:: Inserting code into the parser source.
@end menu
@node Require Decl
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.
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
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
directives:
@deffn {Directive} %code @{@var{code}@}
+@deffnx {Directive} %code @var{qualifier} @{@var{code}@}
@findex %code
-This is the unqualified form of the @code{%code} directive.
-It inserts @var{code} verbatim at a language-dependent default location in the
-output@footnote{The default location is actually skeleton-dependent;
- writers of non-standard skeletons however should choose the default location
- consistently with the behavior of the standard Bison skeletons.}.
-
-@cindex Prologue
-For C/C++, the default location is the parser source code
-file after the usual contents of the parser header file.
-Thus, @code{%code} replaces the traditional Yacc prologue,
-@code{%@{@var{code}%@}}, for most purposes.
-For a detailed discussion, see @ref{Prologue Alternatives}.
-
-For Java, the default location is inside the parser class.
-@end deffn
-
-@deffn {Directive} %code @var{qualifier} @{@var{code}@}
-This is the qualified form of the @code{%code} directive.
-If you need to specify location-sensitive verbatim @var{code} that does not
-belong at the default location selected by the unqualified @code{%code} form,
-use this form instead.
-
-@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:
-
-@itemize @bullet
-@item requires
-@findex %code requires
-
-@itemize @bullet
-@item Language(s): C, C++
-
-@item Purpose: This is the best place to write dependency code required for
-@code{YYSTYPE} and @code{YYLTYPE}.
-In other words, it's the best place to define types referenced in @code{%union}
-directives, and it's the best place to override Bison's default @code{YYSTYPE}
-and @code{YYLTYPE} definitions.
-
-@item Location(s): The parser header file and the parser source code file
-before the Bison-generated @code{YYSTYPE} and @code{YYLTYPE} definitions.
-@end itemize
-
-@item provides
-@findex %code provides
-
-@itemize @bullet
-@item Language(s): C, C++
-
-@item Purpose: This is the best place to write additional definitions and
-declarations that should be provided to other modules.
-
-@item Location(s): The parser header file and the parser source code file after
-the Bison-generated @code{YYSTYPE}, @code{YYLTYPE}, and token definitions.
-@end itemize
-
-@item top
-@findex %code top
-
-@itemize @bullet
-@item Language(s): C, C++
-
-@item Purpose: The unqualified @code{%code} or @code{%code requires} should
-usually be more appropriate than @code{%code top}.
-However, occasionally it is necessary to insert code much nearer the top of the
-parser source code file.
-For example:
-
-@smallexample
-%code top @{
- #define _GNU_SOURCE
- #include <stdio.h>
-@}
-@end smallexample
-
-@item Location(s): Near the top of the parser source code file.
-@end itemize
-
-@item imports
-@findex %code imports
-
-@itemize @bullet
-@item Language(s): Java
-
-@item Purpose: This is the best place to write Java import directives.
-
-@item Location(s): The parser Java file after any Java package directive and
-before any class definitions.
-@end itemize
-@end itemize
-
-@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}.
+Insert @var{code} verbatim into the output parser source at the
+default location or at the location specified by @var{qualifier}.
+@xref{%code Summary}.
@end deffn
@deffn {Directive} %debug
-In the parser file, define the macro @code{YYDEBUG} to 1 if it is not
-already defined, so that the debugging facilities are compiled.
-@xref{Tracing, ,Tracing Your Parser}.
+In the parser implementation file, define the macro @code{YYDEBUG} to
+1 if it is not already defined, so that the debugging facilities are
+compiled. @xref{Tracing, ,Tracing Your Parser}.
@end deffn
@deffn {Directive} %define @var{variable}
+@deffnx {Directive} %define @var{variable} @var{value}
@deffnx {Directive} %define @var{variable} "@var{value}"
Define a variable to adjust Bison's behavior.
-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}, @pxref{Decl Summary,,%skeleton}).
It is an error if a @var{variable} is defined by @code{%define} multiple
-times, but @ref{Bison Options,,-D @var{name}[=@var{value}]}.
+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
@itemize @bullet
@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
+@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: Specifies the kind of states that are permitted to
+@item Purpose: Specify the kind of states that are permitted to
contain default reductions.
-That is, in such a state, Bison declares the reduction with the largest
-lookahead set to be the default reduction and then removes that
+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 advantages of default reductions are discussed below.
-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.
-
-(This feature is experimental.
+(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"}.
-For @acronym{LALR} and @acronym{IELR} parsers (@pxref{Decl
-Summary,,lr.type}) by default, all states are permitted to contain
-default reductions.
-The advantage is that parser table sizes can be significantly reduced.
-The reason Bison does not by default attempt to address the disadvantage
-of delayed syntax error detection is that this disadvantage is already
-inherent in @acronym{LALR} and @acronym{IELR} parser tables.
-That is, unlike in a canonical @acronym{LR} state, the lookahead sets of
-reductions in an @acronym{LALR} or @acronym{IELR} state can contain
-tokens that are syntactically incorrect for some left contexts.
-
-@item @code{"consistent"}.
+@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 only recognizes the ability to ignore the lookahead
-token when such a reduction is encoded as a default reduction.
-Thus, if default reductions are permitted in and only in consistent
-states, then a canonical @acronym{LR} parser reports a syntax error as
-soon as it @emph{needs} the syntactically unacceptable token from the
-scanner.
-
-@item @code{"accepting"}.
+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
-By default, the only default reduction permitted in a canonical
-@acronym{LR} parser is the accept action in the accepting state, which
-the parser reaches only after reading all tokens from the input.
-Thus, the default canonical @acronym{LR} parser reports a syntax error
-as soon as it @emph{reaches} the syntactically unacceptable token
-without performing any extra reductions.
+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.
+@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: Requests that Bison allow unreachable parser states to remain in
-the parser tables.
+@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 @acronym{LALR}
-@cindex @acronym{IELR}
-@cindex @acronym{LR}
+@cindex LALR
+@cindex IELR
+@cindex LR
@itemize @bullet
@item Language(s): all
-@item Purpose: Specifies the type of parser tables within the
-@acronym{LR}(1) family.
+@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 @acronym{LALR} parser tables by default for
-historical reasons, @acronym{IELR} or canonical @acronym{LR} is almost
+@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 @acronym{LALR} parser tables can suffer from
+The trouble is that LALR parser tables can suffer from
mysterious conflicts and thus may not accept the full set of sentences
-that @acronym{IELR} and canonical @acronym{LR} accept.
+that IELR and canonical LR accept.
@xref{Mystery Conflicts}, for details.
-However, there are at least two scenarios where @acronym{LALR} may be
+However, there are at least two scenarios where LALR may be
worthwhile:
@itemize
-@cindex @acronym{GLR} with @acronym{LALR}
-@item When employing @acronym{GLR} parsers (@pxref{GLR Parsers}), if you
+@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 @acronym{LALR} parser tables is guaranteed not
+In this case, the use of LALR parser tables is guaranteed not
to alter the language accepted by the parser.
-@acronym{LALR} parser tables are the smallest parser tables Bison can
+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 @acronym{IELR} or
-canonical @acronym{LR} parser table generation algorithm.
-@acronym{LALR} can be a quick way to generate parser tables in order to
+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 @acronym{IELR} and canonical @acronym{LR}.
+from IELR and canonical LR.
@end itemize
-@item @code{"IELR"}.
-@acronym{IELR} is a minimal @acronym{LR} algorithm.
-That is, given any grammar (@acronym{LR} or non-@acronym{LR}),
-@acronym{IELR} and canonical @acronym{LR} always accept exactly the same
+@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 @acronym{LALR}, the number of parser states is often an
-order of magnitude less for @acronym{IELR} than for canonical
-@acronym{LR}.
-More importantly, because canonical @acronym{LR}'s extra parser states
-may contain duplicate conflicts in the case of non-@acronym{LR}
-grammars, the number of conflicts for @acronym{IELR} is often an order
+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"}.
+@item @code{canonical-lr}.
@cindex delayed syntax errors
@cindex syntax errors delayed
-The only advantage of canonical @acronym{LR} over @acronym{IELR} is
-that, for every left context of every canonical @acronym{LR} state, the
-set of tokens accepted by that state is the exact set of tokens that is
-syntactically acceptable in that left context.
-Thus, the only difference in parsing behavior is that the canonical
-@acronym{LR} parser can report a syntax error as soon as possible
-without performing any unnecessary reductions.
-@xref{Decl Summary,,lr.default-reductions}, for further details.
-Even when canonical @acronym{LR} behavior is ultimately desired,
-@acronym{IELR}'s elimination of duplicate conflicts should still
-facilitate the development of a grammar.
+@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"}
+@item Default Value: @code{lalr}
@end itemize
@item 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
@deffn {Directive} %defines
-Write a header file containing macro definitions for the token type
-names defined in the grammar as well as a few other declarations.
-If the parser output file is named @file{@var{name}.c} then this file
-is named @file{@var{name}.h}.
+Write a parser header file containing macro definitions for the token
+type names defined in the grammar as well as a few other declarations.
+If the parser implementation file is named @file{@var{name}.c} then
+the parser header file is named @file{@var{name}.h}.
-For C parsers, the output header declares @code{YYSTYPE} unless
+For C parsers, the parser header file declares @code{YYSTYPE} unless
@code{YYSTYPE} is already defined as a macro or you have used a
-@code{<@var{type}>} tag without using @code{%union}.
-Therefore, if you are using a @code{%union}
-(@pxref{Multiple Types, ,More Than One Value Type}) with components that
-require other definitions, or if you have defined a @code{YYSTYPE} macro
-or type definition
-(@pxref{Value Type, ,Data Types of Semantic Values}), you need to
-arrange for these definitions to be propagated to all modules, e.g., by
-putting them in a prerequisite header that is included both by your
-parser and by any other module that needs @code{YYSTYPE}.
-
-Unless your parser is pure, the output header declares @code{yylval}
-as an external variable. @xref{Pure Decl, ,A Pure (Reentrant)
-Parser}.
-
-If you have also used locations, the output header declares
+@code{<@var{type}>} tag without using @code{%union}. Therefore, if
+you are using a @code{%union} (@pxref{Multiple Types, ,More Than One
+Value Type}) with components that require other definitions, or if you
+have defined a @code{YYSTYPE} macro or type definition (@pxref{Value
+Type, ,Data Types of Semantic Values}), you need to arrange for these
+definitions to be propagated to all modules, e.g., by putting them in
+a prerequisite header that is included both by your parser and by any
+other module that needs @code{YYSTYPE}.
+
+Unless your parser is pure, the parser header file declares
+@code{yylval} as an external variable. @xref{Pure Decl, ,A Pure
+(Reentrant) Parser}.
+
+If you have also used locations, the parser header file declares
@code{YYLTYPE} and @code{yylloc} using a protocol similar to that of
-the @code{YYSTYPE} macro and @code{yylval}. @xref{Locations, ,Tracking
-Locations}.
+the @code{YYSTYPE} macro and @code{yylval}. @xref{Locations,
+,Tracking Locations}.
-This output file is normally essential if you wish to put the definition
-of @code{yylex} in a separate source file, because @code{yylex}
-typically needs to be able to refer to the above-mentioned declarations
-and to the token type codes. @xref{Token Values, ,Semantic Values of
-Tokens}.
+This parser header file is normally essential if you wish to put the
+definition of @code{yylex} in a separate source file, because
+@code{yylex} typically needs to be able to refer to the
+above-mentioned declarations and to the token type codes. @xref{Token
+Values, ,Semantic Values of Tokens}.
@findex %code requires
@findex %code provides
If you have declared @code{%code requires} or @code{%code provides}, the output
header also contains their code.
-@xref{Decl Summary, ,%code}.
+@xref{%code Summary}.
@end deffn
@deffn {Directive} %defines @var{defines-file}
@end deffn
@deffn {Directive} %file-prefix "@var{prefix}"
-Specify a prefix to use for all Bison output file names. The names are
-chosen as if the input file were named @file{@var{prefix}.y}.
+Specify a prefix to use for all Bison output file names. The names
+are chosen as if the grammar file were named @file{@var{prefix}.y}.
@end deffn
@deffn {Directive} %language "@var{language}"
@deffn {Directive} %no-lines
Don't generate any @code{#line} preprocessor commands in the parser
-file. Ordinarily Bison writes these commands in the parser file so that
-the C compiler and debuggers will associate errors and object code with
-your source file (the grammar file). This directive causes them to
-associate errors with the parser file, treating it an independent source
-file in its own right.
+implementation file. Ordinarily Bison writes these commands in the
+parser implementation file so that the C compiler and debuggers will
+associate errors and object code with your source file (the grammar
+file). This directive causes them to associate errors with the parser
+implementation file, treating it as an independent source file in its
+own right.
@end deffn
@deffn {Directive} %output "@var{file}"
-Specify @var{file} for the parser file.
+Specify @var{file} for the parser implementation file.
@end deffn
@deffn {Directive} %pure-parser
@end deffn
@deffn {Directive} %token-table
-Generate an array of token names in the parser file. The name of the
-array is @code{yytname}; @code{yytname[@var{i}]} is the name of the
-token whose internal Bison token code number is @var{i}. The first
-three elements of @code{yytname} correspond to the predefined tokens
-@code{"$end"},
-@code{"error"}, and @code{"$undefined"}; after these come the symbols
-defined in the grammar file.
+Generate an array of token names in the parser implementation file.
+The name of the array is @code{yytname}; @code{yytname[@var{i}]} is
+the name of the token whose internal Bison token code number is
+@var{i}. The first three elements of @code{yytname} correspond to the
+predefined tokens @code{"$end"}, @code{"error"}, and
+@code{"$undefined"}; after these come the symbols defined in the
+grammar file.
The name in the table includes all the characters needed to represent
the token in Bison. For single-character literals and literal
including its naming conventions. @xref{Bison Options}, for more.
@end deffn
+@node %code Summary
+@subsection %code Summary
+@deffn {Directive} %code @{@var{code}@}
+@findex %code
+This is the unqualified form of the @code{%code} directive.
+It inserts @var{code} verbatim at a language-dependent default location in the
+output@footnote{The default location is actually skeleton-dependent;
+ writers of non-standard skeletons however should choose the default location
+ consistently with the behavior of the standard Bison skeletons.}.
+
+@cindex Prologue
+For C/C++, the default location is the parser implementation file
+after the usual contents of the parser header file. Thus,
+@code{%code} replaces the traditional Yacc prologue,
+@code{%@{@var{code}%@}}, for most purposes. For a detailed
+discussion, see @ref{Prologue Alternatives}.
+
+For Java, the default location is inside the parser class.
+@end deffn
+
+@deffn {Directive} %code @var{qualifier} @{@var{code}@}
+This is the qualified form of the @code{%code} directive.
+If you need to specify location-sensitive verbatim @var{code} that does not
+belong at the default location selected by the unqualified @code{%code} form,
+use this form instead.
+
+@var{qualifier} identifies the purpose of @var{code} and thus the location(s)
+where Bison should generate it.
+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
+@findex %code requires
+
+@itemize @bullet
+@item Language(s): C, C++
+
+@item Purpose: This is the best place to write dependency code required for
+@code{YYSTYPE} and @code{YYLTYPE}.
+In other words, it's the best place to define types referenced in @code{%union}
+directives, and it's the best place to override Bison's default @code{YYSTYPE}
+and @code{YYLTYPE} definitions.
+
+@item Location(s): The parser header file and the parser implementation file
+before the Bison-generated @code{YYSTYPE} and @code{YYLTYPE}
+definitions.
+@end itemize
+
+@item provides
+@findex %code provides
+
+@itemize @bullet
+@item Language(s): C, C++
+
+@item Purpose: This is the best place to write additional definitions and
+declarations that should be provided to other modules.
+
+@item Location(s): The parser header file and the parser implementation
+file after the Bison-generated @code{YYSTYPE}, @code{YYLTYPE}, and
+token definitions.
+@end itemize
+
+@item top
+@findex %code top
+
+@itemize @bullet
+@item Language(s): C, C++
+
+@item Purpose: The unqualified @code{%code} or @code{%code requires}
+should usually be more appropriate than @code{%code top}. However,
+occasionally it is necessary to insert code much nearer the top of the
+parser implementation file. For example:
+
+@smallexample
+%code top @{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+@}
+@end smallexample
+
+@item Location(s): Near the top of the parser implementation file.
+@end itemize
+
+@item imports
+@findex %code imports
+
+@itemize @bullet
+@item Language(s): Java
+
+@item Purpose: This is the best place to write Java import directives.
+
+@item Location(s): The parser Java file after any Java package directive and
+before any class definitions.
+@end itemize
+@end itemize
+
+@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}.
+@end deffn
+
@node Multiple Parsers
@section Multiple Parsers in the Same Program
renamed, but defining this in different ways in different parsers causes
no trouble (@pxref{Value Type, ,Data Types of Semantic Values}).
-The @samp{-p} option works by adding macro definitions to the beginning
-of the parser source file, defining @code{yyparse} as
-@code{@var{prefix}parse}, and so on. This effectively substitutes one
-name for the other in the entire parser file.
+The @samp{-p} option works by adding macro definitions to the
+beginning of the parser implementation file, defining @code{yyparse}
+as @code{@var{prefix}parse}, and so on. This effectively substitutes
+one name for the other in the entire parser implementation file.
@node Interface
@chapter Parser C-Language Interface
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)
this function automatically; you must write it so that @code{yyparse} can
call it. The function is sometimes referred to as a lexical scanner.
-In simple programs, @code{yylex} is often defined at the end of the Bison
-grammar file. If @code{yylex} is defined in a separate source file, you
-need to arrange for the token-type macro definitions to be available there.
-To do this, use the @samp{-d} option when you run Bison, so that it will
-write these macro definitions into a separate header file
-@file{@var{name}.tab.h} which you can include in the other source files
-that need it. @xref{Invocation, ,Invoking Bison}.
+In simple programs, @code{yylex} is often defined at the end of the
+Bison grammar file. If @code{yylex} is defined in a separate source
+file, you need to arrange for the token-type macro definitions to be
+available there. To do this, use the @samp{-d} option when you run
+Bison, so that it will write these macro definitions into the separate
+parser header file, @file{@var{name}.tab.h}, which you can include in
+the other source files that need it. @xref{Invocation, ,Invoking
+Bison}.
@menu
* Calling Convention:: How @code{yyparse} calls @code{yylex}.
signifies end-of-input.
When a token is referred to in the grammar rules by a name, that name
-in the parser file becomes a C macro whose definition is the proper
-numeric code for that token type. So @code{yylex} can use the name
-to indicate that type. @xref{Symbols}.
+in the parser implementation file becomes a C macro whose definition
+is the proper numeric code for that token type. So @code{yylex} can
+use the name to indicate that type. @xref{Symbols}.
When a token is referred to in the grammar rules by a character literal,
the numeric code for that character is also the code for the token type.
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.
Algol 60 and is called the ``dangling @code{else}'' ambiguity.
To avoid warnings from Bison about predictable, legitimate shift/reduce
-conflicts, use the @code{%expect @var{n}} declaration. There will be no
-warning as long as the number of shift/reduce conflicts is exactly @var{n}.
+conflicts, use the @code{%expect @var{n}} declaration.
+There will be no warning as long as the number of shift/reduce conflicts
+is exactly @var{n}, and Bison will report an error if there is a
+different number.
@xref{Expect Decl, ,Suppressing Conflict Warnings}.
The definition of @code{if_stmt} above is solely to blame for the
conflict, but the conflict does not actually appear without additional
-rules. Here is a complete Bison input file that actually manifests the
-conflict:
+rules. Here is a complete Bison grammar file that actually manifests
+the conflict:
@example
@group
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).
+@code{ID} follows. In other words, this grammar is LR(1).
-@cindex @acronym{LR}(1)
-@cindex @acronym{LALR}(1)
+@cindex LR(1)
+@cindex LALR(1)
However, for historical reasons, Bison cannot by default handle all
-@acronym{LR}(1) grammars.
+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
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).
+occurrence means that the grammar is not LALR(1).
For many practical grammars (specifically those that fall into the
-non-@acronym{LR}(1) class), the limitations of @acronym{LALR}(1) result in
+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 @acronym{IELR}(1) or canonical @acronym{LR}(1) would suffice, but
+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 @acronym{IELR}(1) and canonical @acronym{LR}(1) implementations
+(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 @acronym{LALR}(1)'s limitations, you
+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
;
@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
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
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{LR}(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{LR}(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{LR}(1) portions of a
+structure should generally be adequate. On LR(1) portions of a
grammar, in particular, it is only slightly slower than with the
-deterministic @acronym{LR}(1) Bison parser.
+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).
Do not allow @code{YYINITDEPTH} to be greater than @code{YYMAXDEPTH}.
@c FIXME: C++ output.
-Because of semantical differences between C and C++, the deterministic
+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
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
it is nonzero, all integers are parsed in hexadecimal, and tokens starting
with letters are parsed as integers if possible.
-The declaration of @code{hexflag} shown in the prologue of the parser file
-is needed to make it accessible to the actions (@pxref{Prologue, ,The Prologue}).
-You must also write the code in @code{yylex} to obey the flag.
+The declaration of @code{hexflag} shown in the prologue of the grammar
+file is needed to make it accessible to the actions (@pxref{Prologue,
+,The Prologue}). You must also write the code in @code{yylex} to obey
+the flag.
@node Tie-in Recovery
@section Lexical Tie-ins and Error Recovery
The textual file is generated when the options @option{--report} or
@option{--verbose} are specified, see @xref{Invocation, , Invoking
Bison}. Its name is made by removing @samp{.tab.c} or @samp{.c} from
-the parser output file name, and adding @samp{.output} instead.
-Therefore, if the input file is @file{foo.y}, then the parser file is
-called @file{foo.tab.c} by default. As a consequence, the verbose
-output file is called @file{foo.output}.
+the parser implementation file name, and adding @samp{.output}
+instead. Therefore, if the grammar file is @file{foo.y}, then the
+parser implementation file is called @file{foo.tab.c} by default. As
+a consequence, the verbose output file is called @file{foo.output}.
The following grammar file, @file{calc.y}, will be used in the sequel:
@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
something undesirable happens, and you will see which parts of the
grammar are to blame.
-The parser file is a C program and you can use C debuggers on it, but it's
-not easy to interpret what it is doing. The parser function is a
-finite-state machine interpreter, and aside from the actions it executes
-the same code over and over. Only the values of variables show where in
-the grammar it is working.
+The parser implementation file is a C program and you can use C
+debuggers on it, but it's not easy to interpret what it is doing. The
+parser function is a finite-state machine interpreter, and aside from
+the actions it executes the same code over and over. Only the values
+of variables show where in the grammar it is working.
@findex YYPRINT
The debugging information normally gives the token type of each token
@end example
Here @var{infile} is the grammar file name, which usually ends in
-@samp{.y}. The parser file's name is made by replacing the @samp{.y}
-with @samp{.tab.c} and removing any leading directory. Thus, the
-@samp{bison foo.y} file name yields
-@file{foo.tab.c}, and the @samp{bison hack/foo.y} file name yields
-@file{foo.tab.c}. It's also possible, in case you are writing
-C++ code instead of C in your grammar file, to name it @file{foo.ypp}
-or @file{foo.y++}. Then, the output files will take an extension like
-the given one as input (respectively @file{foo.tab.cpp} and
-@file{foo.tab.c++}).
-This feature takes effect with all options that manipulate file names like
+@samp{.y}. The parser implementation file's name is made by replacing
+the @samp{.y} with @samp{.tab.c} and removing any leading directory.
+Thus, the @samp{bison foo.y} file name yields @file{foo.tab.c}, and
+the @samp{bison hack/foo.y} file name yields @file{foo.tab.c}. It's
+also possible, in case you are writing C++ code instead of C in your
+grammar file, to name it @file{foo.ypp} or @file{foo.y++}. Then, the
+output files will take an extension like the given one as input
+(respectively @file{foo.tab.cpp} and @file{foo.tab.c++}). This
+feature takes effect with all options that manipulate file names like
@samp{-o} or @samp{-d}.
For example :
@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.
@item -y
@itemx --yacc
-Act more like the traditional Yacc command. This can cause
-different diagnostics to be generated, and may change behavior in
-other minor ways. Most importantly, imitate Yacc's output
-file name conventions, so that the parser 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 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}:
+Act more like the traditional Yacc command. This can cause different
+diagnostics to be generated, and may change behavior in other minor
+ways. Most importantly, imitate Yacc's output file name conventions,
+so that the parser implementation file is called @file{y.tab.c}, and
+the other outputs are called @file{y.output} and @file{y.tab.h}.
+Also, if generating a deterministic parser in C, generate
+@code{#define} statements in addition to an @code{enum} to associate
+token numbers with token names. Thus, the following shell script can
+substitute for Yacc, and the Bison distribution contains such a script
+for compatibility with POSIX:
@example
#! /bin/sh
@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
@table @option
@item -t
@itemx --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.
-@xref{Tracing, ,Tracing Your Parser}.
+In the parser implementation file, define the macro @code{YYDEBUG} to
+1 if it is not already defined, so that the debugging facilities are
+compiled. @xref{Tracing, ,Tracing Your Parser}.
@item -D @var{name}[=@var{value}]
@itemx --define=@var{name}[=@var{value}]
-@item -F @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
@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.
+make files unless you are confident that it is safe to quietly ignore
+any conflicting @code{%define} that may be added to the grammar file.
@item -L @var{language}
@itemx --language=@var{language}
@item -l
@itemx --no-lines
-Don't put any @code{#line} preprocessor commands in the parser file.
-Ordinarily Bison puts them in the parser file so that the C compiler
-and debuggers will associate errors with your source file, the
-grammar file. This option causes them to associate errors with the
-parser file, treating it as an independent source file in its own right.
+Don't put any @code{#line} preprocessor commands in the parser
+implementation file. Ordinarily Bison puts them in the parser
+implementation file so that the C compiler and debuggers will
+associate errors with your source file, the grammar file. This option
+causes them to associate errors with the parser implementation file,
+treating it as an independent source file in its own right.
@item -S @var{file}
@itemx --skeleton=@var{file}
@item -o @var{file}
@itemx --output=@var{file}
-Specify the @var{file} for the parser file.
+Specify the @var{file} for the parser implementation file.
The other output files' names are constructed from @var{file} as
described under the @samp{-v} and @samp{-d} options.
@itemx --graph[=@var{file}]
Output a graphical representation of the parser's
automaton computed by Bison, in @uref{http://www.graphviz.org/, Graphviz}
-@uref{http://www.graphviz.org/doc/info/lang.html, @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}.
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
@c - initial action
The C++ deterministic parser is selected using the skeleton directive,
-@samp{%skeleton "lalr1.c"}, or the synonymous command-line option
-@option{--skeleton=lalr1.c}.
+@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}
@item @var{file}.hh
@itemx @var{file}.cc
-(Assuming the extension of the input file was @samp{.yy}.) The
+(Assuming the extension of the grammar file was @samp{.yy}.) The
declaration and implementation of the C++ parser class. The basename
and extension of these two files follow the same rules as with regular C
parsers (@pxref{Invocation}).
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.
@node Calc++ Parser
@subsubsection Calc++ Parser
-The parser definition file @file{calc++-parser.yy} starts by asking for
-the C++ deterministic parser skeleton, the creation of the parser header
-file, and specifies the name of the parser class.
-Because the C++ skeleton changed several times, it is safer to require
-the version you designed the grammar for.
+The grammar file @file{calc++-parser.yy} starts by asking for the C++
+deterministic parser skeleton, the creation of the parser header file,
+and specifies the name of the parser class. Because the C++ skeleton
+changed several times, it is safer to require the version you designed
+the grammar for.
@comment file: calc++-parser.yy
@example
driver's header needs detailed knowledge about the parser class (in
particular its inner types), it is the parser's header which will simply
use a forward declaration of the driver.
-@xref{Decl Summary, ,%code}.
+@xref{%code Summary}.
@comment file: calc++-parser.yy
@example
@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.
directive or the @option{-L java}/@option{--language=java} option.
@c FIXME: Documented bug.
-When generating a Java parser, @code{bison @var{basename}.y} will create
-a single Java source file named @file{@var{basename}.java}. Using an
-input file without a @file{.y} suffix is currently broken. The basename
-of the output file can be changed by the @code{%file-prefix} directive
-or the @option{-p}/@option{--name-prefix} option. The entire output file
-name can be changed by the @code{%output} directive or the
-@option{-o}/@option{--output} option. The output file contains a single
-class for the parser.
+When generating a Java parser, @code{bison @var{basename}.y} will
+create a single Java source file named @file{@var{basename}.java}
+containing the parser implementation. Using a grammar file without a
+@file{.y} suffix is currently broken. The basename of the parser
+implementation file can be changed by the @code{%file-prefix}
+directive or the @option{-p}/@option{--name-prefix} option. The
+entire parser implementation file name can be changed by the
+@code{%output} directive or the @option{-o}/@option{--output} option.
+The parser implementation file contains a single class for the parser.
You can create documentation for generated parsers using Javadoc.
Push parsers are currently unsupported in Java and @code{%define
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
@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}".}
@xref{Error Recovery}.
@end deffn
-@deffn {Statement} {return YYFAIL;}
-Print an error message and start error recovery.
-@xref{Error Recovery}.
-@end deffn
-
@deftypefn {Function} {boolean} recovering ()
Return whether error recovery is being done. In this state, the parser
reads token until it reaches a known state, and then restarts normal
@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.
@c Don't insert spaces, or check the DVI output.
@deffn {Delimiter} %@{@var{code}%@}
-All code listed between @samp{%@{} and @samp{%@}} is copied directly to
-the output file uninterpreted. Such code forms the prologue of the input
-file. @xref{Grammar Outline, ,Outline of a Bison
+All code listed between @samp{%@{} and @samp{%@}} is copied verbatim
+to the parser implementation file. Such code forms the prologue of
+the grammar file. @xref{Grammar Outline, ,Outline of a Bison
Grammar}.
@end deffn
@deffn {Directive} %code @{@var{code}@}
@deffnx {Directive} %code @var{qualifier} @{@var{code}@}
Insert @var{code} verbatim into output parser source.
-@xref{Decl Summary,,%code}.
+@xref{%code Summary}.
@end deffn
@deffn {Directive} %debug
@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} %defines
-Bison declaration to create a header file meant for the scanner.
-@xref{Decl Summary}.
+Bison declaration to create a parser header file, which is usually
+meant for the scanner. @xref{Decl Summary}.
@end deffn
@deffn {Directive} %defines @var{defines-file}
@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}"
@deffn {Directive} %no-lines
Bison declaration to avoid generating @code{#line} directives in the
-parser file. @xref{Decl Summary}.
+parser implementation file. @xref{Decl Summary}.
@end deffn
@deffn {Directive} %nonassoc
@end deffn
@deffn {Directive} %output "@var{file}"
-Bison declaration to set the name of the parser file. @xref{Decl
-Summary}.
+Bison declaration to set the name of the parser implementation file.
+@xref{Decl Summary}.
@end deffn
@deffn {Directive} %parse-param @{@var{argument-declaration}@}
@end deffn
@deffn {Directive} %token-table
-Bison declaration to include a token name table in the parser file.
-@xref{Decl Summary}.
+Bison declaration to include a token name table in the parser
+implementation file. @xref{Decl Summary}.
@end deffn
@deffn {Directive} %type
The accepting state is thus a consistent state.
@xref{Understanding,,}.
-@item Backus-Naur Form (@acronym{BNF}; also called ``Backus Normal Form'')
+@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.
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{LR}(1). It resolves situations that Bison's
+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 @acronym{IELR}(1)
-A minimal @acronym{LR}(1) parser table generation algorithm.
-That is, given any context-free grammar, @acronym{IELR}(1) generates
+@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
-@acronym{LR}(1) but with nearly the same number of parser states as
-@acronym{LALR}(1).
+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 @acronym{LR}(1)'s extra parser
+More importantly, because canonical LR(1)'s extra parser
states may contain duplicate conflicts in the case of
-non-@acronym{LR}(1) grammars, the number of conflicts for
-@acronym{IELR}(1) is often an order of magnitude less as well.
+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 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 by default; a subset of @acronym{LR}(1).
+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.
@c fill-column: 76
@c End:
-@c LocalWords: texinfo setfilename settitle setchapternewpage finalout
-@c LocalWords: ifinfo smallbook shorttitlepage titlepage GPL FIXME iftex
-@c LocalWords: akim fn cp syncodeindex vr tp synindex dircategory direntry
-@c LocalWords: ifset vskip pt filll insertcopying sp ISBN Etienne Suvasa
-@c LocalWords: ifnottex yyparse detailmenu GLR RPN Calc var Decls Rpcalc
-@c LocalWords: rpcalc Lexer Expr ltcalc mfcalc yylex
-@c LocalWords: yyerror pxref LR yylval cindex dfn LALR samp gpl BNF xref
-@c LocalWords: const int paren ifnotinfo AC noindent emph expr stmt findex
-@c LocalWords: glr YYSTYPE TYPENAME prog dprec printf decl init stmtMerge
-@c LocalWords: pre STDC GNUC endif yy YY alloca lf stddef stdlib YYDEBUG
-@c LocalWords: NUM exp subsubsection kbd Ctrl ctype EOF getchar isdigit
-@c LocalWords: ungetc stdin scanf sc calc ulator ls lm cc NEG prec yyerrok
-@c LocalWords: longjmp fprintf stderr yylloc YYLTYPE cos ln
-@c LocalWords: smallexample symrec val tptr FNCT fnctptr func struct sym
-@c LocalWords: fnct putsym getsym fname arith fncts atan ptr malloc sizeof
-@c LocalWords: strlen strcpy fctn strcmp isalpha symbuf realloc isalnum
-@c LocalWords: ptypes itype YYPRINT trigraphs yytname expseq vindex dtype
-@c LocalWords: Rhs YYRHSLOC LE nonassoc op deffn typeless yynerrs
-@c LocalWords: yychar yydebug msg YYNTOKENS YYNNTS YYNRULES YYNSTATES
-@c LocalWords: cparse clex deftypefun NE defmac YYACCEPT YYABORT param
-@c LocalWords: strncmp intval tindex lvalp locp llocp typealt YYBACKUP
-@c LocalWords: YYEMPTY YYEOF YYRECOVERING yyclearin GE def UMINUS maybeword
-@c LocalWords: Johnstone Shamsa Sadaf Hussain Tomita TR uref YYMAXDEPTH
-@c LocalWords: YYINITDEPTH stmnts ref stmnt initdcl maybeasm notype
-@c LocalWords: hexflag STR exdent itemset asis DYYDEBUG YYFPRINTF args
-@c LocalWords: infile ypp yxx outfile itemx tex leaderfill
-@c LocalWords: hbox hss hfill tt ly yyin fopen fclose ofirst gcc ll
-@c LocalWords: nbar yytext fst snd osplit ntwo strdup AST
-@c LocalWords: YYSTACK DVI fdl printindex IELR
+@c LocalWords: texinfo setfilename settitle setchapternewpage finalout texi FSF
+@c LocalWords: ifinfo smallbook shorttitlepage titlepage GPL FIXME iftex FSF's
+@c LocalWords: akim fn cp syncodeindex vr tp synindex dircategory direntry Naur
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