* 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.
* Glossary:: Basic concepts are explained.
* Copying This Manual:: License for copying this manual.
+* Bibliography:: Publications cited in this manual.
* Index:: Cross-references to the text.
@detailmenu
* Pure Decl:: Requesting a reentrant parser.
* Push Decl:: Requesting a push parser.
* Decl Summary:: Table of all Bison declarations.
+* %define Summary:: Defining variables to adjust Bison's behavior.
+* %code Summary:: Inserting code into the parser source.
Parser C-Language Interface
* Contextual Precedence:: When an operator's precedence depends on context.
* Parser States:: The parser is a finite-state-machine with stack.
* Reduce/Reduce:: When two rules are applicable in the same situation.
-* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
+* Mysterious Conflicts:: Conflicts that look unjustified.
+* Tuning LR:: How to tune fundamental aspects of LR-based parsing.
* Generalized LR Parsing:: Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted. How to avoid it.
* Precedence Examples:: How these features are used in the previous example.
* How Precedence:: How they work.
+Tuning LR
+
+* LR Table Construction:: Choose a different construction algorithm.
+* Default Reductions:: Disable default reductions.
+* LAC:: Correct lookahead sets in the parser states.
+* Unreachable States:: Keep unreachable parser states for debugging.
+
Handling Context Dependencies
* Semantic Tokens:: Token parsing can depend on the semantic context.
@cindex introduction
@dfn{Bison} is a general-purpose parser generator that converts an
-annotated context-free grammar into a deterministic LR or
-generalized LR (GLR) parser employing
-LALR(1), 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.
-
-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.
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 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
BNF is a context-free grammar. The input to Bison is
essentially machine-readable BNF.
-@cindex LALR(1) grammars
-@cindex IELR(1) grammars
-@cindex LR(1) grammars
-There are various important subclasses of context-free grammars.
-Although it can handle almost all context-free grammars, Bison is
-optimized for what are called LR(1) grammars.
-In brief, in these grammars, it must be possible to tell how to parse
-any portion of an input string with just a single token of lookahead.
-For historical reasons, Bison by default is limited by the additional
-restrictions of LALR(1), which is hard to explain simply.
-@xref{Mystery Conflicts, ,Mysterious Reduce/Reduce Conflicts}, for
-more information on this.
-As an experimental feature, you can escape these additional restrictions by
-requesting IELR(1) or canonical LR(1) parser tables.
-@xref{Decl Summary,,lr.type}, to learn how.
+@cindex LALR grammars
+@cindex IELR grammars
+@cindex LR grammars
+There are various important subclasses of context-free grammars. Although
+it can handle almost all context-free grammars, Bison is optimized for what
+are called LR(1) grammars. In brief, in these grammars, it must be possible
+to tell how to parse any portion of an input string with just a single token
+of lookahead. For historical reasons, Bison by default is limited by the
+additional restrictions of LALR(1), which is hard to explain simply.
+@xref{Mysterious Conflicts}, for more information on this. As an
+experimental feature, you can escape these additional restrictions by
+requesting IELR(1) or canonical LR(1) parser tables. @xref{LR Table
+Construction}, to learn how.
@cindex GLR parsing
@cindex generalized LR (GLR) parsing
@end example
The parser can be turned into a GLR parser, while also telling Bison
-to be silent about the one known reduce/reduce conflict, by
-adding these two declarations to the Bison input file (before the first
+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
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-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.
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
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}.
+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 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.
+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 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}
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
a rule are tricky and used only for special purposes (@pxref{Mid-Rule
Actions, ,Actions in Mid-Rule}).
-The C code in an action can refer to the semantic values of the components
-matched by the rule with the construct @code{$@var{n}}, which stands for
-the value of the @var{n}th component. The semantic value for the grouping
-being constructed is @code{$$}. In addition, the semantic values of
-symbols can be accessed with the named references construct
-@code{$@var{name}} or @code{$[@var{name}]}. Bison translates both of these
-constructs into expressions of the appropriate type when it copies the
-actions into the parser file. @code{$$} (or @code{$@var{name}}, when it
-stands for the current grouping) is translated to a modifiable
-lvalue, so it can be assigned to.
+The C code in an action can refer to the semantic values of the
+components matched by the rule with the construct @code{$@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:
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.
+* %define Summary:: Defining variables to adjust Bison's behavior.
+* %code Summary:: Inserting code into the parser source.
@end menu
@node Require Decl
Normally, Bison generates a pull parser.
The following Bison declaration says that you want the parser to be a push
-parser (@pxref{Decl Summary,,%define api.push-pull}):
+parser (@pxref{%define Summary,,api.push-pull}):
@example
%define api.push-pull push
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.}.
+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
-@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}.
+@deffn {Directive} %debug
+Instrument the output parser for traces. Obsoleted by @samp{%define
+parse.trace}.
+@xref{Tracing, ,Tracing Your Parser}.
+@end deffn
-For Java, the default location is inside the parser class.
+@deffn {Directive} %define @var{variable}
+@deffnx {Directive} %define @var{variable} @var{value}
+@deffnx {Directive} %define @var{variable} "@var{value}"
+Define a variable to adjust Bison's behavior. @xref{%define Summary}.
@end deffn
-@deffn {Directive} %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.
+@deffn {Directive} %defines
+Write a parser header file containing macro definitions for the token
+type names defined in the grammar as well as a few other declarations.
+If the parser implementation file is named @file{@var{name}.c} then
+the parser header file is named @file{@var{name}.h}.
-@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:
+For C parsers, the parser header file declares @code{YYSTYPE} unless
+@code{YYSTYPE} is already defined as a macro or you have used a
+@code{<@var{type}>} tag without using @code{%union}. Therefore, if
+you are using a @code{%union} (@pxref{Multiple Types, ,More Than One
+Value Type}) with components that require other definitions, or if you
+have defined a @code{YYSTYPE} macro or type definition (@pxref{Value
+Type, ,Data Types of Semantic Values}), you need to arrange for these
+definitions to be propagated to all modules, e.g., by putting them in
+a prerequisite header that is included both by your parser and by any
+other module that needs @code{YYSTYPE}.
+
+Unless your parser is pure, the parser header file declares
+@code{yylval} as an external variable. @xref{Pure Decl, ,A Pure
+(Reentrant) Parser}.
+
+If you have also used locations, the parser header file declares
+@code{YYLTYPE} and @code{yylloc} using a protocol similar to that of
+the @code{YYSTYPE} macro and @code{yylval}. @xref{Locations,
+,Tracking Locations}.
+
+This parser header file is normally essential if you wish to put the
+definition of @code{yylex} in a separate source file, because
+@code{yylex} typically needs to be able to refer to the
+above-mentioned declarations and to the token type codes. @xref{Token
+Values, ,Semantic Values of Tokens}.
-@itemize @bullet
-@item requires
@findex %code requires
+@findex %code provides
+If you have declared @code{%code requires} or @code{%code provides}, the output
+header also contains their code.
+@xref{%code Summary}.
+@end deffn
-@itemize @bullet
-@item Language(s): C, C++
+@deffn {Directive} %defines @var{defines-file}
+Same as above, but save in the file @var{defines-file}.
+@end deffn
-@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.
+@deffn {Directive} %destructor
+Specify how the parser should reclaim the memory associated to
+discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
-@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
+@deffn {Directive} %file-prefix "@var{prefix}"
+Specify a prefix to use for all Bison output file names. The names
+are chosen as if the grammar file were named @file{@var{prefix}.y}.
+@end deffn
-@item provides
-@findex %code provides
+@deffn {Directive} %language "@var{language}"
+Specify the programming language for the generated parser. Currently
+supported languages include C, C++, and Java.
+@var{language} is case-insensitive.
-@itemize @bullet
-@item Language(s): C, C++
+This directive is experimental and its effect may be modified in future
+releases.
+@end deffn
-@item Purpose: This is the best place to write additional definitions and
-declarations that should be provided to other modules.
+@deffn {Directive} %locations
+Generate the code processing the locations (@pxref{Action Features,
+,Special Features for Use in Actions}). This mode is enabled as soon as
+the grammar uses the special @samp{@@@var{n}} tokens, but if your
+grammar does not use it, using @samp{%locations} allows for more
+accurate syntax error messages.
+@end deffn
-@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
+@deffn {Directive} %name-prefix "@var{prefix}"
+Rename the external symbols used in the parser so that they start with
+@var{prefix} instead of @samp{yy}. The precise list of symbols renamed
+in C parsers
+is @code{yyparse}, @code{yylex}, @code{yyerror}, @code{yynerrs},
+@code{yylval}, @code{yychar}, @code{yydebug}, and
+(if locations are used) @code{yylloc}. If you use a push parser,
+@code{yypush_parse}, @code{yypull_parse}, @code{yypstate},
+@code{yypstate_new} and @code{yypstate_delete} will
+also be renamed. For example, if you use @samp{%name-prefix "c_"}, the
+names become @code{c_parse}, @code{c_lex}, and so on.
+For C++ parsers, see the @samp{%define api.namespace} documentation in this
+section.
+@xref{Multiple Parsers, ,Multiple Parsers in the Same Program}.
+@end deffn
-@item top
-@findex %code top
+@ifset defaultprec
+@deffn {Directive} %no-default-prec
+Do not assign a precedence to rules lacking an explicit @code{%prec}
+modifier (@pxref{Contextual Precedence, ,Context-Dependent
+Precedence}).
+@end deffn
+@end ifset
-@itemize @bullet
-@item Language(s): C, C++
+@deffn {Directive} %no-lines
+Don't generate any @code{#line} preprocessor commands in the parser
+implementation file. Ordinarily Bison writes these commands in the
+parser implementation file so that the C compiler and debuggers will
+associate errors and object code with your source file (the grammar
+file). This directive causes them to associate errors with the parser
+implementation file, treating it as an independent source file in its
+own right.
+@end deffn
-@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:
+@deffn {Directive} %output "@var{file}"
+Specify @var{file} for the parser implementation file.
+@end deffn
-@smallexample
-%code top @{
- #define _GNU_SOURCE
- #include <stdio.h>
-@}
-@end smallexample
+@deffn {Directive} %pure-parser
+Deprecated version of @samp{%define api.pure} (@pxref{%define
+Summary,,api.pure}), for which Bison is more careful to warn about
+unreasonable usage.
+@end deffn
-@item Location(s): Near the top of the parser source code file.
-@end itemize
+@deffn {Directive} %require "@var{version}"
+Require version @var{version} or higher of Bison. @xref{Require Decl, ,
+Require a Version of Bison}.
+@end deffn
-@item imports
-@findex %code imports
+@deffn {Directive} %skeleton "@var{file}"
+Specify the skeleton to use.
-@itemize @bullet
-@item Language(s): Java
+@c You probably don't need this option unless you are developing Bison.
+@c You should use @code{%language} if you want to specify the skeleton for a
+@c different language, because it is clearer and because it will always choose the
+@c correct skeleton for non-deterministic or push parsers.
-@item Purpose: This is the best place to write Java import directives.
+If @var{file} does not contain a @code{/}, @var{file} is the name of a skeleton
+file in the Bison installation directory.
+If it does, @var{file} is an absolute file name or a file name relative to the
+directory of the grammar file.
+This is similar to how most shells resolve commands.
+@end deffn
-@item Location(s): The parser Java file after any Java package directive and
-before any class definitions.
-@end itemize
-@end itemize
+@deffn {Directive} %token-table
+Generate an array of token names in the parser implementation file.
+The name of the array is @code{yytname}; @code{yytname[@var{i}]} is
+the name of the token whose internal Bison token code number is
+@var{i}. The first three elements of @code{yytname} correspond to the
+predefined tokens @code{"$end"}, @code{"error"}, and
+@code{"$undefined"}; after these come the symbols defined in the
+grammar file.
-@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}.
+The name in the table includes all the characters needed to represent
+the token in Bison. For single-character literals and literal
+strings, this includes the surrounding quoting characters and any
+escape sequences. For example, the Bison single-character literal
+@code{'+'} corresponds to a three-character name, represented in C as
+@code{"'+'"}; and the Bison two-character literal string @code{"\\/"}
+corresponds to a five-character name, represented in C as
+@code{"\"\\\\/\""}.
+
+When you specify @code{%token-table}, Bison also generates macro
+definitions for macros @code{YYNTOKENS}, @code{YYNNTS}, and
+@code{YYNRULES}, and @code{YYNSTATES}:
+
+@table @code
+@item YYNTOKENS
+The highest token number, plus one.
+@item YYNNTS
+The number of nonterminal symbols.
+@item YYNRULES
+The number of grammar rules,
+@item YYNSTATES
+The number of parser states (@pxref{Parser States}).
+@end table
@end deffn
-@deffn {Directive} %debug
-Instrument the output parser for traces. Obsoleted by @samp{%define
-parse.trace}.
-@xref{Tracing, ,Tracing Your Parser}.
+@deffn {Directive} %verbose
+Write an extra output file containing verbose descriptions of the
+parser states and what is done for each type of lookahead token in
+that state. @xref{Understanding, , Understanding Your Parser}, for more
+information.
@end deffn
+@deffn {Directive} %yacc
+Pretend the option @option{--yacc} was given, i.e., imitate Yacc,
+including its naming conventions. @xref{Bison Options}, for more.
+@end deffn
+
+
+@node %define Summary
+@subsection %define Summary
+
+There are many features of Bison's behavior that can be controlled by
+assigning the feature a single value. For historical reasons, some
+such features are assigned values by dedicated directives, such as
+@code{%start}, which assigns the start symbol. However, newer such
+features are associated with variables, which are assigned by the
+@code{%define} directive:
+
@deffn {Directive} %define @var{variable}
@deffnx {Directive} %define @var{variable} @var{value}
@deffnx {Directive} %define @var{variable} "@var{value}"
-Define a variable to adjust Bison's behavior.
-
-It is an error if a @var{variable} is defined by @code{%define} multiple
-times, but see @ref{Bison Options,,-D @var{name}[=@var{value}]}.
+Define @var{variable} to @var{value}.
@var{value} must be placed in quotation marks if it contains any
-character other than a letter, underscore, period, dash, or non-initial
-digit.
+character other than a letter, underscore, period, or non-initial dash
+or digit. Omitting @code{"@var{value}"} entirely is always equivalent
+to specifying @code{""}.
+
+It is an error if a @var{variable} is defined by @code{%define}
+multiple times, but see @ref{Bison Options,,-D
+@var{name}[=@var{value}]}.
+@end deffn
-Omitting @code{"@var{value}"} entirely is always equivalent to specifying
-@code{""}.
+The rest of this section summarizes variables and values that
+@code{%define} accepts.
-Some @var{variable}s take Boolean values.
-In this case, Bison will complain if the variable definition does not meet one
-of the following four conditions:
+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}
@c ================================================== lex_symbol
-@item variant
+@item lex_symbol
@findex %define lex_symbol
@itemize @bullet
@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: Specify the kind of states that are permitted to
-contain default reductions.
-That is, in such a state, Bison selects the reduction with the largest
-lookahead set to be the default parser action and then removes that
-lookahead set.
-(The ability to specify where default reductions should be used is
-experimental.
-More user feedback will help to stabilize it.)
-
-@item Accepted Values:
-@itemize
-@item @code{all}.
-This is the traditional Bison behavior.
-The main advantage is a significant decrease in the size of the parser
-tables.
-The disadvantage is that, when the generated parser encounters a
-syntactically unacceptable token, the parser might then perform
-unnecessary default reductions before it can detect the syntax error.
-Such delayed syntax error detection is usually inherent in
-LALR and IELR parser tables anyway due to
-LR state merging (@pxref{Decl Summary,,lr.type}).
-Furthermore, the use of @code{%nonassoc} can contribute to delayed
-syntax error detection even in the case of canonical LR.
-As an experimental feature, delayed syntax error detection can be
-overcome in all cases by enabling LAC (@pxref{Decl
-Summary,,parse.lac}, for details, including a discussion of the effects
-of delayed syntax error detection).
-
-@item @code{consistent}.
-@cindex consistent states
-A consistent state is a state that has only one possible action.
-If that action is a reduction, then the parser does not need to request
-a lookahead token from the scanner before performing that action.
-However, the parser recognizes the ability to ignore the lookahead token
-in this way only when such a reduction is encoded as a default
-reduction.
-Thus, if default reductions are permitted only in consistent states,
-then a canonical LR parser that does not employ
-@code{%nonassoc} detects a syntax error as soon as it @emph{needs} the
-syntactically unacceptable token from the scanner.
-
-@item @code{accepting}.
-@cindex accepting state
-In the accepting state, the default reduction is actually the accept
-action.
-In this case, a canonical LR parser that does not employ
-@code{%nonassoc} detects a syntax error as soon as it @emph{reaches} the
-syntactically unacceptable token in the input.
-That is, it does not perform any extra reductions.
-@end itemize
+contain default reductions. @xref{Default Reductions}. (The ability to
+specify where default reductions should be used is experimental. More user
+feedback will help to stabilize it.)
+@item Accepted Values: @code{most}, @code{consistent}, @code{accepting}
@item Default Value:
@itemize
@item @code{accepting} if @code{lr.type} is @code{canonical-lr}.
-@item @code{all} otherwise.
+@item @code{most} otherwise.
@end itemize
@end itemize
@itemize @bullet
@item Language(s): all
-
@item Purpose: Request that Bison allow unreachable parser states to
-remain in the parser tables.
-Bison considers a state to be unreachable if there exists no sequence of
-transitions from the start state to that state.
-A state can become unreachable during conflict resolution if Bison disables a
-shift action leading to it from a predecessor state.
-Keeping unreachable states is sometimes useful for analysis purposes, but they
-are useless in the generated parser.
-
+remain in the parser tables. @xref{Unreachable States}.
@item Accepted Values: Boolean
-
@item Default Value: @code{false}
-
-@item Caveats:
-
-@itemize @bullet
-
-@item Unreachable states may contain conflicts and may use rules not used in
-any other state.
-Thus, keeping unreachable states may induce warnings that are irrelevant to
-your parser's behavior, and it may eliminate warnings that are relevant.
-Of course, the change in warnings may actually be relevant to a parser table
-analysis that wants to keep unreachable states, so this behavior will likely
-remain in future Bison releases.
-
-@item While Bison is able to remove unreachable states, it is not guaranteed to
-remove other kinds of useless states.
-Specifically, when Bison disables reduce actions during conflict resolution,
-some goto actions may become useless, and thus some additional states may
-become useless.
-If Bison were to compute which goto actions were useless and then disable those
-actions, it could identify such states as unreachable and then remove those
-states.
-However, Bison does not compute which goto actions are useless.
-@end itemize
@end itemize
@c lr.keep-unreachable-states
@item lr.type
@findex %define lr.type
-@cindex LALR
-@cindex IELR
-@cindex LR
@itemize @bullet
@item Language(s): all
@item Purpose: Specify the type of parser tables within the
-LR(1) family.
-(This feature is experimental.
+LR(1) family. @xref{LR Table Construction}. (This feature is experimental.
More user feedback will help to stabilize it.)
-@item Accepted Values:
-@itemize
-@item @code{lalr}.
-While Bison generates LALR parser tables by default for
-historical reasons, IELR or canonical LR is almost
-always preferable for deterministic parsers.
-The trouble is that LALR parser tables can suffer from
-mysterious conflicts and thus may not accept the full set of sentences
-that IELR and canonical LR accept.
-@xref{Mystery Conflicts}, for details.
-However, there are at least two scenarios where LALR may be
-worthwhile:
-@itemize
-@cindex GLR with LALR
-@item When employing GLR parsers (@pxref{GLR Parsers}), if you
-do not resolve any conflicts statically (for example, with @code{%left}
-or @code{%prec}), then the parser explores all potential parses of any
-given input.
-In this case, the use of LALR parser tables is guaranteed not
-to alter the language accepted by the parser.
-LALR parser tables are the smallest parser tables Bison can
-currently generate, so they may be preferable.
-Nevertheless, once you begin to resolve conflicts statically,
-GLR begins to behave more like a deterministic parser, and so
-IELR and canonical LR can be helpful to avoid
-LALR's mysterious behavior.
-
-@item Occasionally during development, an especially malformed grammar
-with a major recurring flaw may severely impede the IELR or
-canonical LR parser table generation algorithm.
-LALR can be a quick way to generate parser tables in order to
-investigate such problems while ignoring the more subtle differences
-from IELR and canonical LR.
-@end itemize
-
-@item @code{ielr}.
-IELR is a minimal LR algorithm.
-That is, given any grammar (LR or non-LR),
-IELR and canonical LR always accept exactly the same
-set of sentences.
-However, as for LALR, the number of parser states is often an
-order of magnitude less for IELR than for canonical
-LR.
-More importantly, because canonical LR's extra parser states
-may contain duplicate conflicts in the case of non-LR
-grammars, the number of conflicts for IELR is often an order
-of magnitude less as well.
-This can significantly reduce the complexity of developing of a grammar.
-
-@item @code{canonical-lr}.
-@cindex delayed syntax errors
-@cindex syntax errors delayed
-@cindex LAC
-@findex %nonassoc
-While inefficient, canonical LR parser tables can be an
-interesting means to explore a grammar because they have a property that
-IELR and LALR tables do not.
-That is, if @code{%nonassoc} is not used and default reductions are left
-disabled (@pxref{Decl Summary,,lr.default-reductions}), then, for every
-left context of every canonical LR state, the set of tokens
-accepted by that state is guaranteed to be the exact set of tokens that
-is syntactically acceptable in that left context.
-It might then seem that an advantage of canonical LR parsers
-in production is that, under the above constraints, they are guaranteed
-to detect a syntax error as soon as possible without performing any
-unnecessary reductions.
-However, IELR parsers using LAC (@pxref{Decl
-Summary,,parse.lac}) are also able to achieve this behavior without
-sacrificing @code{%nonassoc} or default reductions.
-@end itemize
+@item Accepted Values: @code{lalr}, @code{ielr}, @code{canonical-lr}
@item Default Value: @code{lalr}
@end itemize
Error messages passed to @code{yyerror} are simply @w{@code{"syntax
error"}}.
@item @code{verbose}
-Error messages report the unexpected token, and possibly the expected
-ones.
+Error messages report the unexpected token, and possibly the expected ones.
+However, this report can often be incorrect when LAC is not enabled
+(@pxref{LAC}).
@end itemize
@item Default Value:
@c ================================================== parse.lac
@item parse.lac
@findex %define parse.lac
-@cindex LAC
-@cindex lookahead correction
@itemize
-@item Languages(s): C
+@item Languages(s): C (deterministic parsers only)
@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.
-
+syntax error handling. @xref{LAC}.
@item Accepted Values: @code{none}, @code{full}
-
@item Default Value: @code{none}
@end itemize
@c parse.lac
@item Languages(s): C, C++
@item Purpose: Require parser instrumentation for tracing.
-In C/C++, define the macro @code{YYDEBUG} to 1 in the parser file if it
-is not already defined, so that the debugging facilities are compiled.
-@xref{Tracing, ,Tracing Your Parser}.
+In C/C++, define the macro @code{YYDEBUG} to 1 in the parser implementation
+file if it is not already defined, so that the debugging facilities are
+compiled. @xref{Tracing, ,Tracing Your Parser}.
@item Accepted Values: Boolean
@code{false}
@end itemize
@c variant
+@end table
-@end table
-@end deffn
-@c ---------------------------------------------------------- %define
+@node %code Summary
+@subsection %code Summary
+@findex %code
+@cindex Prologue
-@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}.
+The @code{%code} directive inserts code verbatim into the output
+parser source at any of a predefined set of locations. It thus serves
+as a flexible and user-friendly alternative to the traditional Yacc
+prologue, @code{%@{@var{code}%@}}. This section summarizes the
+functionality of @code{%code} for the various target languages
+supported by Bison. For a detailed discussion of how to use
+@code{%code} in place of @code{%@{@var{code}%@}} for C/C++ and why it
+is advantageous to do so, @pxref{Prologue Alternatives}.
-For C parsers, the output header 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{YYLTYPE} and @code{yylloc} using a protocol similar to that of
-the @code{YYSTYPE} macro and @code{yylval}. @xref{Locations, ,Tracking
-Locations}.
+@deffn {Directive} %code @{@var{code}@}
+This is the unqualified form of the @code{%code} directive. It
+inserts @var{code} verbatim at a language-dependent default location
+in the parser implementation.
-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}.
+For C/C++, the default location is the parser implementation file
+after the usual contents of the parser header file. Thus, the
+unqualified form replaces @code{%@{@var{code}%@}} for most purposes.
-@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}.
+For Java, the default location is inside the parser class.
@end deffn
-@deffn {Directive} %defines @var{defines-file}
-Same as above, but save in the file @var{defines-file}.
+@deffn {Directive} %code @var{qualifier} @{@var{code}@}
+This is the qualified form of the @code{%code} directive.
+@var{qualifier} identifies the purpose of @var{code} and thus the
+location(s) where Bison should insert it. That is, if you need to
+specify location-sensitive @var{code} that does not belong at the
+default location selected by the unqualified @code{%code} form, use
+this form instead.
@end deffn
-@deffn {Directive} %destructor
-Specify how the parser should reclaim the memory associated to
-discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}.
-@end deffn
+For any particular qualifier or for the unqualified form, if there are
+multiple occurrences of the @code{%code} directive, Bison concatenates
+the specified code in the order in which it appears in the grammar
+file.
-@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}.
-@end deffn
+Not all qualifiers are accepted for all target languages. Unaccepted
+qualifiers produce an error. Some of the accepted qualifiers are:
-@deffn {Directive} %language "@var{language}"
-Specify the programming language for the generated parser. Currently
-supported languages include C, C++, and Java.
-@var{language} is case-insensitive.
+@table @code
+@item requires
+@findex %code requires
-This directive is experimental and its effect may be modified in future
-releases.
-@end deffn
+@itemize @bullet
+@item Language(s): C, C++
-@deffn {Directive} %locations
-Generate the code processing the locations (@pxref{Action Features,
-,Special Features for Use in Actions}). This mode is enabled as soon as
-the grammar uses the special @samp{@@@var{n}} tokens, but if your
-grammar does not use it, using @samp{%locations} allows for more
-accurate syntax error messages.
-@end deffn
+@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.
-@deffn {Directive} %name-prefix "@var{prefix}"
-Rename the external symbols used in the parser so that they start with
-@var{prefix} instead of @samp{yy}. The precise list of symbols renamed
-in C parsers
-is @code{yyparse}, @code{yylex}, @code{yyerror}, @code{yynerrs},
-@code{yylval}, @code{yychar}, @code{yydebug}, and
-(if locations are used) @code{yylloc}. If you use a push parser,
-@code{yypush_parse}, @code{yypull_parse}, @code{yypstate},
-@code{yypstate_new} and @code{yypstate_delete} will
-also be renamed. For example, if you use @samp{%name-prefix "c_"}, the
-names become @code{c_parse}, @code{c_lex}, and so on.
-For C++ parsers, see the @samp{%define api.namespace} documentation in this
-section.
-@xref{Multiple Parsers, ,Multiple Parsers in the Same Program}.
-@end deffn
+@item Location(s): The parser header file and the parser implementation file
+before the Bison-generated @code{YYSTYPE} and @code{YYLTYPE}
+definitions.
+@end itemize
-@ifset defaultprec
-@deffn {Directive} %no-default-prec
-Do not assign a precedence to rules lacking an explicit @code{%prec}
-modifier (@pxref{Contextual Precedence, ,Context-Dependent
-Precedence}).
-@end deffn
-@end ifset
+@item provides
+@findex %code provides
-@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.
-@end deffn
+@itemize @bullet
+@item Language(s): C, C++
-@deffn {Directive} %output "@var{file}"
-Specify @var{file} for the parser file.
-@end deffn
+@item Purpose: This is the best place to write additional definitions and
+declarations that should be provided to other modules.
-@deffn {Directive} %pure-parser
-Deprecated version of @samp{%define api.pure} (@pxref{Decl Summary, ,%define}),
-for which Bison is more careful to warn about unreasonable usage.
-@end deffn
+@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
-@deffn {Directive} %require "@var{version}"
-Require version @var{version} or higher of Bison. @xref{Require Decl, ,
-Require a Version of Bison}.
-@end deffn
+@item top
+@findex %code top
-@deffn {Directive} %skeleton "@var{file}"
-Specify the skeleton to use.
+@itemize @bullet
+@item Language(s): C, C++
-@c You probably don't need this option unless you are developing Bison.
-@c You should use @code{%language} if you want to specify the skeleton for a
-@c different language, because it is clearer and because it will always choose the
-@c correct skeleton for non-deterministic or push parsers.
+@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:
-If @var{file} does not contain a @code{/}, @var{file} is the name of a skeleton
-file in the Bison installation directory.
-If it does, @var{file} is an absolute file name or a file name relative to the
-directory of the grammar file.
-This is similar to how most shells resolve commands.
-@end deffn
+@smallexample
+%code top @{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+@}
+@end smallexample
-@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.
+@item Location(s): Near the top of the parser implementation file.
+@end itemize
-The name in the table includes all the characters needed to represent
-the token in Bison. For single-character literals and literal
-strings, this includes the surrounding quoting characters and any
-escape sequences. For example, the Bison single-character literal
-@code{'+'} corresponds to a three-character name, represented in C as
-@code{"'+'"}; and the Bison two-character literal string @code{"\\/"}
-corresponds to a five-character name, represented in C as
-@code{"\"\\\\/\""}.
+@item imports
+@findex %code imports
-When you specify @code{%token-table}, Bison also generates macro
-definitions for macros @code{YYNTOKENS}, @code{YYNNTS}, and
-@code{YYNRULES}, and @code{YYNSTATES}:
+@itemize @bullet
+@item Language(s): Java
-@table @code
-@item YYNTOKENS
-The highest token number, plus one.
-@item YYNNTS
-The number of nonterminal symbols.
-@item YYNRULES
-The number of grammar rules,
-@item YYNSTATES
-The number of parser states (@pxref{Parser States}).
-@end table
-@end deffn
+@item Purpose: This is the best place to write Java import directives.
-@deffn {Directive} %verbose
-Write an extra output file containing verbose descriptions of the
-parser states and what is done for each type of lookahead token in
-that state. @xref{Understanding, , Understanding Your Parser}, for more
-information.
-@end deffn
+@item Location(s): The parser Java file after any Java package directive and
+before any class definitions.
+@end itemize
+@end table
-@deffn {Directive} %yacc
-Pretend the option @option{--yacc} was given, i.e., imitate Yacc,
-including its naming conventions. @xref{Bison Options}, for more.
-@end deffn
+Though we say the insertion locations are language-dependent, they are
+technically skeleton-dependent. Writers of non-standard skeletons
+however should choose their locations consistently with the behavior
+of the standard Bison skeletons.
@node Multiple Parsers
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
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.
@w{@code{"syntax error"}}.
@findex %define parse.error
-If you invoke @samp{%define parse.error verbose} in the Bison
-declarations section (@pxref{Bison Declarations, ,The Bison Declarations
-Section}), then Bison provides a more verbose and specific error message
-string instead of just plain @w{@code{"syntax error"}}.
+If you invoke @samp{%define parse.error verbose} in the Bison declarations
+section (@pxref{Bison Declarations, ,The Bison Declarations Section}), then
+Bison provides a more verbose and specific error message string instead of
+just plain @w{@code{"syntax error"}}. However, that message sometimes
+contains incorrect information if LAC is not enabled (@pxref{LAC}).
The parser can detect one other kind of error: memory exhaustion. This
can happen when the input contains constructions that are very deeply
* Contextual Precedence:: When an operator's precedence depends on context.
* Parser States:: The parser is a finite-state-machine with stack.
* Reduce/Reduce:: When two rules are applicable in the same situation.
-* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
+* Mysterious Conflicts:: Conflicts that look unjustified.
+* Tuning LR:: How to tune fundamental aspects of LR-based parsing.
* Generalized LR Parsing:: Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted. How to avoid it.
@end menu
The definition of @code{if_stmt} above is solely to blame for the
conflict, but the conflict does not actually appear without additional
-rules. Here is a complete Bison input file that actually manifests the
-conflict:
+rules. Here is a complete Bison grammar file that actually manifests
+the conflict:
@example
@group
;
@end example
-@node Mystery Conflicts
-@section Mysterious Reduce/Reduce Conflicts
+@node Mysterious Conflicts
+@section Mysterious Conflicts
+@cindex Mysterious Conflicts
Sometimes reduce/reduce conflicts can occur that don't look warranted.
Here is an example:
a @code{name} if a comma or colon follows, or a @code{type} if another
@code{ID} follows. In other words, this grammar is LR(1).
-@cindex LR(1)
-@cindex LALR(1)
+@cindex LR
+@cindex LALR
However, for historical reasons, Bison cannot by default handle all
LR(1) grammars.
In this grammar, two contexts, that after an @code{ID} at the beginning
the two contexts causes a conflict later. In parser terminology, this
occurrence means that the grammar is not LALR(1).
-For many practical grammars (specifically those that fall into the
-non-LR(1) class), the limitations of LALR(1) result in
-difficulties beyond just mysterious reduce/reduce conflicts.
-The best way to fix all these problems is to select a different parser
-table generation algorithm.
-Either IELR(1) or canonical LR(1) would suffice, but
-the former is more efficient and easier to debug during development.
-@xref{Decl Summary,,lr.type}, for details.
-(Bison's IELR(1) and canonical LR(1) implementations
-are experimental.
-More user feedback will help to stabilize them.)
+@cindex IELR
+@cindex canonical LR
+For many practical grammars (specifically those that fall into the non-LR(1)
+class), the limitations of LALR(1) result in difficulties beyond just
+mysterious reduce/reduce conflicts. The best way to fix all these problems
+is to select a different parser table construction algorithm. Either
+IELR(1) or canonical LR(1) would suffice, but the former is more efficient
+and easier to debug during development. @xref{LR Table Construction}, for
+details. (Bison's IELR(1) and canonical LR(1) implementations are
+experimental. More user feedback will help to stabilize them.)
If you instead wish to work around LALR(1)'s limitations, you
can often fix a mysterious conflict by identifying the two parser states
@end example
For a more detailed exposition of LALR(1) parsers and parser
-generators, please see:
-Frank DeRemer and Thomas Pennello, Efficient Computation of
-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}.
+generators, @pxref{Bibliography,,DeRemer 1982}.
+
+@node Tuning LR
+@section Tuning LR
+
+The default behavior of Bison's LR-based parsers is chosen mostly for
+historical reasons, but that behavior is often not robust. For example, in
+the previous section, we discussed the mysterious conflicts that can be
+produced by LALR(1), Bison's default parser table construction algorithm.
+Another example is Bison's @code{%define parse.error verbose} directive,
+which instructs the generated parser to produce verbose syntax error
+messages, which can sometimes contain incorrect information.
+
+In this section, we explore several modern features of Bison that allow you
+to tune fundamental aspects of the generated LR-based parsers. Some of
+these features easily eliminate shortcomings like those mentioned above.
+Others can be helpful purely for understanding your parser.
+
+Most of the features discussed in this section are still experimental. More
+user feedback will help to stabilize them.
+
+@menu
+* LR Table Construction:: Choose a different construction algorithm.
+* Default Reductions:: Disable default reductions.
+* LAC:: Correct lookahead sets in the parser states.
+* Unreachable States:: Keep unreachable parser states for debugging.
+@end menu
+
+@node LR Table Construction
+@subsection LR Table Construction
+@cindex Mysterious Conflict
+@cindex LALR
+@cindex IELR
+@cindex canonical LR
+@findex %define lr.type
+
+For historical reasons, Bison constructs LALR(1) parser tables by default.
+However, LALR does not possess the full language-recognition power of LR.
+As a result, the behavior of parsers employing LALR parser tables is often
+mysterious. We presented a simple example of this effect in @ref{Mysterious
+Conflicts}.
+
+As we also demonstrated in that example, the traditional approach to
+eliminating such mysterious behavior is to restructure the grammar.
+Unfortunately, doing so correctly is often difficult. Moreover, merely
+discovering that LALR causes mysterious behavior in your parser can be
+difficult as well.
+
+Fortunately, Bison provides an easy way to eliminate the possibility of such
+mysterious behavior altogether. You simply need to activate a more powerful
+parser table construction algorithm by using the @code{%define lr.type}
+directive.
+
+@deffn {Directive} {%define lr.type @var{TYPE}}
+Specify the type of parser tables within the LR(1) family. The accepted
+values for @var{TYPE} are:
+
+@itemize
+@item @code{lalr} (default)
+@item @code{ielr}
+@item @code{canonical-lr}
+@end itemize
+
+(This feature is experimental. More user feedback will help to stabilize
+it.)
+@end deffn
+
+For example, to activate IELR, you might add the following directive to you
+grammar file:
+
+@example
+%define lr.type ielr
+@end example
+
+@noindent For the example in @ref{Mysterious Conflicts}, the mysterious
+conflict is then eliminated, so there is no need to invest time in
+comprehending the conflict or restructuring the grammar to fix it. If,
+during future development, the grammar evolves such that all mysterious
+behavior would have disappeared using just LALR, you need not fear that
+continuing to use IELR will result in unnecessarily large parser tables.
+That is, IELR generates LALR tables when LALR (using a deterministic parsing
+algorithm) is sufficient to support the full language-recognition power of
+LR. Thus, by enabling IELR at the start of grammar development, you can
+safely and completely eliminate the need to consider LALR's shortcomings.
+
+While IELR is almost always preferable, there are circumstances where LALR
+or the canonical LR parser tables described by Knuth
+(@pxref{Bibliography,,Knuth 1965}) can be useful. Here we summarize the
+relative advantages of each parser table construction algorithm within
+Bison:
+
+@itemize
+@item LALR
+
+There are at least two scenarios where LALR can be worthwhile:
+
+@itemize
+@item GLR without static conflict resolution.
+
+@cindex GLR with LALR
+When employing GLR parsers (@pxref{GLR Parsers}), if you do not resolve any
+conflicts statically (for example, with @code{%left} or @code{%prec}), then
+the parser explores all potential parses of any given input. In this case,
+the choice of parser table construction algorithm is guaranteed not to alter
+the language accepted by the parser. LALR parser tables are the smallest
+parser tables Bison can currently construct, so they may then be preferable.
+Nevertheless, once you begin to resolve conflicts statically, GLR behaves
+more like a deterministic parser in the syntactic contexts where those
+conflicts appear, and so either IELR or canonical LR can then be helpful to
+avoid LALR's mysterious behavior.
+
+@item Malformed grammars.
+
+Occasionally during development, an especially malformed grammar with a
+major recurring flaw may severely impede the IELR or canonical LR parser
+table construction algorithm. LALR can be a quick way to construct parser
+tables in order to investigate such problems while ignoring the more subtle
+differences from IELR and canonical LR.
+@end itemize
+
+@item IELR
+
+IELR (Inadequacy Elimination LR) is a minimal LR algorithm. That is, given
+any grammar (LR or non-LR), parsers using IELR or canonical LR parser tables
+always accept exactly the same set of sentences. However, like LALR, IELR
+merges parser states during parser table construction so that the number of
+parser states is often an order of magnitude less than for canonical LR.
+More importantly, because canonical LR's extra parser states may contain
+duplicate conflicts in the case of non-LR grammars, the number of conflicts
+for IELR is often an order of magnitude less as well. This effect can
+significantly reduce the complexity of developing a grammar.
+
+@item Canonical LR
+
+@cindex delayed syntax error detection
+@cindex LAC
+@findex %nonassoc
+While inefficient, canonical LR parser tables can be an interesting means to
+explore a grammar because they possess a property that IELR and LALR tables
+do not. That is, if @code{%nonassoc} is not used and default reductions are
+left disabled (@pxref{Default Reductions}), then, for every left context of
+every canonical LR state, the set of tokens accepted by that state is
+guaranteed to be the exact set of tokens that is syntactically acceptable in
+that left context. It might then seem that an advantage of canonical LR
+parsers in production is that, under the above constraints, they are
+guaranteed to detect a syntax error as soon as possible without performing
+any unnecessary reductions. However, IELR parsers that use LAC are also
+able to achieve this behavior without sacrificing @code{%nonassoc} or
+default reductions. For details and a few caveats of LAC, @pxref{LAC}.
+@end itemize
+
+For a more detailed exposition of the mysterious behavior in LALR parsers
+and the benefits of IELR, @pxref{Bibliography,,Denny 2008 March}, and
+@ref{Bibliography,,Denny 2010 November}.
+
+@node Default Reductions
+@subsection Default Reductions
+@cindex default reductions
+@findex %define lr.default-reductions
+@findex %nonassoc
+
+After parser table construction, Bison identifies the reduction with the
+largest lookahead set in each parser state. To reduce the size of the
+parser state, traditional Bison behavior is to remove that lookahead set and
+to assign that reduction to be the default parser action. Such a reduction
+is known as a @dfn{default reduction}.
+
+Default reductions affect more than the size of the parser tables. They
+also affect the behavior of the parser:
+
+@itemize
+@item Delayed @code{yylex} invocations.
+
+@cindex delayed yylex invocations
+@cindex consistent states
+@cindex defaulted states
+A @dfn{consistent state} is a state that has only one possible parser
+action. If that action is a reduction and is encoded as a default
+reduction, then that consistent state is called a @dfn{defaulted state}.
+Upon reaching a defaulted state, a Bison-generated parser does not bother to
+invoke @code{yylex} to fetch the next token before performing the reduction.
+In other words, whether default reductions are enabled in consistent states
+determines how soon a Bison-generated parser invokes @code{yylex} for a
+token: immediately when it @emph{reaches} that token in the input or when it
+eventually @emph{needs} that token as a lookahead to determine the next
+parser action. Traditionally, default reductions are enabled, and so the
+parser exhibits the latter behavior.
+
+The presence of defaulted states is an important consideration when
+designing @code{yylex} and the grammar file. That is, if the behavior of
+@code{yylex} can influence or be influenced by the semantic actions
+associated with the reductions in defaulted states, then the delay of the
+next @code{yylex} invocation until after those reductions is significant.
+For example, the semantic actions might pop a scope stack that @code{yylex}
+uses to determine what token to return. Thus, the delay might be necessary
+to ensure that @code{yylex} does not look up the next token in a scope that
+should already be considered closed.
+
+@item Delayed syntax error detection.
+
+@cindex delayed syntax error detection
+When the parser fetches a new token by invoking @code{yylex}, it checks
+whether there is an action for that token in the current parser state. The
+parser detects a syntax error if and only if either (1) there is no action
+for that token or (2) the action for that token is the error action (due to
+the use of @code{%nonassoc}). However, if there is a default reduction in
+that state (which might or might not be a defaulted state), then it is
+impossible for condition 1 to exist. That is, all tokens have an action.
+Thus, the parser sometimes fails to detect the syntax error until it reaches
+a later state.
+
+@cindex LAC
+@c If there's an infinite loop, default reductions can prevent an incorrect
+@c sentence from being rejected.
+While default reductions never cause the parser to accept syntactically
+incorrect sentences, the delay of syntax error detection can have unexpected
+effects on the behavior of the parser. However, the delay can be caused
+anyway by parser state merging and the use of @code{%nonassoc}, and it can
+be fixed by another Bison feature, LAC. We discuss the effects of delayed
+syntax error detection and LAC more in the next section (@pxref{LAC}).
+@end itemize
+
+For canonical LR, the only default reduction that Bison enables by default
+is the accept action, which appears only in the accepting state, which has
+no other action and is thus a defaulted state. However, the default accept
+action does not delay any @code{yylex} invocation or syntax error detection
+because the accept action ends the parse.
+
+For LALR and IELR, Bison enables default reductions in nearly all states by
+default. There are only two exceptions. First, states that have a shift
+action on the @code{error} token do not have default reductions because
+delayed syntax error detection could then prevent the @code{error} token
+from ever being shifted in that state. However, parser state merging can
+cause the same effect anyway, and LAC fixes it in both cases, so future
+versions of Bison might drop this exception when LAC is activated. Second,
+GLR parsers do not record the default reduction as the action on a lookahead
+token for which there is a conflict. The correct action in this case is to
+split the parse instead.
+
+To adjust which states have default reductions enabled, use the
+@code{%define lr.default-reductions} directive.
+
+@deffn {Directive} {%define lr.default-reductions @var{WHERE}}
+Specify the kind of states that are permitted to contain default reductions.
+The accepted values of @var{WHERE} are:
+@itemize
+@item @code{most} (default for LALR and IELR)
+@item @code{consistent}
+@item @code{accepting} (default for canonical LR)
+@end itemize
+
+(The ability to specify where default reductions are permitted is
+experimental. More user feedback will help to stabilize it.)
+@end deffn
+
+@node LAC
+@subsection LAC
+@findex %define parse.lac
+@cindex LAC
+@cindex lookahead correction
+
+Canonical LR, IELR, and LALR can suffer from a couple of problems upon
+encountering a syntax error. First, the parser might perform additional
+parser stack reductions before discovering the syntax error. Such
+reductions can perform user semantic actions that are unexpected because
+they are based on an invalid token, and they cause error recovery to begin
+in a different syntactic context than the one in which the invalid token was
+encountered. Second, when verbose error messages are enabled (@pxref{Error
+Reporting}), the expected token list in the syntax error message can both
+contain invalid tokens and omit valid tokens.
+
+The culprits for the above problems are @code{%nonassoc}, default reductions
+in inconsistent states (@pxref{Default Reductions}), and parser state
+merging. Because IELR and LALR merge parser states, they suffer the most.
+Canonical LR can suffer only if @code{%nonassoc} is used or if default
+reductions are enabled for inconsistent states.
+
+LAC (Lookahead Correction) is a new mechanism within the parsing algorithm
+that solves these problems for canonical LR, IELR, and LALR without
+sacrificing @code{%nonassoc}, default reductions, or state merging. You can
+enable LAC with the @code{%define parse.lac} directive.
+
+@deffn {Directive} {%define parse.lac @var{VALUE}}
+Enable LAC to improve syntax error handling.
+@itemize
+@item @code{none} (default)
+@item @code{full}
+@end itemize
+(This feature is experimental. More user feedback will help to stabilize
+it. Moreover, it is currently only available for deterministic parsers in
+C.)
+@end deffn
+
+Conceptually, the LAC mechanism is straight-forward. Whenever the parser
+fetches a new token from the scanner so that it can determine the next
+parser action, it immediately suspends normal parsing and performs an
+exploratory parse using a temporary copy of the normal parser state stack.
+During this exploratory parse, the parser does not perform user semantic
+actions. If the exploratory parse reaches a shift action, normal parsing
+then resumes on the normal parser stacks. If the exploratory parse reaches
+an error instead, the parser reports a syntax error. If verbose syntax
+error messages are enabled, the parser must then discover the list of
+expected tokens, so it performs a separate exploratory parse for each token
+in the grammar.
+
+There is one subtlety about the use of LAC. That is, when in a consistent
+parser state with a default reduction, the parser will not attempt to fetch
+a token from the scanner because no lookahead is needed to determine the
+next parser action. Thus, whether default reductions are enabled in
+consistent states (@pxref{Default Reductions}) affects how soon the parser
+detects a syntax error: immediately when it @emph{reaches} an erroneous
+token or when it eventually @emph{needs} that token as a lookahead to
+determine the next parser action. The latter behavior is probably more
+intuitive, so Bison currently provides no way to achieve the former behavior
+while default reductions are enabled in consistent states.
+
+Thus, when LAC is in use, for some fixed decision of whether to enable
+default reductions in consistent states, canonical LR and IELR behave almost
+exactly the same for both syntactically acceptable and syntactically
+unacceptable input. While LALR still does not support the full
+language-recognition power of canonical LR and IELR, LAC at least enables
+LALR's syntax error handling to correctly reflect LALR's
+language-recognition power.
+
+There are a few caveats to consider when using LAC:
+
+@itemize
+@item Infinite parsing loops.
+
+IELR plus LAC does have one shortcoming relative to canonical LR. Some
+parsers generated by Bison can loop infinitely. LAC does not fix infinite
+parsing loops that occur between encountering a syntax error and detecting
+it, but enabling canonical LR or disabling default reductions sometimes
+does.
+
+@item Verbose error message limitations.
+
+Because of internationalization considerations, Bison-generated parsers
+limit the size of the expected token list they are willing to report in a
+verbose syntax error message. If the number of expected tokens exceeds that
+limit, the list is simply dropped from the message. Enabling LAC can
+increase the size of the list and thus cause the parser to drop it. Of
+course, dropping the list is better than reporting an incorrect list.
+
+@item Performance.
+
+Because LAC requires many parse actions to be performed twice, it can have a
+performance penalty. However, not all parse actions must be performed
+twice. Specifically, during a series of default reductions in consistent
+states and shift actions, the parser never has to initiate an exploratory
+parse. Moreover, the most time-consuming tasks in a parse are often the
+file I/O, the lexical analysis performed by the scanner, and the user's
+semantic actions, but none of these are performed during the exploratory
+parse. Finally, the base of the temporary stack used during an exploratory
+parse is a pointer into the normal parser state stack so that the stack is
+never physically copied. In our experience, the performance penalty of LAC
+has proven insignificant for practical grammars.
+@end itemize
+
+While the LAC algorithm shares techniques that have been recognized in the
+parser community for years, for the publication that introduces LAC,
+@pxref{Bibliography,,Denny 2010 May}.
+
+@node Unreachable States
+@subsection Unreachable States
+@findex %define lr.keep-unreachable-states
+@cindex unreachable states
+
+If there exists no sequence of transitions from the parser's start state to
+some state @var{s}, then Bison considers @var{s} to be an @dfn{unreachable
+state}. A state can become unreachable during conflict resolution if Bison
+disables a shift action leading to it from a predecessor state.
+
+By default, Bison removes unreachable states from the parser after conflict
+resolution because they are useless in the generated parser. However,
+keeping unreachable states is sometimes useful when trying to understand the
+relationship between the parser and the grammar.
+
+@deffn {Directive} {%define lr.keep-unreachable-states @var{VALUE}}
+Request that Bison allow unreachable states to remain in the parser tables.
+@var{VALUE} must be a Boolean. The default is @code{false}.
+@end deffn
+
+There are a few caveats to consider:
+
+@itemize @bullet
+@item Missing or extraneous warnings.
+
+Unreachable states may contain conflicts and may use rules not used in any
+other state. Thus, keeping unreachable states may induce warnings that are
+irrelevant to your parser's behavior, and it may eliminate warnings that are
+relevant. Of course, the change in warnings may actually be relevant to a
+parser table analysis that wants to keep unreachable states, so this
+behavior will likely remain in future Bison releases.
+
+@item Other useless states.
+
+While Bison is able to remove unreachable states, it is not guaranteed to
+remove other kinds of useless states. Specifically, when Bison disables
+reduce actions during conflict resolution, some goto actions may become
+useless, and thus some additional states may become useless. If Bison were
+to compute which goto actions were useless and then disable those actions,
+it could identify such states as unreachable and then remove those states.
+However, Bison does not compute which goto actions are useless.
+@end itemize
@node Generalized LR Parsing
@section Generalized LR (GLR) Parsing
The same is true of languages that require more than one symbol of
lookahead, since the parser lacks the information necessary to make a
decision at the point it must be made in a shift-reduce parser.
-Finally, as previously mentioned (@pxref{Mystery Conflicts}),
+Finally, as previously mentioned (@pxref{Mysterious Conflicts}),
there are languages where Bison's default choice of how to
summarize the input seen so far loses necessary information.
grammar, in particular, it is only slightly slower than with the
deterministic LR(1) Bison parser.
-For a more detailed exposition of GLR parsers, please see: Elizabeth
-Scott, Adrian Johnstone and Shamsa Sadaf Hussain, Tomita-Style
-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).
+For a more detailed exposition of GLR parsers, @pxref{Bibliography,,Scott
+2000}.
@node Memory Management
@section Memory Management, and How to Avoid Memory Exhaustion
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 variable @samp{parse.trace}
@findex %define parse.trace
-Add the @samp{%define parse.trace} directive (@pxref{Decl Summary,
-,Bison Declaration Summary}), or pass the @option{-Dparse.trace} option
+Add the @samp{%define parse.trace} directive (@pxref{%define
+Summary,,parse.trace}), or pass the @option{-Dparse.trace} option
(@pxref{Bison Options}). This is a Bison extension, which is especially
-useful for languages that don't use a preprocessor. Unless
-POSIX and Yacc portability matter to you, this is the
-preferred solution.
+useful for languages that don't use a preprocessor. Unless POSIX and Yacc
+portability matter to you, this is the preferred solution.
@end table
We suggest that you always enable the trace option so that debugging is
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 :
@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 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
be false alarms in existing grammars employing the Yacc constructs
@code{$0} or @code{$-@var{n}} (where @var{n} is some positive integer).
-
@item yacc
Incompatibilities with POSIX Yacc.
+@item other
+All warnings not categorized above. These warnings are enabled by default.
+
+This category is provided merely for the sake of completeness. Future
+releases of Bison may move warnings from this category to new, more specific
+categories.
+
@item all
All the warnings.
@item none
@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}]
@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
+(@pxref{%define Summary}) except that Bison processes multiple
definitions for the same @var{name} as follows:
@itemize
@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.
When run, @command{bison} will create several entities in the @samp{yy}
namespace.
@findex %define api.namespace
-Use the @samp{%define api.namespace} directive to change the namespace
-name, see
-@ref{Decl Summary}.
-The various classes are generated in the following files:
+Use the @samp{%define api.namespace} directive to change the namespace name,
+see @ref{%define Summary,,api.namespace}. The various classes are generated
+in the following files:
@table @file
@item position.hh
@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}).
types can be used without pointers.
To enable variant-based semantic values, set @code{%define} variable
-@code{variant} (@pxref{Decl Summary, , variant}). Once this defined,
+@code{variant} (@pxref{%define Summary,, variant}). Once this defined,
@code{%union} is ignored, and instead of using the name of the fields of the
@code{%union} to ``type'' the symbols, use genuine types.
@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
mutual dependency will be broken using forward declarations. Because the
driver's header needs detailed knowledge about the parser class (in
particular its inner types), it is the parser's header which will use a
-forward declaration of the driver. @xref{Decl Summary, ,%code}.
+forward declaration of the driver. @xref{%code Summary}.
@comment file: calc++-parser.yy
@example
@end example
@noindent
-Use the following two directives to enable parser tracing and verbose
-error messages.
+Use the following two directives to enable parser tracing and verbose error
+messages. However, verbose error messages can contain incorrect information
+(@pxref{LAC}).
@comment file: calc++-parser.yy
@example
@noindent
The token numbered as 0 corresponds to end of file; the following line
allows for nicer error messages referring to ``end of file'' instead of
-``$end''. Similarly user friendly names are provided for each symbol.
-To avoid name clashes in the generated files (@pxref{Calc++ Scanner}),
-prefix tokens with @code{TOK_} (@pxref{Decl Summary,, api.tokens.prefix}).
+``$end''. Similarly user friendly names are provided for each symbol. To
+avoid name clashes in the generated files (@pxref{Calc++ Scanner}), prefix
+tokens with @code{TOK_} (@pxref{%define Summary,,api.tokens.prefix}).
@comment file: calc++-parser.yy
@example
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.
@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}.
+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
@end deffn
@end ifset
-@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}.
+@deffn {Directive} %define @var{variable}
+@deffnx {Directive} %define @var{variable} @var{value}
+@deffnx {Directive} %define @var{variable} "@var{value}"
+Define a variable to adjust Bison's behavior. @xref{%define Summary}.
@end deffn
@deffn {Directive} %defines
-Bison declaration to create a 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}
@end deffn
@deffn {Directive} %error-verbose
-An obsolete directive standing for @samp{%define parse.error verbose}.
+An obsolete directive standing for @samp{%define parse.error verbose}
+(@pxref{Error Reporting, ,The Error Reporting Function @code{yyerror}}).
@end deffn
@deffn {Directive} %file-prefix "@var{prefix}"
@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} %param @{@var{argument-declaration}@} @dots{}
@end deffn
@deffn {Directive} %pure-parser
-Deprecated version of @samp{%define api.pure} (@pxref{Decl Summary, ,%define}),
-for which Bison is more careful to warn about unreasonable usage.
+Deprecated version of @samp{%define api.pure} (@pxref{%define
+Summary,,api.pure}), for which Bison is more careful to warn about
+unreasonable usage.
@end deffn
@deffn {Directive} %require "@var{version}"
@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
@cindex glossary
@table @asis
-@item Accepting State
+@item Accepting state
A state whose only action is the accept action.
The accepting state is thus a consistent state.
@xref{Understanding,,}.
committee document contributing to what became the Algol 60 report.
@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
-@item Consistent State
-A state containing only one possible action.
-@xref{Decl Summary,,lr.default-reductions}.
+@item Consistent state
+A state containing only one possible action. @xref{Default Reductions}.
@item Context-free grammars
Grammars specified as rules that can be applied regardless of context.
permitted. @xref{Language and Grammar, ,Languages and Context-Free
Grammars}.
-@item Default Reduction
+@item Default reduction
The reduction that a parser should perform if the current parser state
-contains no other action for the lookahead token.
-In permitted parser states, Bison declares the reduction with the
-largest lookahead set to be the default reduction and removes that
-lookahead set.
-@xref{Decl Summary,,lr.default-reductions}.
+contains no other action for the lookahead token. In permitted parser
+states, Bison declares the reduction with the largest lookahead set to be
+the default reduction and removes that lookahead set. @xref{Default
+Reductions}.
+
+@item Defaulted state
+A consistent state with a default reduction. @xref{Default Reductions}.
@item Dynamic allocation
Allocation of memory that occurs during execution, rather than at
for example, `expression' or `declaration' in C@.
@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
-@item IELR(1)
-A minimal LR(1) parser table generation algorithm.
-That is, given any context-free grammar, IELR(1) generates
-parser tables with the full language recognition power of canonical
-LR(1) but with nearly the same number of parser states as
-LALR(1).
-This reduction in parser states is often an order of magnitude.
-More importantly, because canonical LR(1)'s extra parser
-states may contain duplicate conflicts in the case of
-non-LR(1) grammars, the number of conflicts for
-IELR(1) is often an order of magnitude less as well.
-This can significantly reduce the complexity of developing of a grammar.
-@xref{Decl Summary,,lr.type}.
+@item IELR(1) (Inadequacy Elimination LR(1))
+A minimal LR(1) parser table construction algorithm. That is, given any
+context-free grammar, IELR(1) generates parser tables with the full
+language-recognition power of canonical LR(1) but with nearly the same
+number of parser states as LALR(1). This reduction in parser states is
+often an order of magnitude. More importantly, because canonical LR(1)'s
+extra parser states may contain duplicate conflicts in the case of non-LR(1)
+grammars, the number of conflicts for IELR(1) is often an order of magnitude
+less as well. This can significantly reduce the complexity of developing a
+grammar. @xref{LR Table Construction}.
@item Infix operator
An arithmetic operator that is placed between the operands on which it
@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
+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}.
+syntax error message. @xref{LAC}.
@item Language construct
One of the typical usage schemas of the language. For example, one of
@item LALR(1)
The class of context-free grammars that Bison (like most other parser
generators) can handle by default; a subset of LR(1).
-@xref{Mystery Conflicts, ,Mysterious Reduce/Reduce Conflicts}.
+@xref{Mysterious Conflicts}.
@item LR(1)
The class of context-free grammars in which at most one token of
A grammar symbol that has no rules in the grammar and therefore is
grammatically indivisible. The piece of text it represents is a token.
@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
+
+@item Unreachable state
+A parser state to which there does not exist a sequence of transitions from
+the parser's start state. A state can become unreachable during conflict
+resolution. @xref{Unreachable States}.
@end table
@node Copying This Manual
@appendix Copying This Manual
@include fdl.texi
+@node Bibliography
+@unnumbered Bibliography
+
+@table @asis
+@item [Denny 2008]
+Joel E. Denny and Brian A. Malloy, IELR(1): Practical LR(1) Parser Tables
+for Non-LR(1) Grammars with Conflict Resolution, in @cite{Proceedings of the
+2008 ACM Symposium on Applied Computing} (SAC'08), ACM, New York, NY, USA,
+pp.@: 240--245. @uref{http://dx.doi.org/10.1145/1363686.1363747}
+
+@item [Denny 2010 May]
+Joel E. Denny, PSLR(1): Pseudo-Scannerless Minimal LR(1) for the
+Deterministic Parsing of Composite Languages, Ph.D. Dissertation, Clemson
+University, Clemson, SC, USA (May 2010).
+@uref{http://proquest.umi.com/pqdlink?did=2041473591&Fmt=7&clientId=79356&RQT=309&VName=PQD}
+
+@item [Denny 2010 November]
+Joel E. Denny and Brian A. Malloy, The IELR(1) Algorithm for Generating
+Minimal LR(1) Parser Tables for Non-LR(1) Grammars with Conflict Resolution,
+in @cite{Science of Computer Programming}, Vol.@: 75, Issue 11 (November
+2010), pp.@: 943--979. @uref{http://dx.doi.org/10.1016/j.scico.2009.08.001}
+
+@item [DeRemer 1982]
+Frank DeRemer and Thomas Pennello, Efficient Computation of LALR(1)
+Look-Ahead Sets, in @cite{ACM Transactions on Programming Languages and
+Systems}, Vol.@: 4, No.@: 4 (October 1982), pp.@:
+615--649. @uref{http://dx.doi.org/10.1145/69622.357187}
+
+@item [Knuth 1965]
+Donald E. Knuth, On the Translation of Languages from Left to Right, in
+@cite{Information and Control}, Vol.@: 8, Issue 6 (December 1965), pp.@:
+607--639. @uref{http://dx.doi.org/10.1016/S0019-9958(65)90426-2}
+
+@item [Scott 2000]
+Elizabeth Scott, Adrian Johnstone, and Shamsa Sadaf Hussain,
+@cite{Tomita-Style Generalised LR Parsers}, Royal Holloway, University of
+London, Department of Computer Science, TR-00-12 (December 2000).
+@uref{http://www.cs.rhul.ac.uk/research/languages/publications/tomita_style_1.ps}
+@end table
+
@node Index
@unnumbered Index
@c LocalWords: YYSTACK DVI fdl printindex IELR nondeterministic nonterminals ps
@c LocalWords: subexpressions declarator nondeferred config libintl postfix LAC
@c LocalWords: preprocessor nonpositive unary nonnumeric typedef extern rhs
-@c LocalWords: yytokentype filename destructor multicharacter nonnull EBCDIC
+@c LocalWords: yytokentype destructor multicharacter nonnull EBCDIC
@c LocalWords: lvalue nonnegative XNUM CHR chr TAGLESS tagless stdout api TOK
@c LocalWords: destructors Reentrancy nonreentrant subgrammar nonassociative
@c LocalWords: deffnx namespace xml goto lalr ielr runtime lex yacc yyps env
@c LocalWords: YYENABLE bindtextdomain Makefile DEFS CPPFLAGS DBISON DeRemer
@c LocalWords: autoreconf Pennello multisets nondeterminism Generalised baz
@c LocalWords: redeclare automata Dparse localedir datadir XSLT midrule Wno
-@c LocalWords: makefiles Graphviz multitable headitem hh basename Doxygen fno
+@c LocalWords: Graphviz multitable headitem hh basename Doxygen fno
@c LocalWords: doxygen ival sval deftypemethod deallocate pos deftypemethodx
@c LocalWords: Ctor defcv defcvx arg accessors arithmetics CPP ifndef CALCXX
@c LocalWords: lexer's calcxx bool LPAREN RPAREN deallocation cerrno climits
@c LocalWords: superclasses boolean getErrorVerbose setErrorVerbose deftypecv
@c LocalWords: getDebugStream setDebugStream getDebugLevel setDebugLevel url
@c LocalWords: bisonVersion deftypecvx bisonSkeleton getStartPos getEndPos
-@c LocalWords: getLVal defvar deftypefn deftypefnx gotos msgfmt
+@c LocalWords: getLVal defvar deftypefn deftypefnx gotos msgfmt Corbett
@c LocalWords: subdirectory Solaris nonassociativity
@c Local Variables:
projects / bison.git / blobdiff