Writing @acronym{GLR} Parsers
-* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars
-* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities
-* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler
+* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars
+* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities
+* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler
Examples
Bison Declarations
+* Require Decl:: Requiring a Bison version.
* Token Decl:: Declaring terminal symbols.
* Precedence Decl:: Declaring terminals with precedence and associativity.
* Union Decl:: Declaring the set of all semantic value types.
merged result.
@menu
-* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars
-* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities
-* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler
+* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars
+* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities
+* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler
@end menu
@node Simple GLR Parsers
Grammars}).
@menu
+* Require Decl:: Requiring a Bison version.
* Token Decl:: Declaring terminal symbols.
* Precedence Decl:: Declaring terminals with precedence and associativity.
* Union Decl:: Declaring the set of all semantic value types.
* Decl Summary:: Table of all Bison declarations.
@end menu
+@node Require Decl
+@subsection Require a Version of Bison
+@cindex version requirement
+@cindex requiring a version of Bison
+@findex %require
+
+You may require the minimum version of Bison to process the grammar. If
+the requirement is not met, @command{bison} exits with an error (exit
+status 63).
+
+@example
+%require "@var{version}"
+@end example
+
@node Token Decl
@subsection Token Type Names
@cindex declaring token type names
%parse-param @{ char const *file_name @};
%initial-action
@{
- @@$.begin.filename = @@$.end.filename = file_name;
+ @@$.initialize (file_name);
@};
@end example
@cindex freeing discarded symbols
@findex %destructor
-Some symbols can be discarded by the parser. During error
-recovery (@pxref{Error Recovery}), symbols already pushed
-on the stack and tokens coming from the rest of the file
-are discarded until the parser falls on its feet. If the parser
-runs out of memory, all the symbols on the stack must be discarded.
-Even if the parser succeeds, it must discard the start symbol.
+Some symbols can be discarded by the parser. During error recovery
+(@pxref{Error Recovery}), symbols already pushed on the stack and tokens
+coming from the rest of the file are discarded until the parser falls on
+its feet. If the parser runs out of memory, all the symbols on the
+stack must be discarded. Even if the parser succeeds, it must discard
+the start symbol.
When discarded symbols convey heap based information, this memory is
lost. While this behavior can be tolerable for batch parsers, such as
-in traditional compilers, it is unacceptable for programs like shells
-or protocol implementations that may parse and execute indefinitely.
+in traditional compilers, it is unacceptable for programs like shells or
+protocol implementations that may parse and execute indefinitely.
The @code{%destructor} directive defines code that
is called when a symbol is discarded.
@deffn {Directive} %destructor @{ @var{code} @} @var{symbols}
@findex %destructor
-Invoke @var{code} whenever the parser discards one of the
-@var{symbols}. Within @var{code}, @code{$$} designates the semantic
-value associated with the discarded symbol. The additional
-parser parameters are also available
-(@pxref{Parser Function, , The Parser Function @code{yyparse}}).
+Invoke @var{code} whenever the parser discards one of the @var{symbols}.
+Within @var{code}, @code{$$} designates the semantic value associated
+with the discarded symbol. The additional parser parameters are also
+available (@pxref{Parser Function, , The Parser Function
+@code{yyparse}}).
@strong{Warning:} as of Bison 2.1, this feature is still
experimental, as there has not been enough user feedback. In particular,
%expect @var{n}
@end example
-Here @var{n} is a decimal integer. The declaration says there should be
-no warning if there are @var{n} shift/reduce conflicts and no
-reduce/reduce conflicts. The usual warning is
-given if there are either more or fewer conflicts, or if there are any
-reduce/reduce conflicts.
+Here @var{n} is a decimal integer. The declaration says there should
+be @var{n} shift/reduce conflicts and no reduce/reduce conflicts.
+Bison reports an error if the number of shift/reduce conflicts differs
+from @var{n}, or if there are any reduce/reduce conflicts.
-For normal @acronym{LALR}(1) parsers, reduce/reduce conflicts are more serious,
-and should be eliminated entirely. Bison will always report
-reduce/reduce conflicts for these parsers. With @acronym{GLR} parsers, however,
-both shift/reduce and reduce/reduce are routine (otherwise, there
-would be no need to use @acronym{GLR} parsing). Therefore, it is also possible
-to specify an expected number of reduce/reduce conflicts in @acronym{GLR}
-parsers, using the declaration:
+For normal @acronym{LALR}(1) parsers, reduce/reduce conflicts are more
+serious, and should be eliminated entirely. Bison will always report
+reduce/reduce conflicts for these parsers. With @acronym{GLR}
+parsers, however, both kinds of conflicts are routine; otherwise,
+there would be no need to use @acronym{GLR} parsing. Therefore, it is
+also possible to specify an expected number of reduce/reduce conflicts
+in @acronym{GLR} parsers, using the declaration:
@example
%expect-rr @var{n}
@item
Add an @code{%expect} declaration, copying the number @var{n} from the
-number which Bison printed.
+number which Bison printed. With @acronym{GLR} parsers, add an
+@code{%expect-rr} declaration as well.
@end itemize
-Now Bison will stop annoying you if you do not change the number of
-conflicts, but it will warn you again if changes in the grammar result
-in more or fewer conflicts.
+Now Bison will warn you if you introduce an unexpected conflict, but
+will keep silent otherwise.
@node Start Decl
@subsection The Start-Symbol
(Reentrant) Parser}).
@end deffn
+@deffn {Directive} %require "@var{version}"
+Require version @var{version} or higher of Bison. @xref{Require Decl, ,
+Require a Version of Bison}.
+@end deffn
+
@deffn {Directive} %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
@node Calc++ Parser
@subsection Calc++ Parser
-The parser definition file @file{calc++-parser.yy} starts by asking
-for the C++ LALR(1) skeleton, the creation of the parser header file, and
-specifies the name of the parser class.
+The parser definition file @file{calc++-parser.yy} starts by asking for
+the C++ LALR(1) skeleton, the creation of the parser header file, and
+specifies the name of the parser class. Because the C++ skeleton
+changed several times, it is safer to require the version you designed
+the grammar for.
@comment file: calc++-parser.yy
@example
%skeleton "lalr1.cc" /* -*- C++ -*- */
+%require "2.1a"
%defines
%define "parser_class_name" "calcxx_parser"
@end example
@xref{Pure Decl, ,A Pure (Reentrant) Parser}.
@end deffn
+@deffn {Directive} %require "@var{version}"
+Require version @var{version} or higher of Bison. @xref{Require Decl, ,
+Require a Version of Bison}.
+@end deffn
+
@deffn {Directive} %right
Bison declaration to assign right associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
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