@c @clear shorttitlepage-enabled
@c @set shorttitlepage-enabled
+@c Set following if you want to document %default-prec and %no-default-prec.
+@c This feature is experimental and may change in future Bison versions.
+@c @set defaultprec
+
@c ISPELL CHECK: done, 14 Jan 1993 --bob
@c Check COPYRIGHT dates. should be updated in the titlepage, ifinfo
@value{UPDATED}), the @acronym{GNU} parser generator.
Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998,
-1999, 2000, 2001, 2002, 2003 Free Software Foundation, Inc.
+1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.
@quotation
Permission is granted to copy, distribute and/or modify this document
* Union Decl:: Declaring the set of all semantic value types.
* Type Decl:: Declaring the choice of type for a nonterminal symbol.
* Destructor Decl:: Declaring how symbols are freed.
-* Expect Decl:: Suppressing warnings about shift/reduce conflicts.
+* Expect Decl:: Suppressing warnings about parsing conflicts.
* Start Decl:: Specifying the start symbol.
* Pure Decl:: Requesting a reentrant parser.
* Decl Summary:: Table of all Bison declarations.
practical conditions for using Bison match the practical conditions for
using the other @acronym{GNU} tools.
-This exception applies only when Bison is generating C code for a
+This exception applies only when Bison is generating C code for an
@acronym{LALR}(1) parser; otherwise, the @acronym{GPL} terms operate
as usual. You can
tell whether the exception applies to your @samp{.c} output file by
%@}
%union @{
- long n;
+ long int n;
tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
@}
read your program will be confused.
All the escape sequences used in string literals in C can be used in
-Bison as well. However, unlike Standard C, trigraphs have no special
+Bison as well, except that you must not use a null character within a
+string literal. Also, unlike Standard C, trigraphs have no special
meaning in Bison string literals, nor is backslash-newline allowed. A
literal string token must contain two or more characters; for a token
containing just one character, use a character token (see above).
In most programs, you will need different data types for different kinds
of tokens and groupings. For example, a numeric constant may need type
-@code{int} or @code{long}, while a string constant needs type @code{char *},
+@code{int} or @code{long int}, while a string constant needs type @code{char *},
and an identifier might need a pointer to an entry in the symbol table.
To use more than one data type for semantic values in one parser, Bison
The C code in an action can refer to the semantic values of the components
matched by the rule with the construct @code{$@var{n}}, which stands for
the value of the @var{n}th component. The semantic value for the grouping
-being constructed is @code{$$}. (Bison translates both of these constructs
-into array element references when it copies the actions into the parser
-file.)
+being constructed is @code{$$}. Bison translates both of these
+constructs into expressions of the appropriate type when it copies the
+actions into the parser file. @code{$$} is translated to a modifiable
+lvalue, so it can be assigned to.
Here is a typical example:
* Union Decl:: Declaring the set of all semantic value types.
* Type Decl:: Declaring the choice of type for a nonterminal symbol.
* Destructor Decl:: Declaring how symbols are freed.
-* Expect Decl:: Suppressing warnings about shift/reduce conflicts.
+* Expect Decl:: Suppressing warnings about parsing conflicts.
* Start Decl:: Specifying the start symbol.
* Pure Decl:: Requesting a reentrant parser.
* Decl Summary:: Table of all Bison declarations.
Precedence}.
You can explicitly specify the numeric code for a token type by appending
-an integer value in the field immediately following the token name:
+a decimal or hexadecimal integer value in the field immediately
+following the token name:
@example
%token NUM 300
+%token XNUM 0x12d // a GNU extension
@end example
@noindent
@cindex warnings, preventing
@cindex conflicts, suppressing warnings of
@findex %expect
+@findex %expect-rr
Bison normally warns if there are any conflicts in the grammar
(@pxref{Shift/Reduce, ,Shift/Reduce Conflicts}), but most real grammars
given if there are either more or fewer conflicts, or if there are any
reduce/reduce conflicts.
+For normal 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 GLR parsers, however,
+both shift/reduce and reduce/reduce are routine (otherwise, there
+would be no need to use GLR parsing). Therefore, it is also possible
+to specify an expected number of reduce/reduce conflicts in GLR
+parsers, using the declaration:
+
+@example
+%expect-rr @var{n}
+@end example
+
In general, using @code{%expect} involves these steps:
@itemize @bullet
Using it in a way that would be associative is a syntax error.
@end deffn
+@ifset defaultprec
@deffn {Directive} %default-prec
Assign a precedence to rules lacking an explicit @code{%prec} modifier
(@pxref{Contextual Precedence, ,Context-Dependent Precedence}).
@end deffn
+@end ifset
@deffn {Directive} %type
Declare the type of semantic values for a nonterminal symbol
Program}.
@end deffn
+@ifset defaultprec
@deffn {Directive} %no-default-prec
Do not assign a precedence to rules lacking an explicit @code{%prec}
modifier (@pxref{Contextual Precedence, ,Context-Dependent
Precedence}).
@end deffn
+@end ifset
@deffn {Directive} %no-parser
Do not include any C code in the parser file; generate tables only. The
@end group
@end example
+@ifset defaultprec
If you forget to append @code{%prec UMINUS} to the rule for unary
minus, Bison silently assumes that minus has its usual precedence.
This kind of problem can be tricky to debug, since one typically
The effect of @code{%no-default-prec;} can be reversed by giving
@code{%default-prec;}, which is the default.
+@end ifset
@node Parser States
@section Parser States
@example
#! /bin/sh
-bison -y "$@"
+bison -y "$@@"
@end example
@end table
functions like @samp{yy_switch_to_buffer} that manipulate multiple
input buffers.
+If your Flex-generated scanner uses start conditions (@pxref{Start
+conditions, , Start conditions, flex, The Flex Manual}), you might
+also want to reset the scanner's state, i.e., go back to the initial
+start condition, through a call to @samp{BEGIN (0)}.
+
@node Strings are Destroyed
@section Strings are Destroyed
Equip the parser for debugging. @xref{Decl Summary}.
@end deffn
+@ifset defaultprec
@deffn {Directive} %default-prec
Assign a precedence to rules that lack an explicit @samp{%prec}
modifier. @xref{Contextual Precedence, ,Context-Dependent
Precedence}.
@end deffn
+@end ifset
@deffn {Directive} %defines
Bison declaration to create a header file meant for the scanner.
Bison declaration to rename the external symbols. @xref{Decl Summary}.
@end deffn
+@ifset defaultprec
@deffn {Directive} %no-default-prec
Do not assign a precedence to rules that lack an explicit @samp{%prec}
modifier. @xref{Contextual Precedence, ,Context-Dependent
Precedence}.
@end deffn
+@end ifset
@deffn {Directive} %no-lines
Bison declaration to avoid generating @code{#line} directives in the
projects / bison.git / blobdiff