@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
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
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).
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:
Using it in a way that would be associative is a syntax error.
@end deffn
+@ifset defaultprec
@deffn {Directive} %default-prec
-Specify whether to assign a precedence to rules lacking an
-explicit @code{%prec} modifier
+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
parser file contains just @code{#define} directives and static variable
@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
discovers the mistake only by testing the code.
-The @code{%default-prec 0;} declaration makes it easier to discover
+The @code{%no-default-prec;} declaration makes it easier to discover
this kind of problem systematically. It causes rules that lack a
@code{%prec} modifier to have no precedence, even if the last terminal
symbol mentioned in their components has a declared precedence.
-If @code{%default-prec 0;} is in effect, you must specify @code{%prec}
+If @code{%no-default-prec;} is in effect, you must specify @code{%prec}
for all rules that participate in precedence conflict resolution.
Then you will see any shift/reduce conflict until you tell Bison how
to resolve it, either by changing your grammar or by adding an
explicit precedence. This will probably add declarations to the
grammar, but it helps to protect against incorrect rule precedences.
-The effect of @code{%default-prec 0;} can be reversed by giving
-@code{%default-prec 1;}, which is the default.
+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
Equip the parser for debugging. @xref{Decl Summary}.
@end deffn
-@deffn {Directive} %default-prec @var{state};
-Bison declaration to specify whether to assign a precedence to rules
-that lack an explicit @samp{%prec} modifier. @xref{Contextual
-Precedence, ,Context-Dependent Precedence}.
+@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
parser file. @xref{Decl Summary}.
projects / bison.git / blobdiff