* Grammar Outline:: Overall layout of the grammar file.
* Symbols:: Terminal and nonterminal symbols.
* Rules:: How to write grammar rules.
-* Recursion:: Writing recursive rules.
* Semantics:: Semantic values and actions.
* Tracking Locations:: Locations and actions.
* Named References:: Using named references in actions.
* Grammar Rules:: Syntax and usage of the grammar rules section.
* Epilogue:: Syntax and usage of the epilogue.
+Grammar Rules
+
+* Rules Syntax:: Syntax of the rules.
+* Empty Rules:: Symbols that can match the empty string.
+* Recursion:: Writing recursive rules.
+
+
Defining Language Semantics
* Value Type:: Specifying one data type for all semantic values.
%%
prog:
- /* Nothing. */
+ %empty
| prog stmt @{ printf ("\n"); @}
;
@example
@group
input:
- /* empty */
+ %empty
| input line
;
@end group
@example
input:
- /* empty */
+ %empty
| input line
;
@end example
colon and the first @samp{|}; this means that @code{input} can match an
empty string of input (no tokens). We write the rules this way because it
is legitimate to type @kbd{Ctrl-d} right after you start the calculator.
-It's conventional to put an empty alternative first and write the comment
-@samp{/* empty */} in it.
+It's conventional to put an empty alternative first and to use the
+(optional) @code{%empty} directive, or to write the comment @samp{/* empty
+*/} in it (@pxref{Empty Rules}).
The second alternate rule (@code{input line}) handles all nontrivial input.
It means, ``After reading any number of lines, read one more line if
%% /* The grammar follows. */
@group
input:
- /* empty */
+ %empty
| input line
;
@end group
@example
@group
input:
- /* empty */
+ %empty
| input line
;
@end group
%@{
#include <stdio.h> /* For printf, etc. */
#include <math.h> /* For pow, used in the grammar. */
- #include "calc.h" /* Contains definition of `symrec'. */
+ #include "calc.h" /* Contains definition of 'symrec'. */
int yylex (void);
void yyerror (char const *);
%@}
%% /* The grammar follows. */
@group
input:
- /* empty */
+ %empty
| input line
;
@end group
@group
typedef struct symrec symrec;
-/* The symbol table: a chain of `struct symrec'. */
+/* The symbol table: a chain of 'struct symrec'. */
extern symrec *sym_table;
symrec *putsym (char const *, int);
@end group
@group
-/* The symbol table: a chain of `struct symrec'. */
+/* The symbol table: a chain of 'struct symrec'. */
symrec *sym_table;
@end group
* Grammar Outline:: Overall layout of the grammar file.
* Symbols:: Terminal and nonterminal symbols.
* Rules:: How to write grammar rules.
-* Recursion:: Writing recursive rules.
* Semantics:: Semantic values and actions.
* Tracking Locations:: Locations and actions.
* Named References:: Using named references in actions.
#include <stdio.h>
/* WARNING: The following code really belongs
- * in a `%code requires'; see below. */
+ * in a '%code requires'; see below. */
#include "ptypes.h"
#define YYLTYPE YYLTYPE
one of your tokens with a @code{%token} declaration.
@node Rules
-@section Syntax of Grammar Rules
+@section Grammar Rules
+
+A Bison grammar is a list of rules.
+
+@menu
+* Rules Syntax:: Syntax of the rules.
+* Empty Rules:: Symbols that can match the empty string.
+* Recursion:: Writing recursive rules.
+@end menu
+
+@node Rules Syntax
+@subsection Syntax of Grammar Rules
@cindex rule syntax
@cindex grammar rule syntax
@cindex syntax of grammar rules
@noindent
They are still considered distinct rules even when joined in this way.
-If @var{components} in a rule is empty, it means that @var{result} can
-match the empty string. For example, here is how to define a
-comma-separated sequence of zero or more @code{exp} groupings:
+@node Empty Rules
+@subsection Empty Rules
+@cindex empty rule
+@cindex rule, empty
+@findex %empty
+
+A rule is said to be @dfn{empty} if its right-hand side (@var{components})
+is empty. It means that @var{result} can match the empty string. For
+example, here is how to define an optional semicolon:
+
+@example
+semicolon.opt: | ";";
+@end example
+
+@noindent
+It is easy not to see an empty rule, especially when @code{|} is used. The
+@code{%empty} directive allows to make explicit that a rule is empty on
+purpose:
@example
@group
-expseq:
- /* empty */
-| expseq1
+semicolon.opt:
+ %empty
+| ";"
;
@end group
+@end example
+
+Flagging a non-empty rule with @code{%empty} is an error. If run with
+@option{-Wempty-rule}, @command{bison} will report empty rules without
+@code{%empty}. Using @code{%empty} enables this warning, unless
+@option{-Wno-empty-rule} was specified.
+
+The @code{%empty} directive is a Bison extension, it does not work with
+Yacc. To remain compatible with POSIX Yacc, it is customary to write a
+comment @samp{/* empty */} in each rule with no components:
+@example
@group
-expseq1:
- exp
-| expseq1 ',' exp
+semicolon.opt:
+ /* empty */
+| ";"
;
@end group
@end example
-@noindent
-It is customary to write a comment @samp{/* empty */} in each rule
-with no components.
@node Recursion
-@section Recursive Rules
+@subsection Recursive Rules
@cindex recursive rule
+@cindex rule, recursive
A rule is called @dfn{recursive} when its @var{result} nonterminal
appears also on its right hand side. Nearly all Bison grammars need to
@group
bar:
- /* empty */ @{ previous_expr = $0; @}
+ %empty @{ previous_expr = $0; @}
;
@end group
@end example
@group
%type <context> let
%destructor @{ pop_context ($$); @} let
+@end group
%%
+@group
stmt:
let stmt
@{
$$ = $2;
pop_context ($let);
@};
+@end group
+@group
let:
"let" '(' var ')'
@{
is translated into:
@example
-$@@1: /* empty */ @{ a(); @};
-$@@2: /* empty */ @{ c(); @};
-$@@3: /* empty */ @{ d(); @};
+$@@1: %empty @{ a(); @};
+$@@2: %empty @{ c(); @};
+$@@3: %empty @{ d(); @};
exp: $@@1 "b" $@@2 $@@3 "e" @{ f(); @};
@end example
is translated into
@example
-@@1: /* empty */ @{ a(); @};
-@@2: /* empty */ @{ $$ = c(); @};
-$@@3: /* empty */ @{ d(); @};
+@@1: %empty @{ a(); @};
+@@2: %empty @{ $$ = c(); @};
+$@@3: %empty @{ d(); @};
exp: @@1 "b" @@2 $@@3 "e" @{ f = $1; @}
@end example
@example
@group
subroutine:
- /* empty */ @{ prepare_for_local_variables (); @}
+ %empty @{ prepare_for_local_variables (); @}
;
@end group
@end deffn
@deffn {Directive} %defines @var{defines-file}
-Same as above, but save in the file @var{defines-file}.
+Same as above, but save in the file @file{@var{defines-file}}.
@end deffn
@deffn {Directive} %destructor
@end deffn
@deffn {Directive} %output "@var{file}"
-Specify @var{file} for the parser implementation file.
+Generate the parser implementation in @file{@var{file}}.
@end deffn
@deffn {Directive} %pure-parser
return 0;
@dots{}
if (c == '+' || c == '-')
- return c; /* Assume token type for `+' is '+'. */
+ return c; /* Assume token type for '+' is '+'. */
@dots{}
return INT; /* Return the type of the token. */
@dots{}
@example
@group
sequence:
- /* empty */ @{ printf ("empty sequence\n"); @}
+ %empty @{ printf ("empty sequence\n"); @}
| maybeword
| sequence word @{ printf ("added word %s\n", $2); @}
;
@group
maybeword:
- /* empty */ @{ printf ("empty maybeword\n"); @}
-| word @{ printf ("single word %s\n", $1); @}
+ %empty @{ printf ("empty maybeword\n"); @}
+| word @{ printf ("single word %s\n", $1); @}
;
@end group
@end example
@example
@group
sequence:
- /* empty */ @{ printf ("empty sequence\n"); @}
+ %empty @{ printf ("empty sequence\n"); @}
| sequence word @{ printf ("added word %s\n", $2); @}
;
@end group
@example
@group
sequence:
- /* empty */
+ %empty
| sequence words
| sequence redirects
;
@group
words:
- /* empty */
+ %empty
| words word
;
@end group
@group
redirects:
- /* empty */
+ %empty
| redirects redirect
;
@end group
@example
sequence:
- /* empty */
+ %empty
| sequence word
| sequence redirect
;
@example
@group
sequence:
- /* empty */
+ %empty
| sequence words
| sequence redirects
;
%%
@group
sequence:
- /* empty */
+ %empty
| sequence word %prec "sequence"
| sequence redirect %prec "sequence"
;
%%
@group
sequence:
- /* empty */
+ %empty
| sequence word %prec "word"
| sequence redirect %prec "redirect"
;
@example
stmts:
- /* empty string */
+ %empty
| stmts '\n'
| stmts exp '\n'
| stmts error '\n'
short option. It is followed by a cross key alphabetized by long
option.
-@c Please, keep this ordered as in `bison --help'.
+@c Please, keep this ordered as in 'bison --help'.
@noindent
Operations modes:
@table @option
Deprecated constructs whose support will be removed in future versions of
Bison.
+@item empty-rule
+Empty rules without @code{%empty}. @xref{Empty Rules}. Disabled by
+default, but enabled by uses of @code{%empty}, unless
+@option{-Wno-empty-rule} was specified.
+
@item precedence
Useless precedence and associativity directives. Disabled by default.
categories.
@item all
-All the warnings.
+All the warnings except @code{yacc}.
+
@item none
Turn off all the warnings.
+
@item error
See @option{-Werror}, below.
@end table
unit: assignments exp @{ driver.result = $2; @};
assignments:
- /* Nothing. */ @{@}
+ %empty @{@}
| assignments assignment @{@};
assignment:
version. If you have trouble compiling, you should also include a
transcript of the build session, starting with the invocation of
`configure'. Depending on the nature of the bug, you may be asked to
-send additional files as well (such as `config.h' or `config.cache').
+send additional files as well (such as @file{config.h} or @file{config.cache}).
Patches are most welcome, but not required. That is, do not hesitate to
send a bug report just because you cannot provide a fix.
GLR Parsers}.
@end deffn
+@deffn {Directive} %empty
+Bison declaration to declare make explicit that a rule has an empty
+right-hand side. @xref{Empty Rules}.
+@end deffn
+
@deffn {Symbol} $end
The predefined token marking the end of the token stream. It cannot be
used in the grammar.
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