This manual (@value{UPDATED}) is for GNU Bison (version
@value{VERSION}), the GNU parser generator.
-Copyright @copyright{} 1988-1993, 1995, 1998-2012 Free Software
+Copyright @copyright{} 1988-1993, 1995, 1998-2013 Free Software
Foundation, Inc.
@quotation
* 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
/* Reverse polish notation calculator. */
+@group
%@{
#define YYSTYPE double
#include <stdio.h>
int yylex (void);
void yyerror (char const *);
%@}
+@end group
%token NUM
@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 *);
%@}
%type <val> exp
@group
-%right '='
+%precedence '='
%left '-' '+'
%left '*' '/'
%precedence NEG /* negation--unary minus */
%% /* 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.
@code{%union} declaration.
@example
+@group
%@{
#define _GNU_SOURCE
#include <stdio.h>
#include "ptypes.h"
%@}
+@end group
+@group
%union @{
long int n;
tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
@}
+@end group
+@group
%@{
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
%@}
+@end group
@dots{}
@end example
Look again at the example of the previous section:
@example
+@group
%@{
#define _GNU_SOURCE
#include <stdio.h>
#include "ptypes.h"
%@}
+@end group
+@group
%union @{
long int n;
tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
@}
+@end group
+@group
%@{
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
%@}
+@end group
@dots{}
@end example
#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
@} YYLTYPE;
@}
+@group
%union @{
long int n;
tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
@}
+@end group
+@group
%code @{
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
static void trace_token (enum yytokentype token, YYLTYPE loc);
@}
+@end group
@dots{}
@end example
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
skeleton (@pxref{Decl Summary,,%language}, @pxref{Decl
Summary,,%skeleton}).
Unaccepted @var{variable}s produce an error.
-Some of the accepted @var{variable}s are:
+Some of the accepted @var{variable}s are described below.
-@table @code
-@c ================================================== api.namespace
-@item api.namespace
-@findex %define api.namespace
+@deffn Directive {%define api.namespace} @{@var{namespace}@}
@itemize
@item Languages(s): C++
For example, if you specify:
@example
-%define api.namespace "foo::bar"
+%define api.namespace @{foo::bar@}
@end example
Bison uses @code{foo::bar} verbatim in references such as:
lexical analyzer function. For example, if you specify:
@example
-%define api.namespace "foo"
+%define api.namespace @{foo@}
%name-prefix "bar::"
@end example
The parser namespace is @code{foo} and @code{yylex} is referenced as
@code{bar::lex}.
@end itemize
-@c namespace
+@end deffn
+@c api.namespace
@c ================================================== api.location.type
-@item @code{api.location.type}
-@findex %define api.location.type
+@deffn {Directive} {%define api.location.type} @var{type}
@itemize @bullet
@item Language(s): C++, Java
@item Default Value: none
-@item History: introduced in Bison 2.7
+@item History:
+Introduced in Bison 2.7 for C, C++ and Java. Introduced under the name
+@code{location_type} for C++ in Bison 2.5 and for Java in Bison 2.4.
@end itemize
+@end deffn
@c ================================================== api.prefix
-@item api.prefix
-@findex %define api.prefix
+@deffn {Directive} {%define api.prefix} @var{prefix}
@itemize @bullet
@item Language(s): All
@item History: introduced in Bison 2.6
@end itemize
+@end deffn
@c ================================================== api.pure
-@item api.pure
-@findex %define api.pure
+@deffn Directive {%define api.pure}
@itemize @bullet
@item Language(s): C
@item Default Value: @code{false}
-@item History: the @code{full} value was introduced in Bison 2.7
+@item History:
+the @code{full} value was introduced in Bison 2.7
@end itemize
+@end deffn
@c api.pure
@c ================================================== api.push-pull
-@item api.push-pull
-@findex %define api.push-pull
+@deffn Directive {%define api.push-pull} @var{kind}
@itemize @bullet
@item Language(s): C (deterministic parsers only)
@item Default Value: @code{pull}
@end itemize
+@end deffn
@c api.push-pull
@c ================================================== api.token.constructor
-@item api.token.constructor
-@findex %define api.token.constructor
+@deffn Directive {%define api.token.constructor}
@itemize @bullet
@item Language(s):
@item History:
introduced in Bison 2.8
@end itemize
+@end deffn
@c api.token.constructor
@c ================================================== api.token.prefix
-@item api.token.prefix
-@findex %define api.token.prefix
+@deffn Directive {%define api.token.prefix} @var{prefix}
@itemize
@item Languages(s): all
@item History:
introduced in Bison 2.8
@end itemize
+@end deffn
@c api.token.prefix
@c ================================================== api.value.type
-@item %define api.value.type variant
-@findex %define api.value.type variant
-
+@deffn Directive {%define api.value.type} @var{type}
@itemize @bullet
@item Language(s):
C++
Request variant-based semantic values.
@xref{C++ Variants}.
-@c FIXME: @item Accepted Values:
-@c FIXME: Boolean.
-
-@c @item Default Value:
-@c @code{false}
-@ @end itemize
+@item Default Value:
+FIXME:
+@item History:
+introduced in Bison 2.8. Was introduced for Java only in 2.3b as
+@code{stype}.
+@end itemize
+@end deffn
@c api.value.type
+
+@c ================================================== location_type
+@deffn Directive {%define location_type}
+Obsoleted by @code{api.location.type} since Bison 2.7.
+@end deffn
+
+
@c ================================================== lr.default-reduction
-@item lr.default-reduction
-@findex %define lr.default-reduction
+@deffn Directive {%define lr.default-reduction} @var{when}
@itemize @bullet
@item Language(s): all
introduced as @code{lr.default-reduction} in 2.5, renamed as
@code{lr.default-reduction} in 2.8.
@end itemize
+@end deffn
@c ============================================ lr.keep-unreachable-state
-@item lr.keep-unreachable-state
-@findex %define lr.keep-unreachable-state
+@deffn Directive {%define lr.keep-unreachable-state}
@itemize @bullet
@item Language(s): all
remain in the parser tables. @xref{Unreachable States}.
@item Accepted Values: Boolean
@item Default Value: @code{false}
-@end itemize
+@item History:
introduced as @code{lr.keep_unreachable_states} in 2.3b, renamed as
@code{lr.keep-unreachable-states} in 2.5, and as
@code{lr.keep-unreachable-state} in 2.8.
+@end itemize
+@end deffn
@c lr.keep-unreachable-state
@c ================================================== lr.type
-@item lr.type
-@findex %define lr.type
+@deffn Directive {%define lr.type} @var{type}
@itemize @bullet
@item Language(s): all
@item Default Value: @code{lalr}
@end itemize
-
+@end deffn
@c ================================================== namespace
-@item namespace
-@findex %define namespace
+@deffn Directive %define namespace @{@var{namespace}@}
Obsoleted by @code{api.namespace}
@c namespace
-
+@end deffn
@c ================================================== parse.assert
-@item parse.assert
-@findex %define parse.assert
+@deffn Directive {%define parse.assert}
@itemize
@item Languages(s): C++
@item Default Value: @code{false}
@end itemize
+@end deffn
@c parse.assert
@c ================================================== parse.error
-@item parse.error
-@findex %define parse.error
+@deffn Directive {%define parse.error}
@itemize
@item Languages(s):
all
@item Default Value:
@code{simple}
@end itemize
+@end deffn
@c parse.error
@c ================================================== parse.lac
-@item parse.lac
-@findex %define parse.lac
+@deffn Directive {%define parse.lac}
@itemize
@item Languages(s): C (deterministic parsers only)
@item Accepted Values: @code{none}, @code{full}
@item Default Value: @code{none}
@end itemize
+@end deffn
@c parse.lac
@c ================================================== parse.trace
-@item parse.trace
-@findex %define parse.trace
+@deffn Directive {%define parse.trace}
@itemize
@item Languages(s): C, C++, Java
@item Default Value: @code{false}
@end itemize
+@end deffn
@c parse.trace
-@end table
@node %code Summary
@subsection %code Summary
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.
+
+Consider for instance the following grammar:
+
+@example
+@group
+%nonassoc "="
+%left "+"
+%left "*"
+%precedence "("
+@end group
+%%
+@group
+stmt:
+ exp
+| "var" "=" exp
+;
+@end group
+
+@group
+exp:
+ exp "+" exp
+| exp "*" "num"
+| "(" exp ")"
+| "num"
+;
+@end group
+@end example
+
+Bison reports:
+
+@c cannot leave the location and the [-Wprecedence] for lack of
+@c width in PDF.
+@example
+@group
+warning: useless precedence and associativity for "="
+ %nonassoc "="
+ ^^^
+@end group
+@group
+warning: useless associativity for "*", use %precedence
+ %left "*"
+ ^^^
+@end group
+@group
+warning: useless precedence for "("
+ %precedence "("
+ ^^^
+@end group
+@end example
+
+One would get the exact same parser with the following directives instead:
+
+@example
+@group
+%left "+"
+%precedence "*"
+@end group
+@end example
+
@item other
All warnings not categorized above. These warnings are enabled 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
exp: exp '+' exp @{ $exp = $1 + $2; @};
@end example
-When invoked with @option{-fcaret}, Bison will report:
+When invoked with @option{-fcaret} (or nothing), Bison will report:
@example
@group
@end group
@end example
+Whereas, when invoked with @option{-fno-caret}, Bison will only report:
+
+@example
+@group
+in.y:3.20-23: error: ambiguous reference: ‘$exp’
+in.y:3.1-3: refers to: $exp at $$
+in.y:3.6-8: refers to: $exp at $1
+in.y:3.14-16: refers to: $exp at $3
+in.y:3.32-33: error: $2 of ‘exp’ has no declared type
+@end group
+@end example
+
+This option is activated by default.
+
@end table
@end table
type on all platforms, alignments are enforced for @code{double} whatever
types are actually used. This may waste space in some cases.
-@item
-Our implementation is not conforming with strict aliasing rules. Alias
-analysis is a technique used in optimizing compilers to detect when two
-pointers are disjoint (they cannot ``meet''). Our implementation breaks
-some of the rules that G++ 4.4 uses in its alias analysis, so @emph{strict
-alias analysis must be disabled}. Use the option
-@option{-fno-strict-aliasing} to compile the generated parser.
-
@item
There might be portability issues we are not aware of.
@end itemize
unit: assignments exp @{ driver.result = $2; @};
assignments:
- /* Nothing. */ @{@}
+ %empty @{@}
| assignments assignment @{@};
assignment:
@comment file: calc++-scanner.ll
@example
-%option noyywrap nounput batch debug
+%option noyywrap nounput batch debug noinput
@end example
@noindent
By default, the semantic stack is declared to have @code{Object} members,
which means that the class types you specify can be of any class.
To improve the type safety of the parser, you can declare the common
-superclass of all the semantic values using the @samp{%define stype}
+superclass of all the semantic values using the @samp{%define api.value.type}
directive. For example, after the following declaration:
@example
-%define stype "ASTNode"
+%define api.value.type "ASTNode"
@end example
@noindent
@deftypemethod {Lexer} {Object} getLVal ()
Return the semantic value of the last token that yylex returned.
-The return type can be changed using @samp{%define stype
+The return type can be changed using @samp{%define api.value.type
"@var{class-name}".}
@end deftypemethod
@defvar $$
The semantic value for the grouping made by the current rule. As a
value, this is in the base type (@code{Object} or as specified by
-@samp{%define stype}) as in not cast to the declared subtype because
+@samp{%define api.value.type}) as in not cast to the declared subtype because
casts are not allowed on the left-hand side of Java assignments.
Use an explicit Java cast if the correct subtype is needed.
@xref{Java Semantic Values}.
@item
Java lacks unions, so @code{%union} has no effect. Instead, semantic
values have a common base type: @code{Object} or as specified by
-@samp{%define stype}. Angle brackets on @code{%token}, @code{type},
+@samp{%define api.value.type}. Angle brackets on @code{%token}, @code{type},
@code{$@var{n}} and @code{$$} specify subtypes rather than fields of
an union. The type of @code{$$}, even with angle brackets, is the base
type since Java casts are not allow on the left-hand side of assignments.
@xref{Java Bison Interface}.
@end deffn
-@deffn {Directive} {%define stype} "@var{class}"
+@deffn {Directive} {%define api.value.type} "@var{class}"
The base type of semantic values. Default is @code{Object}.
@xref{Java Semantic Values}.
@end deffn
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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