This manual (@value{UPDATED}) is for GNU Bison (version
@value{VERSION}), the GNU parser generator.
-Copyright @copyright{} 1988-1993, 1995, 1998-2011 Free Software
+Copyright @copyright{} 1988-1993, 1995, 1998-2012 Free Software
Foundation, Inc.
@quotation
the name of an identifier, etc.).
* Semantic Actions:: Each rule can have an action containing C code.
* GLR Parsers:: Writing parsers for general context-free languages.
-* Locations Overview:: Tracking Locations.
+* Locations:: Overview of location tracking.
* Bison Parser:: What are Bison's input and output,
how is the output used?
* Stages:: Stages in writing and running Bison grammars.
Bison Grammar Files
-* 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.
-* Locations:: Locations and actions.
-* Declarations:: All kinds of Bison declarations are described here.
-* Multiple Parsers:: Putting more than one Bison parser in one program.
+* 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.
+* Declarations:: All kinds of Bison declarations are described here.
+* Multiple Parsers:: Putting more than one Bison parser in one program.
Outline of a Bison Grammar
* Mid-Rule Actions:: Most actions go at the end of a rule.
This says when, why and how to use the exceptional
action in the middle of a rule.
-* Named References:: Using named references in actions.
Tracking Locations
* Contextual Precedence:: When an operator's precedence depends on context.
* Parser States:: The parser is a finite-state-machine with stack.
* Reduce/Reduce:: When two rules are applicable in the same situation.
-* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
+* Mysterious Conflicts:: Conflicts that look unjustified.
* Tuning LR:: How to tune fundamental aspects of LR-based parsing.
* Generalized LR Parsing:: Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted. How to avoid it.
the name of an identifier, etc.).
* Semantic Actions:: Each rule can have an action containing C code.
* GLR Parsers:: Writing parsers for general context-free languages.
-* Locations Overview:: Tracking Locations.
+* Locations:: Overview of location tracking.
* Bison Parser:: What are Bison's input and output,
how is the output used?
* Stages:: Stages in writing and running Bison grammars.
to tell how to parse any portion of an input string with just a single token
of lookahead. For historical reasons, Bison by default is limited by the
additional restrictions of LALR(1), which is hard to explain simply.
-@xref{Mystery Conflicts, ,Mysterious Reduce/Reduce Conflicts}, for more
-information on this. As an experimental feature, you can escape these
-additional restrictions by requesting IELR(1) or canonical LR(1) parser
-tables. @xref{LR Table Construction}, to learn how.
+@xref{Mysterious Conflicts}, for more information on this. As an
+experimental feature, you can escape these additional restrictions by
+requesting IELR(1) or canonical LR(1) parser tables. @xref{LR Table
+Construction}, to learn how.
@cindex GLR parsing
@cindex generalized LR (GLR) parsing
Here is a simple C function subdivided into tokens:
-@ifinfo
@example
int /* @r{keyword `int'} */
square (int x) /* @r{identifier, open-paren, keyword `int',}
@r{identifier, semicolon} */
@} /* @r{close-brace} */
@end example
-@end ifinfo
-@ifnotinfo
-@example
-int /* @r{keyword `int'} */
-square (int x) /* @r{identifier, open-paren, keyword `int', identifier, close-paren} */
-@{ /* @r{open-brace} */
- return x * x; /* @r{keyword `return', identifier, asterisk, identifier, semicolon} */
-@} /* @r{close-brace} */
-@end example
-@end ifnotinfo
The syntactic groupings of C include the expression, the statement, the
declaration, and the function definition. These are represented in the
used in every rule.
@example
-stmt: RETURN expr ';'
- ;
+stmt: RETURN expr ';' ;
@end example
@noindent
two subexpressions:
@example
-expr: expr '+' expr @{ $$ = $1 + $3; @}
- ;
+expr: expr '+' expr @{ $$ = $1 + $3; @} ;
@end example
@noindent
%%
@group
-type_decl : TYPE ID '=' type ';'
- ;
+type_decl: TYPE ID '=' type ';' ;
@end group
@group
-type : '(' id_list ')'
- | expr DOTDOT expr
- ;
+type:
+ '(' id_list ')'
+| expr DOTDOT expr
+;
@end group
@group
-id_list : ID
- | id_list ',' ID
- ;
+id_list:
+ ID
+| id_list ',' ID
+;
@end group
@group
-expr : '(' expr ')'
- | expr '+' expr
- | expr '-' expr
- | expr '*' expr
- | expr '/' expr
- | ID
- ;
+expr:
+ '(' expr ')'
+| expr '+' expr
+| expr '-' expr
+| expr '*' expr
+| expr '/' expr
+| ID
+;
@end group
@end example
%%
-prog :
- | prog stmt @{ printf ("\n"); @}
- ;
+prog:
+ /* Nothing. */
+| prog stmt @{ printf ("\n"); @}
+;
-stmt : expr ';' %dprec 1
- | decl %dprec 2
- ;
+stmt:
+ expr ';' %dprec 1
+| decl %dprec 2
+;
-expr : ID @{ printf ("%s ", $$); @}
- | TYPENAME '(' expr ')'
- @{ printf ("%s <cast> ", $1); @}
- | expr '+' expr @{ printf ("+ "); @}
- | expr '=' expr @{ printf ("= "); @}
- ;
+expr:
+ ID @{ printf ("%s ", $$); @}
+| TYPENAME '(' expr ')'
+ @{ printf ("%s <cast> ", $1); @}
+| expr '+' expr @{ printf ("+ "); @}
+| expr '=' expr @{ printf ("= "); @}
+;
-decl : TYPENAME declarator ';'
- @{ printf ("%s <declare> ", $1); @}
- | TYPENAME declarator '=' expr ';'
- @{ printf ("%s <init-declare> ", $1); @}
- ;
+decl:
+ TYPENAME declarator ';'
+ @{ printf ("%s <declare> ", $1); @}
+| TYPENAME declarator '=' expr ';'
+ @{ printf ("%s <init-declare> ", $1); @}
+;
-declarator : ID @{ printf ("\"%s\" ", $1); @}
- | '(' declarator ')'
- ;
+declarator:
+ ID @{ printf ("\"%s\" ", $1); @}
+| '(' declarator ')'
+;
@end example
@noindent
follows:
@example
-stmt : expr ';' %merge <stmtMerge>
- | decl %merge <stmtMerge>
- ;
+stmt:
+ expr ';' %merge <stmtMerge>
+| decl %merge <stmtMerge>
+;
@end example
@noindent
@example
%@{
- #if __STDC_VERSION__ < 199901 && ! defined __GNUC__ && ! defined inline
- #define inline
+ #if (__STDC_VERSION__ < 199901 && ! defined __GNUC__ \
+ && ! defined inline)
+ # define inline
#endif
%@}
@end example
-@node Locations Overview
+@node Locations
@section Locations
@cindex location
@cindex textual location
Bison provides a mechanism for handling these locations.
Each token has a semantic value. In a similar fashion, each token has an
-associated location, but the type of locations is the same for all tokens and
-groupings. Moreover, the output parser is equipped with a default data
-structure for storing locations (@pxref{Locations}, for more details).
+associated location, but the type of locations is the same for all tokens
+and groupings. Moreover, the output parser is equipped with a default data
+structure for storing locations (@pxref{Tracking Locations}, for more
+details).
Like semantic values, locations can be reached in actions using a dedicated
set of constructs. In the example above, the location of the whole grouping
@cindex simple examples
@cindex examples, simple
-Now we show and explain three sample programs written using Bison: a
+Now we show and explain several sample programs written using Bison: a
reverse polish notation calculator, an algebraic (infix) notation
-calculator, and a multi-function calculator. All three have been tested
-under BSD Unix 4.3; each produces a usable, though limited, interactive
-desk-top calculator.
+calculator --- later extended to track ``locations'' ---
+and a multi-function calculator. All
+produce usable, though limited, interactive desk-top calculators.
These examples are simple, but Bison grammars for real programming
languages are written the same way. You can copy these examples into a
Here are the grammar rules for the reverse polish notation calculator.
@example
-input: /* empty */
- | input line
+@group
+input:
+ /* empty */
+| input line
;
+@end group
-line: '\n'
- | exp '\n' @{ printf ("\t%.10g\n", $1); @}
+@group
+line:
+ '\n'
+| exp '\n' @{ printf ("%.10g\n", $1); @}
;
+@end group
-exp: NUM @{ $$ = $1; @}
- | exp exp '+' @{ $$ = $1 + $2; @}
- | exp exp '-' @{ $$ = $1 - $2; @}
- | exp exp '*' @{ $$ = $1 * $2; @}
- | exp exp '/' @{ $$ = $1 / $2; @}
- /* Exponentiation */
- | exp exp '^' @{ $$ = pow ($1, $2); @}
- /* Unary minus */
- | exp 'n' @{ $$ = -$1; @}
+@group
+exp:
+ NUM @{ $$ = $1; @}
+| exp exp '+' @{ $$ = $1 + $2; @}
+| exp exp '-' @{ $$ = $1 - $2; @}
+| exp exp '*' @{ $$ = $1 * $2; @}
+| exp exp '/' @{ $$ = $1 / $2; @}
+| exp exp '^' @{ $$ = pow ($1, $2); @} /* Exponentiation */
+| exp 'n' @{ $$ = -$1; @} /* Unary minus */
;
+@end group
%%
@end example
Consider the definition of @code{input}:
@example
-input: /* empty */
- | input line
+input:
+ /* empty */
+| input line
;
@end example
Now consider the definition of @code{line}:
@example
-line: '\n'
- | exp '\n' @{ printf ("\t%.10g\n", $1); @}
+line:
+ '\n'
+| exp '\n' @{ printf ("%.10g\n", $1); @}
;
@end example
followed by a plus-sign. The third handles subtraction, and so on.
@example
-exp: NUM
- | exp exp '+' @{ $$ = $1 + $2; @}
- | exp exp '-' @{ $$ = $1 - $2; @}
- @dots{}
- ;
+exp:
+ NUM
+| exp exp '+' @{ $$ = $1 + $2; @}
+| exp exp '-' @{ $$ = $1 - $2; @}
+@dots{}
+;
@end example
We have used @samp{|} to join all the rules for @code{exp}, but we could
equally well have written them separately:
@example
-exp: NUM ;
-exp: exp exp '+' @{ $$ = $1 + $2; @} ;
-exp: exp exp '-' @{ $$ = $1 - $2; @} ;
- @dots{}
+exp: NUM ;
+exp: exp exp '+' @{ $$ = $1 + $2; @};
+exp: exp exp '-' @{ $$ = $1 - $2; @};
+@dots{}
@end example
Most of the rules have actions that compute the value of the expression in
For example, this:
@example
-exp : NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{} ;
+exp: NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{} ;
@end example
@noindent
means the same thing as this:
@example
-exp: NUM
- | exp exp '+' @{ $$ = $1 + $2; @}
- | @dots{}
+exp:
+ NUM
+| exp exp '+' @{ $$ = $1 + $2; @}
+| @dots{}
;
@end example
/* Skip white space. */
while ((c = getchar ()) == ' ' || c == '\t')
- ;
+ continue;
@end group
@group
/* Process numbers. */
@example
@group
#include <stdio.h>
+@end group
+@group
/* Called by yyparse on error. */
void
yyerror (char const *s)
@example
/* Infix notation calculator. */
+@group
%@{
#define YYSTYPE double
#include <math.h>
int yylex (void);
void yyerror (char const *);
%@}
+@end group
+@group
/* Bison declarations. */
%token NUM
%left '-' '+'
%left '*' '/'
%left NEG /* negation--unary minus */
%right '^' /* exponentiation */
+@end group
%% /* The grammar follows. */
-input: /* empty */
- | input line
+@group
+input:
+ /* empty */
+| input line
;
+@end group
-line: '\n'
- | exp '\n' @{ printf ("\t%.10g\n", $1); @}
+@group
+line:
+ '\n'
+| exp '\n' @{ printf ("\t%.10g\n", $1); @}
;
+@end group
-exp: NUM @{ $$ = $1; @}
- | exp '+' exp @{ $$ = $1 + $3; @}
- | exp '-' exp @{ $$ = $1 - $3; @}
- | exp '*' exp @{ $$ = $1 * $3; @}
- | exp '/' exp @{ $$ = $1 / $3; @}
- | '-' exp %prec NEG @{ $$ = -$2; @}
- | exp '^' exp @{ $$ = pow ($1, $3); @}
- | '(' exp ')' @{ $$ = $2; @}
+@group
+exp:
+ NUM @{ $$ = $1; @}
+| exp '+' exp @{ $$ = $1 + $3; @}
+| exp '-' exp @{ $$ = $1 - $3; @}
+| exp '*' exp @{ $$ = $1 * $3; @}
+| exp '/' exp @{ $$ = $1 / $3; @}
+| '-' exp %prec NEG @{ $$ = -$2; @}
+| exp '^' exp @{ $$ = pow ($1, $3); @}
+| '(' exp ')' @{ $$ = $2; @}
;
+@end group
%%
@end example
@example
@group
-line: '\n'
- | exp '\n' @{ printf ("\t%.10g\n", $1); @}
- | error '\n' @{ yyerrok; @}
+line:
+ '\n'
+| exp '\n' @{ printf ("\t%.10g\n", $1); @}
+| error '\n' @{ yyerrok; @}
;
@end group
@end example
@example
@group
-input : /* empty */
- | input line
+input:
+ /* empty */
+| input line
;
@end group
@group
-line : '\n'
- | exp '\n' @{ printf ("%d\n", $1); @}
+line:
+ '\n'
+| exp '\n' @{ printf ("%d\n", $1); @}
;
@end group
@group
-exp : NUM @{ $$ = $1; @}
- | exp '+' exp @{ $$ = $1 + $3; @}
- | exp '-' exp @{ $$ = $1 - $3; @}
- | exp '*' exp @{ $$ = $1 * $3; @}
+exp:
+ NUM @{ $$ = $1; @}
+| exp '+' exp @{ $$ = $1 + $3; @}
+| exp '-' exp @{ $$ = $1 - $3; @}
+| exp '*' exp @{ $$ = $1 * $3; @}
@end group
@group
- | exp '/' exp
- @{
- if ($3)
- $$ = $1 / $3;
- else
- @{
- $$ = 1;
- fprintf (stderr, "%d.%d-%d.%d: division by zero",
- @@3.first_line, @@3.first_column,
- @@3.last_line, @@3.last_column);
- @}
- @}
+| exp '/' exp
+ @{
+ if ($3)
+ $$ = $1 / $3;
+ else
+ @{
+ $$ = 1;
+ fprintf (stderr, "%d.%d-%d.%d: division by zero",
+ @@3.first_line, @@3.first_column,
+ @@3.last_line, @@3.last_column);
+ @}
+ @}
@end group
@group
- | '-' exp %prec NEG @{ $$ = -$2; @}
- | exp '^' exp @{ $$ = pow ($1, $3); @}
- | '(' exp ')' @{ $$ = $2; @}
+| '-' exp %prec NEG @{ $$ = -$2; @}
+| exp '^' exp @{ $$ = pow ($1, $3); @}
+| '(' exp ')' @{ $$ = $2; @}
@end group
@end example
if (c == EOF)
return 0;
+@group
/* Return a single char, and update location. */
if (c == '\n')
@{
++yylloc.last_column;
return c;
@}
+@end group
@end example
Basically, the lexical analyzer performs the same processing as before:
Here are the C and Bison declarations for the multi-function calculator.
-@smallexample
+@comment file: mfcalc.y
+@example
@group
%@{
#include <math.h> /* For math functions, cos(), sin(), etc. */
%right '^' /* exponentiation */
@end group
%% /* The grammar follows. */
-@end smallexample
+@end example
The above grammar introduces only two new features of the Bison language.
These features allow semantic values to have various data types
Most of them are copied directly from @code{calc}; three rules,
those which mention @code{VAR} or @code{FNCT}, are new.
-@smallexample
+@comment file: mfcalc.y
+@example
@group
-input: /* empty */
- | input line
+input:
+ /* empty */
+| input line
;
@end group
@group
line:
- '\n'
- | exp '\n' @{ printf ("\t%.10g\n", $1); @}
- | error '\n' @{ yyerrok; @}
+ '\n'
+| exp '\n' @{ printf ("%.10g\n", $1); @}
+| error '\n' @{ yyerrok; @}
;
@end group
@group
-exp: NUM @{ $$ = $1; @}
- | VAR @{ $$ = $1->value.var; @}
- | VAR '=' exp @{ $$ = $3; $1->value.var = $3; @}
- | FNCT '(' exp ')' @{ $$ = (*($1->value.fnctptr))($3); @}
- | exp '+' exp @{ $$ = $1 + $3; @}
- | exp '-' exp @{ $$ = $1 - $3; @}
- | exp '*' exp @{ $$ = $1 * $3; @}
- | exp '/' exp @{ $$ = $1 / $3; @}
- | '-' exp %prec NEG @{ $$ = -$2; @}
- | exp '^' exp @{ $$ = pow ($1, $3); @}
- | '(' exp ')' @{ $$ = $2; @}
+exp:
+ NUM @{ $$ = $1; @}
+| VAR @{ $$ = $1->value.var; @}
+| VAR '=' exp @{ $$ = $3; $1->value.var = $3; @}
+| FNCT '(' exp ')' @{ $$ = (*($1->value.fnctptr))($3); @}
+| exp '+' exp @{ $$ = $1 + $3; @}
+| exp '-' exp @{ $$ = $1 - $3; @}
+| exp '*' exp @{ $$ = $1 * $3; @}
+| exp '/' exp @{ $$ = $1 / $3; @}
+| '-' exp %prec NEG @{ $$ = -$2; @}
+| exp '^' exp @{ $$ = pow ($1, $3); @}
+| '(' exp ')' @{ $$ = $2; @}
;
@end group
/* End of grammar. */
%%
-@end smallexample
+@end example
@node Mfcalc Symbol Table
@subsection The @code{mfcalc} Symbol Table
definition, which is kept in the header @file{calc.h}, is as follows. It
provides for either functions or variables to be placed in the table.
-@smallexample
+@comment file: calc.h
+@example
@group
/* Function type. */
typedef double (*func_t) (double);
symrec *putsym (char const *, int);
symrec *getsym (char const *);
@end group
-@end smallexample
+@end example
The new version of @code{main} includes a call to @code{init_table}, a
function that initializes the symbol table. Here it is, and
@code{init_table} as well:
-@smallexample
+@example
#include <stdio.h>
@group
init_table (void)
@{
int i;
- symrec *ptr;
for (i = 0; arith_fncts[i].fname != 0; i++)
@{
- ptr = putsym (arith_fncts[i].fname, FNCT);
+ symrec *ptr = putsym (arith_fncts[i].fname, FNCT);
ptr->value.fnctptr = arith_fncts[i].fnct;
@}
@}
return yyparse ();
@}
@end group
-@end smallexample
+@end example
By simply editing the initialization list and adding the necessary include
files, you can add additional functions to the calculator.
The function @code{getsym} is passed the name of the symbol to look up. If
found, a pointer to that symbol is returned; otherwise zero is returned.
-@smallexample
+@comment file: mfcalc.y
+@example
+#include <stdlib.h> /* malloc. */
+#include <string.h> /* strlen. */
+
+@group
symrec *
putsym (char const *sym_name, int sym_type)
@{
- symrec *ptr;
- ptr = (symrec *) malloc (sizeof (symrec));
+ symrec *ptr = (symrec *) malloc (sizeof (symrec));
ptr->name = (char *) malloc (strlen (sym_name) + 1);
strcpy (ptr->name,sym_name);
ptr->type = sym_type;
sym_table = ptr;
return ptr;
@}
+@end group
+@group
symrec *
getsym (char const *sym_name)
@{
return ptr;
return 0;
@}
-@end smallexample
+@end group
+@end example
The function @code{yylex} must now recognize variables, numeric values, and
the single-character arithmetic operators. Strings of alphanumeric
No change is needed in the handling of numeric values and arithmetic
operators in @code{yylex}.
-@smallexample
+@comment file: mfcalc.y
+@example
@group
#include <ctype.h>
@end group
int c;
/* Ignore white space, get first nonwhite character. */
- while ((c = getchar ()) == ' ' || c == '\t');
+ while ((c = getchar ()) == ' ' || c == '\t')
+ continue;
if (c == EOF)
return 0;
/* Char starts an identifier => read the name. */
if (isalpha (c))
@{
- symrec *s;
+ /* Initially make the buffer long enough
+ for a 40-character symbol name. */
+ static size_t length = 40;
static char *symbuf = 0;
- static int length = 0;
+ symrec *s;
int i;
@end group
-@group
- /* Initially make the buffer long enough
- for a 40-character symbol name. */
- if (length == 0)
- length = 40, symbuf = (char *)malloc (length + 1);
+ if (!symbuf)
+ symbuf = (char *) malloc (length + 1);
i = 0;
do
-@end group
@group
@{
/* If buffer is full, make it bigger. */
return c;
@}
@end group
-@end smallexample
+@end example
This program is both powerful and flexible. You may easily add new
functions, and it is a simple job to modify this code to install
@xref{Invocation, ,Invoking Bison}.
@menu
-* 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.
-* Locations:: Locations and actions.
-* Declarations:: All kinds of Bison declarations are described here.
-* Multiple Parsers:: Putting more than one Bison parser in one program.
+* 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.
+* Declarations:: All kinds of Bison declarations are described here.
+* Multiple Parsers:: Putting more than one Bison parser in one program.
@end menu
@node Grammar Outline
can be done with two @var{Prologue} blocks, one before and one after the
@code{%union} declaration.
-@smallexample
+@example
%@{
#define _GNU_SOURCE
#include <stdio.h>
%@}
@dots{}
-@end smallexample
+@end example
When in doubt, it is usually safer to put prologue code before all
Bison declarations, rather than after. For example, any definitions
Look again at the example of the previous section:
-@smallexample
+@example
%@{
#define _GNU_SOURCE
#include <stdio.h>
%@}
@dots{}
-@end smallexample
+@end example
@noindent
Notice that there are two @var{Prologue} sections here, but there's a
Let's go ahead and add the new @code{YYLTYPE} definition and the
@code{trace_token} prototype at the same time:
-@smallexample
+@example
%code top @{
#define _GNU_SOURCE
#include <stdio.h>
@}
@dots{}
-@end smallexample
+@end example
@noindent
In this way, @code{%code top} and the unqualified @code{%code} achieve the same
definitions.
Thus, they belong in one or more @code{%code requires}:
-@smallexample
+@example
+@group
%code top @{
#define _GNU_SOURCE
#include <stdio.h>
@}
+@end group
+@group
%code requires @{
#include "ptypes.h"
@}
+@end group
+@group
%union @{
long int n;
tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
@}
+@end group
+@group
%code requires @{
#define YYLTYPE YYLTYPE
typedef struct YYLTYPE
char *filename;
@} YYLTYPE;
@}
+@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 smallexample
+@end example
@noindent
Now Bison will insert @code{#include "ptypes.h"} and the new
sufficient. Instead, move its prototype from the unqualified
@code{%code} to a @code{%code provides}:
-@smallexample
+@example
+@group
%code top @{
#define _GNU_SOURCE
#include <stdio.h>
@}
+@end group
+@group
%code requires @{
#include "ptypes.h"
@}
+@end group
+@group
%union @{
long int n;
tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
@}
+@end group
+@group
%code requires @{
#define YYLTYPE YYLTYPE
typedef struct YYLTYPE
char *filename;
@} YYLTYPE;
@}
+@end group
+@group
%code provides @{
void trace_token (enum yytokentype token, YYLTYPE loc);
@}
+@end group
+@group
%code @{
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
@}
+@end group
@dots{}
-@end smallexample
+@end example
@noindent
Bison will insert the @code{trace_token} prototype into both the
For example, you may organize semantic-type-related directives by semantic
type:
-@smallexample
+@example
+@group
%code requires @{ #include "type1.h" @}
%union @{ type1 field1; @}
%destructor @{ type1_free ($$); @} <field1>
%printer @{ type1_print ($$); @} <field1>
+@end group
+@group
%code requires @{ #include "type2.h" @}
%union @{ type2 field2; @}
%destructor @{ type2_free ($$); @} <field2>
%printer @{ type2_print ($$); @} <field2>
-@end smallexample
+@end group
+@end example
@noindent
You could even place each of the above directive groups in the rules section of
@example
@group
-@var{result}: @var{components}@dots{}
- ;
+@var{result}: @var{components}@dots{};
@end group
@end example
@example
@group
-exp: exp '+' exp
- ;
+exp: exp '+' exp;
@end group
@end example
@example
@group
-@var{result}: @var{rule1-components}@dots{}
- | @var{rule2-components}@dots{}
- @dots{}
- ;
+@var{result}:
+ @var{rule1-components}@dots{}
+| @var{rule2-components}@dots{}
+@dots{}
+;
@end group
@end example
@example
@group
-expseq: /* empty */
- | expseq1
- ;
+expseq:
+ /* empty */
+| expseq1
+;
@end group
@group
-expseq1: exp
- | expseq1 ',' exp
- ;
+expseq1:
+ exp
+| expseq1 ',' exp
+;
@end group
@end example
@example
@group
-expseq1: exp
- | expseq1 ',' exp
- ;
+expseq1:
+ exp
+| expseq1 ',' exp
+;
@end group
@end example
@example
@group
-expseq1: exp
- | exp ',' expseq1
- ;
+expseq1:
+ exp
+| exp ',' expseq1
+;
@end group
@end example
@example
@group
-expr: primary
- | primary '+' primary
- ;
+expr:
+ primary
+| primary '+' primary
+;
@end group
@group
-primary: constant
- | '(' expr ')'
- ;
+primary:
+ constant
+| '(' expr ')'
+;
@end group
@end example
* Mid-Rule Actions:: Most actions go at the end of a rule.
This says when, why and how to use the exceptional
action in the middle of a rule.
-* Named References:: Using named references in actions.
@end menu
@node Value Type
@example
@group
-exp: @dots{}
- | exp '+' exp
- @{ $$ = $1 + $3; @}
+exp:
+@dots{}
+| exp '+' exp @{ $$ = $1 + $3; @}
@end group
@end example
@example
@group
-exp[result]: @dots{}
- | exp[left] '+' exp[right]
- @{ $result = $left + $right; @}
+exp[result]:
+@dots{}
+| exp[left] '+' exp[right] @{ $result = $left + $right; @}
@end group
@end example
useful semantic value associated with the @samp{+} token, it could be
referred to as @code{$2}.
-@xref{Named References,,Using Named References}, for more information
-about using the named references construct.
+@xref{Named References}, for more information about using the named
+references construct.
Note that the vertical-bar character @samp{|} is really a rule
separator, and actions are attached to a single rule. This is a
@example
@group
-foo: expr bar '+' expr @{ @dots{} @}
- | expr bar '-' expr @{ @dots{} @}
- ;
+foo:
+ expr bar '+' expr @{ @dots{} @}
+| expr bar '-' expr @{ @dots{} @}
+;
@end group
@group
-bar: /* empty */
- @{ previous_expr = $0; @}
- ;
+bar:
+ /* empty */ @{ previous_expr = $0; @}
+;
@end group
@end example
@example
@group
-exp: @dots{}
- | exp '+' exp
- @{ $$ = $1 + $3; @}
+exp:
+ @dots{}
+| exp '+' exp @{ $$ = $1 + $3; @}
@end group
@end example
@example
@group
-stmt: LET '(' var ')'
- @{ $<context>$ = push_context ();
- declare_variable ($3); @}
- stmt @{ $$ = $6;
- pop_context ($<context>5); @}
+stmt:
+ LET '(' var ')'
+ @{ $<context>$ = push_context (); declare_variable ($3); @}
+ stmt
+ @{ $$ = $6; pop_context ($<context>5); @}
@end group
@end example
%%
-stmt: let stmt
- @{ $$ = $2;
- pop_context ($1); @}
- ;
+stmt:
+ let stmt
+ @{
+ $$ = $2;
+ pop_context ($1);
+ @};
-let: LET '(' var ')'
- @{ $$ = push_context ();
- declare_variable ($3); @}
- ;
+let:
+ LET '(' var ')'
+ @{
+ $$ = push_context ();
+ declare_variable ($3);
+ @};
@end group
@end example
@example
@group
-compound: '@{' declarations statements '@}'
- | '@{' statements '@}'
- ;
+compound:
+ '@{' declarations statements '@}'
+| '@{' statements '@}'
+;
@end group
@end example
@example
@group
-compound: @{ prepare_for_local_variables (); @}
- '@{' declarations statements '@}'
+compound:
+ @{ prepare_for_local_variables (); @}
+ '@{' declarations statements '@}'
@end group
@group
- | '@{' statements '@}'
- ;
+| '@{' statements '@}'
+;
@end group
@end example
@example
@group
-compound: @{ prepare_for_local_variables (); @}
- '@{' declarations statements '@}'
- | @{ prepare_for_local_variables (); @}
- '@{' statements '@}'
- ;
+compound:
+ @{ prepare_for_local_variables (); @}
+ '@{' declarations statements '@}'
+| @{ prepare_for_local_variables (); @}
+ '@{' statements '@}'
+;
@end group
@end example
@example
@group
-compound: '@{' @{ prepare_for_local_variables (); @}
- declarations statements '@}'
- | '@{' statements '@}'
- ;
+compound:
+ '@{' @{ prepare_for_local_variables (); @}
+ declarations statements '@}'
+| '@{' statements '@}'
+;
@end group
@end example
@example
@group
-subroutine: /* empty */
- @{ prepare_for_local_variables (); @}
- ;
-
+subroutine:
+ /* empty */ @{ prepare_for_local_variables (); @}
+;
@end group
@group
-compound: subroutine
- '@{' declarations statements '@}'
- | subroutine
- '@{' statements '@}'
- ;
+compound:
+ subroutine '@{' declarations statements '@}'
+| subroutine '@{' statements '@}'
+;
@end group
@end example
Now Bison can execute the action in the rule for @code{subroutine} without
deciding which rule for @code{compound} it will eventually use.
-@node Named References
-@subsection Using Named References
-@cindex named references
-
-While every semantic value can be accessed with positional references
-@code{$@var{n}} and @code{$$}, it's often much more convenient to refer to
-them by name. First of all, original symbol names may be used as named
-references. For example:
-
-@example
-@group
-invocation: op '(' args ')'
- @{ $invocation = new_invocation ($op, $args, @@invocation); @}
-@end group
-@end example
-
-@noindent
-The positional @code{$$}, @code{@@$}, @code{$n}, and @code{@@n} can be
-mixed with @code{$name} and @code{@@name} arbitrarily. For example:
-
-@example
-@group
-invocation: op '(' args ')'
- @{ $$ = new_invocation ($op, $args, @@$); @}
-@end group
-@end example
-
-@noindent
-However, sometimes regular symbol names are not sufficient due to
-ambiguities:
-
-@example
-@group
-exp: exp '/' exp
- @{ $exp = $exp / $exp; @} // $exp is ambiguous.
-
-exp: exp '/' exp
- @{ $$ = $1 / $exp; @} // One usage is ambiguous.
-
-exp: exp '/' exp
- @{ $$ = $1 / $3; @} // No error.
-@end group
-@end example
-
-@noindent
-When ambiguity occurs, explicitly declared names may be used for values and
-locations. Explicit names are declared as a bracketed name after a symbol
-appearance in rule definitions. For example:
-@example
-@group
-exp[result]: exp[left] '/' exp[right]
- @{ $result = $left / $right; @}
-@end group
-@end example
-
-@noindent
-Explicit names may be declared for RHS and for LHS symbols as well. In order
-to access a semantic value generated by a mid-rule action, an explicit name
-may also be declared by putting a bracketed name after the closing brace of
-the mid-rule action code:
-@example
-@group
-exp[res]: exp[x] '+' @{$left = $x;@}[left] exp[right]
- @{ $res = $left + $right; @}
-@end group
-@end example
-
-@noindent
-
-In references, in order to specify names containing dots and dashes, an explicit
-bracketed syntax @code{$[name]} and @code{@@[name]} must be used:
-@example
-@group
-if-stmt: IF '(' expr ')' THEN then.stmt ';'
- @{ $[if-stmt] = new_if_stmt ($expr, $[then.stmt]); @}
-@end group
-@end example
-
-It often happens that named references are followed by a dot, dash or other
-C punctuation marks and operators. By default, Bison will read
-@code{$name.suffix} as a reference to symbol value @code{$name} followed by
-@samp{.suffix}, i.e., an access to the @samp{suffix} field of the semantic
-value. In order to force Bison to recognize @code{name.suffix} in its entirety
-as the name of a semantic value, bracketed syntax @code{$[name.suffix]}
-must be used.
-
-
-@node Locations
+@node Tracking Locations
@section Tracking Locations
@cindex location
@cindex textual location
In addition, the named references construct @code{@@@var{name}} and
@code{@@[@var{name}]} may also be used to address the symbol locations.
-@xref{Named References,,Using Named References}, for more information
-about using the named references construct.
+@xref{Named References}, for more information about using the named
+references construct.
Here is a basic example using the default data type for locations:
@example
@group
-exp: @dots{}
- | exp '/' exp
- @{
- @@$.first_column = @@1.first_column;
- @@$.first_line = @@1.first_line;
- @@$.last_column = @@3.last_column;
- @@$.last_line = @@3.last_line;
- if ($3)
- $$ = $1 / $3;
- else
- @{
- $$ = 1;
- fprintf (stderr,
- "Division by zero, l%d,c%d-l%d,c%d",
- @@3.first_line, @@3.first_column,
- @@3.last_line, @@3.last_column);
- @}
- @}
+exp:
+ @dots{}
+| exp '/' exp
+ @{
+ @@$.first_column = @@1.first_column;
+ @@$.first_line = @@1.first_line;
+ @@$.last_column = @@3.last_column;
+ @@$.last_line = @@3.last_line;
+ if ($3)
+ $$ = $1 / $3;
+ else
+ @{
+ $$ = 1;
+ fprintf (stderr,
+ "Division by zero, l%d,c%d-l%d,c%d",
+ @@3.first_line, @@3.first_column,
+ @@3.last_line, @@3.last_column);
+ @}
+ @}
@end group
@end example
@example
@group
-exp: @dots{}
- | exp '/' exp
- @{
- if ($3)
- $$ = $1 / $3;
- else
- @{
- $$ = 1;
- fprintf (stderr,
- "Division by zero, l%d,c%d-l%d,c%d",
- @@3.first_line, @@3.first_column,
- @@3.last_line, @@3.last_column);
- @}
- @}
+exp:
+ @dots{}
+| exp '/' exp
+ @{
+ if ($3)
+ $$ = $1 / $3;
+ else
+ @{
+ $$ = 1;
+ fprintf (stderr,
+ "Division by zero, l%d,c%d-l%d,c%d",
+ @@3.first_line, @@3.first_column,
+ @@3.last_line, @@3.last_column);
+ @}
+ @}
@end group
@end example
By default, @code{YYLLOC_DEFAULT} is defined this way:
-@smallexample
+@example
@group
-# define YYLLOC_DEFAULT(Current, Rhs, N) \
- do \
- if (N) \
- @{ \
- (Current).first_line = YYRHSLOC(Rhs, 1).first_line; \
- (Current).first_column = YYRHSLOC(Rhs, 1).first_column; \
- (Current).last_line = YYRHSLOC(Rhs, N).last_line; \
- (Current).last_column = YYRHSLOC(Rhs, N).last_column; \
- @} \
- else \
- @{ \
- (Current).first_line = (Current).last_line = \
- YYRHSLOC(Rhs, 0).last_line; \
- (Current).first_column = (Current).last_column = \
- YYRHSLOC(Rhs, 0).last_column; \
- @} \
- while (0)
+# define YYLLOC_DEFAULT(Cur, Rhs, N) \
+do \
+ if (N) \
+ @{ \
+ (Cur).first_line = YYRHSLOC(Rhs, 1).first_line; \
+ (Cur).first_column = YYRHSLOC(Rhs, 1).first_column; \
+ (Cur).last_line = YYRHSLOC(Rhs, N).last_line; \
+ (Cur).last_column = YYRHSLOC(Rhs, N).last_column; \
+ @} \
+ else \
+ @{ \
+ (Cur).first_line = (Cur).last_line = \
+ YYRHSLOC(Rhs, 0).last_line; \
+ (Cur).first_column = (Cur).last_column = \
+ YYRHSLOC(Rhs, 0).last_column; \
+ @} \
+while (0)
@end group
-@end smallexample
+@end example
+@noindent
where @code{YYRHSLOC (rhs, k)} is the location of the @var{k}th symbol
in @var{rhs} when @var{k} is positive, and the location of the symbol
just before the reduction when @var{k} and @var{n} are both zero.
statement when it is followed by a semicolon.
@end itemize
+@node Named References
+@section Named References
+@cindex named references
+
+As described in the preceding sections, the traditional way to refer to any
+semantic value or location is a @dfn{positional reference}, which takes the
+form @code{$@var{n}}, @code{$$}, @code{@@@var{n}}, and @code{@@$}. However,
+such a reference is not very descriptive. Moreover, if you later decide to
+insert or remove symbols in the right-hand side of a grammar rule, the need
+to renumber such references can be tedious and error-prone.
+
+To avoid these issues, you can also refer to a semantic value or location
+using a @dfn{named reference}. First of all, original symbol names may be
+used as named references. For example:
+
+@example
+@group
+invocation: op '(' args ')'
+ @{ $invocation = new_invocation ($op, $args, @@invocation); @}
+@end group
+@end example
+
+@noindent
+Positional and named references can be mixed arbitrarily. For example:
+
+@example
+@group
+invocation: op '(' args ')'
+ @{ $$ = new_invocation ($op, $args, @@$); @}
+@end group
+@end example
+
+@noindent
+However, sometimes regular symbol names are not sufficient due to
+ambiguities:
+
+@example
+@group
+exp: exp '/' exp
+ @{ $exp = $exp / $exp; @} // $exp is ambiguous.
+
+exp: exp '/' exp
+ @{ $$ = $1 / $exp; @} // One usage is ambiguous.
+
+exp: exp '/' exp
+ @{ $$ = $1 / $3; @} // No error.
+@end group
+@end example
+
+@noindent
+When ambiguity occurs, explicitly declared names may be used for values and
+locations. Explicit names are declared as a bracketed name after a symbol
+appearance in rule definitions. For example:
+@example
+@group
+exp[result]: exp[left] '/' exp[right]
+ @{ $result = $left / $right; @}
+@end group
+@end example
+
+@noindent
+In order to access a semantic value generated by a mid-rule action, an
+explicit name may also be declared by putting a bracketed name after the
+closing brace of the mid-rule action code:
+@example
+@group
+exp[res]: exp[x] '+' @{$left = $x;@}[left] exp[right]
+ @{ $res = $left + $right; @}
+@end group
+@end example
+
+@noindent
+
+In references, in order to specify names containing dots and dashes, an explicit
+bracketed syntax @code{$[name]} and @code{@@[name]} must be used:
+@example
+@group
+if-stmt: "if" '(' expr ')' "then" then.stmt ';'
+ @{ $[if-stmt] = new_if_stmt ($expr, $[then.stmt]); @}
+@end group
+@end example
+
+It often happens that named references are followed by a dot, dash or other
+C punctuation marks and operators. By default, Bison will read
+@samp{$name.suffix} as a reference to symbol value @code{$name} followed by
+@samp{.suffix}, i.e., an access to the @code{suffix} field of the semantic
+value. In order to force Bison to recognize @samp{name.suffix} in its
+entirety as the name of a semantic value, the bracketed syntax
+@samp{$[name.suffix]} must be used.
+
+The named references feature is experimental. More user feedback will help
+to stabilize it.
+
@node Declarations
@section Bison Declarations
@cindex declarations, Bison
@noindent
For example:
-@smallexample
+@example
%union @{ char *string; @}
%token <string> STRING1
%token <string> STRING2
%destructor @{ free ($$); @} <*>
%destructor @{ free ($$); printf ("%d", @@$.first_line); @} STRING1 string1
%destructor @{ printf ("Discarding tagless symbol.\n"); @} <>
-@end smallexample
+@end example
@noindent
guarantees that, when the parser discards any user-defined symbol that has a
However, it may invoke one of them for the end token (token 0) if you
redefine it from @code{$end} to, for example, @code{END}:
-@smallexample
+@example
%token END 0
-@end smallexample
+@end example
@cindex actions in mid-rule
@cindex mid-rule actions
Finally, Bison will never invoke a @code{%destructor} for an unreferenced
mid-rule semantic value (@pxref{Mid-Rule Actions,,Actions in Mid-Rule}).
-That is, Bison does not consider a mid-rule to have a semantic value if you do
-not reference @code{$$} in the mid-rule's action or @code{$@var{n}} (where
-@var{n} is the RHS symbol position of the mid-rule) in any later action in that
-rule.
-However, if you do reference either, the Bison-generated parser will invoke the
-@code{<>} @code{%destructor} whenever it discards the mid-rule symbol.
+That is, Bison does not consider a mid-rule to have a semantic value if you
+do not reference @code{$$} in the mid-rule's action or @code{$@var{n}}
+(where @var{n} is the right-hand side symbol position of the mid-rule) in
+any later action in that rule. However, if you do reference either, the
+Bison-generated parser will invoke the @code{<>} @code{%destructor} whenever
+it discards the mid-rule symbol.
@ignore
@noindent
(Reentrant) Parser}.
If you have also used locations, the parser header file declares
-@code{YYLTYPE} and @code{yylloc} using a protocol similar to that of
-the @code{YYSTYPE} macro and @code{yylval}. @xref{Locations,
-,Tracking Locations}.
+@code{YYLTYPE} and @code{yylloc} using a protocol similar to that of the
+@code{YYSTYPE} macro and @code{yylval}. @xref{Tracking Locations}.
This parser header file is normally essential if you wish to put the
definition of @code{yylex} in a separate source file, because
Some of the accepted @var{variable}s are:
@itemize @bullet
+@c ================================================== api.pure
@item api.pure
@findex %define api.pure
specify where default reductions should be used is experimental. More user
feedback will help to stabilize it.)
-@item Accepted Values: @code{all}, @code{consistent}, @code{accepting}
+@item Accepted Values: @code{most}, @code{consistent}, @code{accepting}
@item Default Value:
@itemize
@item @code{accepting} if @code{lr.type} is @code{canonical-lr}.
-@item @code{all} otherwise.
+@item @code{most} otherwise.
@end itemize
@end itemize
occasionally it is necessary to insert code much nearer the top of the
parser implementation file. For example:
-@smallexample
+@example
%code top @{
#define _GNU_SOURCE
#include <stdio.h>
@}
-@end smallexample
+@end example
@item Location(s): Near the top of the parser implementation file.
@end itemize
@code{token_buffer}, and assuming that the token does not contain any
characters like @samp{"} that require escaping.
-@smallexample
+@example
for (i = 0; i < YYNTOKENS; i++)
@{
if (yytname[i] != 0
&& yytname[i][strlen (token_buffer) + 2] == 0)
break;
@}
-@end smallexample
+@end example
The @code{yytname} table is generated only if you use the
@code{%token-table} declaration. @xref{Decl Summary}.
@subsection Textual Locations of Tokens
@vindex yylloc
-If you are using the @samp{@@@var{n}}-feature (@pxref{Locations, ,
-Tracking Locations}) in actions to keep track of the textual locations
-of tokens and groupings, then you must provide this information in
-@code{yylex}. The function @code{yyparse} expects to find the textual
-location of a token just parsed in the global variable @code{yylloc}.
-So @code{yylex} must store the proper data in that variable.
+If you are using the @samp{@@@var{n}}-feature (@pxref{Tracking Locations})
+in actions to keep track of the textual locations of tokens and groupings,
+then you must provide this information in @code{yylex}. The function
+@code{yyparse} expects to find the textual location of a token just parsed
+in the global variable @code{yylloc}. So @code{yylex} must store the proper
+data in that variable.
By default, the value of @code{yylloc} is a structure and you need only
initialize the members that are going to be used by the actions. The
@deffn {Value} @@$
@findex @@$
-Acts like a structure variable containing information on the textual location
-of the grouping made by the current rule. @xref{Locations, ,
-Tracking Locations}.
+Acts like a structure variable containing information on the textual
+location of the grouping made by the current rule. @xref{Tracking
+Locations}.
@c Check if those paragraphs are still useful or not.
@deffn {Value} @@@var{n}
@findex @@@var{n}
-Acts like a structure variable containing information on the textual location
-of the @var{n}th component of the current rule. @xref{Locations, ,
-Tracking Locations}.
+Acts like a structure variable containing information on the textual
+location of the @var{n}th component of the current rule. @xref{Tracking
+Locations}.
@end deffn
@node Internationalization
* Contextual Precedence:: When an operator's precedence depends on context.
* Parser States:: The parser is a finite-state-machine with stack.
* Reduce/Reduce:: When two rules are applicable in the same situation.
-* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
+* Mysterious Conflicts:: Conflicts that look unjustified.
* Tuning LR:: How to tune fundamental aspects of LR-based parsing.
* Generalized LR Parsing:: Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted. How to avoid it.
@example
@group
-expr: term '+' expr
- | term
- ;
+expr:
+ term '+' expr
+| term
+;
@end group
@group
-term: '(' expr ')'
- | term '!'
- | NUMBER
- ;
+term:
+ '(' expr ')'
+| term '!'
+| NUMBER
+;
@end group
@end example
@example
@group
if_stmt:
- IF expr THEN stmt
- | IF expr THEN stmt ELSE stmt
- ;
+ IF expr THEN stmt
+| IF expr THEN stmt ELSE stmt
+;
@end group
@end example
%%
@end group
@group
-stmt: expr
- | if_stmt
- ;
+stmt:
+ expr
+| if_stmt
+;
@end group
@group
if_stmt:
- IF expr THEN stmt
- | IF expr THEN stmt ELSE stmt
- ;
+ IF expr THEN stmt
+| IF expr THEN stmt ELSE stmt
+;
@end group
-expr: variable
- ;
+expr:
+ variable
+;
@end example
@node Precedence
@example
@group
-expr: expr '-' expr
- | expr '*' expr
- | expr '<' expr
- | '(' expr ')'
- @dots{}
- ;
+expr:
+ expr '-' expr
+| expr '*' expr
+| expr '<' expr
+| '(' expr ')'
+@dots{}
+;
@end group
@end example
@example
@group
-exp: @dots{}
- | exp '-' exp
- @dots{}
- | '-' exp %prec UMINUS
+exp:
+ @dots{}
+| exp '-' exp
+ @dots{}
+| '-' exp %prec UMINUS
@end group
@end example
of zero or more @code{word} groupings.
@example
-sequence: /* empty */
- @{ printf ("empty sequence\n"); @}
- | maybeword
- | sequence word
- @{ printf ("added word %s\n", $2); @}
- ;
+@group
+sequence:
+ /* empty */ @{ printf ("empty sequence\n"); @}
+| maybeword
+| sequence word @{ printf ("added word %s\n", $2); @}
+;
+@end group
-maybeword: /* empty */
- @{ printf ("empty maybeword\n"); @}
- | word
- @{ printf ("single word %s\n", $1); @}
- ;
+@group
+maybeword:
+ /* empty */ @{ printf ("empty maybeword\n"); @}
+| word @{ printf ("single word %s\n", $1); @}
+;
+@end group
@end example
@noindent
proper way to define @code{sequence}:
@example
-sequence: /* empty */
- @{ printf ("empty sequence\n"); @}
- | sequence word
- @{ printf ("added word %s\n", $2); @}
- ;
+sequence:
+ /* empty */ @{ printf ("empty sequence\n"); @}
+| sequence word @{ printf ("added word %s\n", $2); @}
+;
@end example
Here is another common error that yields a reduce/reduce conflict:
@example
-sequence: /* empty */
- | sequence words
- | sequence redirects
- ;
+sequence:
+ /* empty */
+| sequence words
+| sequence redirects
+;
-words: /* empty */
- | words word
- ;
+words:
+ /* empty */
+| words word
+;
-redirects:/* empty */
- | redirects redirect
- ;
+redirects:
+ /* empty */
+| redirects redirect
+;
@end example
@noindent
of sequence:
@example
-sequence: /* empty */
- | sequence word
- | sequence redirect
- ;
+sequence:
+ /* empty */
+| sequence word
+| sequence redirect
+;
@end example
Second, to prevent either a @code{words} or a @code{redirects}
from being empty:
@example
-sequence: /* empty */
- | sequence words
- | sequence redirects
- ;
+@group
+sequence:
+ /* empty */
+| sequence words
+| sequence redirects
+;
+@end group
-words: word
- | words word
- ;
+@group
+words:
+ word
+| words word
+;
+@end group
-redirects:redirect
- | redirects redirect
- ;
+@group
+redirects:
+ redirect
+| redirects redirect
+;
+@end group
@end example
-@node Mystery Conflicts
-@section Mysterious Reduce/Reduce Conflicts
+@node Mysterious Conflicts
+@section Mysterious Conflicts
@cindex Mysterious Conflicts
Sometimes reduce/reduce conflicts can occur that don't look warranted.
%token ID
%%
-def: param_spec return_spec ','
- ;
+def: param_spec return_spec ',';
param_spec:
- type
- | name_list ':' type
- ;
+ type
+| name_list ':' type
+;
@end group
@group
return_spec:
- type
- | name ':' type
- ;
+ type
+| name ':' type
+;
@end group
@group
-type: ID
- ;
+type: ID;
@end group
@group
-name: ID
- ;
+name: ID;
name_list:
- name
- | name ',' name_list
- ;
+ name
+| name ',' name_list
+;
@end group
@end example
%%
@dots{}
return_spec:
- type
- | name ':' type
- /* This rule is never used. */
- | ID BOGUS
- ;
+ type
+| name ':' type
+| ID BOGUS /* This rule is never used. */
+;
@end group
@end example
@example
param_spec:
- type
- | name_list ':' type
- ;
+ type
+| name_list ':' type
+;
return_spec:
- type
- | ID ':' type
- ;
+ type
+| ID ':' type
+;
@end example
For a more detailed exposition of LALR(1) parsers and parser
For historical reasons, Bison constructs LALR(1) parser tables by default.
However, LALR does not possess the full language-recognition power of LR.
As a result, the behavior of parsers employing LALR parser tables is often
-mysterious. We presented a simple example of this effect in @ref{Mystery
+mysterious. We presented a simple example of this effect in @ref{Mysterious
Conflicts}.
As we also demonstrated in that example, the traditional approach to
%define lr.type ielr
@end example
-@noindent For the example in @ref{Mystery Conflicts}, the mysterious
+@noindent For the example in @ref{Mysterious Conflicts}, the mysterious
conflict is then eliminated, so there is no need to invest time in
comprehending the conflict or restructuring the grammar to fix it. If,
during future development, the grammar evolves such that all mysterious
Specify the kind of states that are permitted to contain default reductions.
The accepted values of @var{WHERE} are:
@itemize
-@item @code{all} (default for LALR and IELR)
+@item @code{most} (default for LALR and IELR)
@item @code{consistent}
@item @code{accepting} (default for canonical LR)
@end itemize
experimental. More user feedback will help to stabilize it.)
@end deffn
-FIXME: Because of the exceptions described above, @code{all} is a misnomer.
-Rename to @code{full}.
-
@node LAC
@subsection LAC
@findex %define parse.lac
has proven insignificant for practical grammars.
@end itemize
+While the LAC algorithm shares techniques that have been recognized in the
+parser community for years, for the publication that introduces LAC,
+@pxref{Bibliography,,Denny 2010 May}.
+
@node Unreachable States
@subsection Unreachable States
@findex %define lr.keep-unreachable-states
The same is true of languages that require more than one symbol of
lookahead, since the parser lacks the information necessary to make a
decision at the point it must be made in a shift-reduce parser.
-Finally, as previously mentioned (@pxref{Mystery Conflicts}),
+Finally, as previously mentioned (@pxref{Mysterious Conflicts}),
there are languages where Bison's default choice of how to
summarize the input seen so far loses necessary information.
For example:
@example
-stmnts: /* empty string */
- | stmnts '\n'
- | stmnts exp '\n'
- | stmnts error '\n'
+stmnts:
+ /* empty string */
+| stmnts '\n'
+| stmnts exp '\n'
+| stmnts error '\n'
@end example
The fourth rule in this example says that an error followed by a newline
spurious error message:
@example
-primary: '(' expr ')'
- | '(' error ')'
- @dots{}
- ;
+primary:
+ '(' expr ')'
+| '(' error ')'
+@dots{}
+;
@end example
Error recovery strategies are necessarily guesses. When they guess wrong,
@example
typedef int foo, bar;
int baz (void)
+@group
@{
static bar (bar); /* @r{redeclare @code{bar} as static variable} */
extern foo foo (foo); /* @r{redeclare @code{foo} as function} */
return foo (bar);
@}
+@end group
@end example
Unfortunately, the name being declared is separated from the declaration
duplication, with actions omitted for brevity:
@example
+@group
initdcl:
- declarator maybeasm '='
- init
- | declarator maybeasm
- ;
+ declarator maybeasm '=' init
+| declarator maybeasm
+;
+@end group
+@group
notype_initdcl:
- notype_declarator maybeasm '='
- init
- | notype_declarator maybeasm
- ;
+ notype_declarator maybeasm '=' init
+| notype_declarator maybeasm
+;
+@end group
@end example
@noindent
@dots{}
@end group
@group
-expr: IDENTIFIER
- | constant
- | HEX '('
- @{ hexflag = 1; @}
- expr ')'
- @{ hexflag = 0;
- $$ = $4; @}
- | expr '+' expr
- @{ $$ = make_sum ($1, $3); @}
- @dots{}
- ;
+expr:
+ IDENTIFIER
+| constant
+| HEX '(' @{ hexflag = 1; @}
+ expr ')' @{ hexflag = 0; $$ = $4; @}
+| expr '+' expr @{ $$ = make_sum ($1, $3); @}
+@dots{}
+;
@end group
@group
constant:
- INTEGER
- | STRING
- ;
+ INTEGER
+| STRING
+;
@end group
@end example
tokens until the next semicolon, and then start a new statement, like this:
@example
-stmt: expr ';'
- | IF '(' expr ')' stmt @{ @dots{} @}
- @dots{}
- error ';'
- @{ hexflag = 0; @}
- ;
+stmt:
+ expr ';'
+| IF '(' expr ')' stmt @{ @dots{} @}
+@dots{}
+| error ';' @{ hexflag = 0; @}
+;
@end example
If there is a syntax error in the middle of a @samp{hex (@var{expr})}
@example
@group
-expr: @dots{}
- | '(' expr ')'
- @{ $$ = $2; @}
- | '(' error ')'
- @dots{}
+expr:
+ @dots{}
+| '(' expr ')' @{ $$ = $2; @}
+| '(' error ')'
+@dots{}
@end group
@end example
%left '+' '-'
%left '*'
%%
-exp: exp '+' exp
- | exp '-' exp
- | exp '*' exp
- | exp '/' exp
- | NUM
- ;
+exp:
+ exp '+' exp
+| exp '-' exp
+| exp '*' exp
+| exp '/' exp
+| NUM
+;
useless: STR;
%%
@end example
and reports the uses of the symbols:
@example
+@group
Terminals, with rules where they appear
$end (0) 0
'/' (47) 4
error (256)
NUM (258) 5
+@end group
+@group
Nonterminals, with rules where they appear
$accept (8)
on left: 0
exp (9)
on left: 1 2 3 4 5, on right: 0 1 2 3 4
+@end group
@end example
@noindent
@cindex pointed rule
@cindex rule, pointed
Bison then proceeds onto the automaton itself, describing each state
-with it set of @dfn{items}, also known as @dfn{pointed rules}. Each
-item is a production rule together with a point (marked by @samp{.})
-that the input cursor.
+with its set of @dfn{items}, also known as @dfn{pointed rules}. Each
+item is a production rule together with a point (@samp{.}) marking
+the location of the input cursor.
@example
state 0
symbol (here, @code{exp}). When the parser returns to this state right
after having reduced a rule that produced an @code{exp}, the control
flow jumps to state 2. If there is no such transition on a nonterminal
-symbol, and the lookahead is a @code{NUM}, then this token is shifted on
+symbol, and the lookahead is a @code{NUM}, then this token is shifted onto
the parse stack, and the control flow jumps to state 1. Any other
lookahead triggers a syntax error.''
at the beginning of any rule deriving an @code{exp}. By default Bison
reports the so-called @dfn{core} or @dfn{kernel} of the item set, but if
you want to see more detail you can invoke @command{bison} with
-@option{--report=itemset} to list all the items, include those that can
-be derived:
+@option{--report=itemset} to list the derived items as well:
@example
state 0
@noindent
In state 2, the automaton can only shift a symbol. For instance,
-because of the item @samp{exp -> exp . '+' exp}, if the lookahead if
-@samp{+}, it will be shifted on the parse stack, and the automaton
-control will jump to state 4, corresponding to the item @samp{exp -> exp
-'+' . exp}. Since there is no default action, any other token than
-those listed above will trigger a syntax error.
+because of the item @samp{exp -> exp . '+' exp}, if the lookahead is
+@samp{+} it is shifted onto the parse stack, and the automaton
+jumps to state 4, corresponding to the item @samp{exp -> exp '+' . exp}.
+Since there is no default action, any lookahead not listed triggers a syntax
+error.
@cindex accepting state
The state 3 is named the @dfn{final state}, or the @dfn{accepting
The remaining states are similar:
@example
+@group
state 9
exp -> exp . '+' exp (rule 1)
'/' [reduce using rule 2 (exp)]
$default reduce using rule 2 (exp)
+@end group
+@group
state 10
exp -> exp . '+' exp (rule 1)
'/' [reduce using rule 3 (exp)]
$default reduce using rule 3 (exp)
+@end group
+@group
state 11
exp -> exp . '+' exp (rule 1)
'*' [reduce using rule 4 (exp)]
'/' [reduce using rule 4 (exp)]
$default reduce using rule 4 (exp)
+@end group
@end example
@noindent
Here is an example of @code{YYPRINT} suitable for the multi-function
calculator (@pxref{Mfcalc Declarations, ,Declarations for @code{mfcalc}}):
-@smallexample
+@example
%@{
static void print_token_value (FILE *, int, YYSTYPE);
- #define YYPRINT(file, type, value) print_token_value (file, type, value)
+ #define YYPRINT(file, type, value) \
+ print_token_value (file, type, value)
%@}
@dots{} %% @dots{} %% @dots{}
else if (type == NUM)
fprintf (file, "%d", value.val);
@}
-@end smallexample
+@end example
@c ================================================= Invoking Bison
For example, warn about unset @code{$$} in the mid-rule action in:
@example
- exp: '1' @{ $1 = 1; @} '+' exp @{ $$ = $2 + $4; @};
+exp: '1' @{ $1 = 1; @} '+' exp @{ $$ = $2 + $4; @};
@end example
These warnings are not enabled by default since they sometimes prove to
be false alarms in existing grammars employing the Yacc constructs
@code{$0} or @code{$-@var{n}} (where @var{n} is some positive integer).
-
@item yacc
Incompatibilities with POSIX Yacc.
+@item conflicts-sr
+@itemx conflicts-rr
+S/R and R/R conflicts. These warnings are enabled by default. However, if
+the @code{%expect} or @code{%expect-rr} directive is specified, an
+unexpected number of conflicts is an error, and an expected number of
+conflicts is not reported, so @option{-W} and @option{--warning} then have
+no effect on the conflict report.
+
+@item other
+All warnings not categorized above. These warnings are enabled by default.
+
+This category is provided merely for the sake of completeness. Future
+releases of Bison may move warnings from this category to new, more specific
+categories.
+
@item all
All the warnings.
@item none
@c - %define filename_type "const symbol::Symbol"
When the directive @code{%locations} is used, the C++ parser supports
-location tracking, see @ref{Locations, , Locations Overview}. Two
-auxiliary classes define a @code{position}, a single point in a file,
-and a @code{location}, a range composed of a pair of
-@code{position}s (possibly spanning several files).
+location tracking, see @ref{Tracking Locations}. Two auxiliary classes
+define a @code{position}, a single point in a file, and a @code{location}, a
+range composed of a pair of @code{position}s (possibly spanning several
+files).
@deftypemethod {position} {std::string*} file
The name of the file. It will always be handled as a pointer, the
@end defcv
@defcv {Type} {parser} {token}
-A structure that contains (only) the definition of the tokens as the
-@code{yytokentype} enumeration. To refer to the token @code{FOO}, the
-scanner should use @code{yy::parser::token::FOO}. The scanner can use
+A structure that contains (only) the @code{yytokentype} enumeration, which
+defines the tokens. To refer to the token @code{FOO},
+use @code{yy::parser::token::FOO}. The scanner can use
@samp{typedef yy::parser::token token;} to ``import'' the token enumeration
(@pxref{Calc++ Scanner}).
@end defcv
@comment file: calc++-parser.yy
@example
-%skeleton "lalr1.cc" /* -*- C++ -*- */
+%skeleton "lalr1.cc" /* -*- C++ -*- */
%require "@value{VERSION}"
%defines
%define parser_class_name "calcxx_parser"
%start unit;
unit: assignments exp @{ driver.result = $2; @};
-assignments: assignments assignment @{@}
- | /* Nothing. */ @{@};
+assignments:
+ /* Nothing. */ @{@}
+| assignments assignment @{@};
assignment:
"identifier" ":=" exp
@comment file: calc++-scanner.ll
@example
-%@{ /* -*- C++ -*- */
+%@{ /* -*- C++ -*- */
# include <cstdlib>
# include <cerrno>
# include <climits>
@comment file: calc++-scanner.ll
@example
+@group
%@{
# define YY_USER_ACTION yylloc->columns (yyleng);
%@}
+@end group
%%
%@{
yylloc->step ();
@comment file: calc++-scanner.ll
@example
+@group
void
calcxx_driver::scan_begin ()
@{
yyin = stdin;
else if (!(yyin = fopen (file.c_str (), "r")))
@{
- error (std::string ("cannot open ") + file);
- exit (1);
+ error ("cannot open " + file + ": " + strerror(errno));
+ exit (EXIT_FAILURE);
@}
@}
+@end group
+@group
void
calcxx_driver::scan_end ()
@{
fclose (yyin);
@}
+@end group
@end example
@node Calc++ Top Level
#include <iostream>
#include "calc++-driver.hh"
+@group
int
main (int argc, char *argv[])
@{
else if (!driver.parse (*argv))
std::cout << driver.result << std::endl;
@}
+@end group
@end example
@node Java Parsers
@c - class Position
@c - class Location
-When the directive @code{%locations} is used, the Java parser
-supports location tracking, see @ref{Locations, , Locations Overview}.
-An auxiliary user-defined class defines a @dfn{position}, a single point
-in a file; Bison itself defines a class representing a @dfn{location},
-a range composed of a pair of positions (possibly spanning several
-files). The location class is an inner class of the parser; the name
-is @code{Location} by default, and may also be renamed using
-@code{%define location_type "@var{class-name}"}.
+When the directive @code{%locations} is used, the Java parser supports
+location tracking, see @ref{Tracking Locations}. An auxiliary user-defined
+class defines a @dfn{position}, a single point in a file; Bison itself
+defines a class representing a @dfn{location}, a range composed of a pair of
+positions (possibly spanning several files). The location class is an inner
+class of the parser; the name is @code{Location} by default, and may also be
+renamed using @code{%define location_type "@var{class-name}"}.
The location class treats the position as a completely opaque value.
By default, the class name is @code{Position}, but this can be changed
@node Memory Exhausted
@section Memory Exhausted
-@display
+@quotation
My parser returns with error with a @samp{memory exhausted}
message. What can I do?
-@end display
+@end quotation
This question is already addressed elsewhere, @xref{Recursion,
,Recursive Rules}.
The following phenomenon has several symptoms, resulting in the
following typical questions:
-@display
+@quotation
I invoke @code{yyparse} several times, and on correct input it works
properly; but when a parse error is found, all the other calls fail
too. How can I reset the error flag of @code{yyparse}?
-@end display
+@end quotation
@noindent
or
-@display
+@quotation
My parser includes support for an @samp{#include}-like feature, in
which case I run @code{yyparse} from @code{yyparse}. This fails
-although I did specify @code{%define api.pure}.
-@end display
+although I did specify @samp{%define api.pure}.
+@end quotation
These problems typically come not from Bison itself, but from
Lex-generated scanners. Because these scanners use large buffers for
demonstration, consider the following source file,
@file{first-line.l}:
-@verbatim
-%{
+@example
+@group
+%@{
#include <stdio.h>
#include <stdlib.h>
-%}
+%@}
+@end group
%%
.*\n ECHO; return 1;
%%
+@group
int
yyparse (char const *file)
-{
+@{
yyin = fopen (file, "r");
if (!yyin)
- exit (2);
+ @{
+ perror ("fopen");
+ exit (EXIT_FAILURE);
+ @}
+@end group
+@group
/* One token only. */
yylex ();
if (fclose (yyin) != 0)
- exit (3);
+ @{
+ perror ("fclose");
+ exit (EXIT_FAILURE);
+ @}
return 0;
-}
+@}
+@end group
+@group
int
main (void)
-{
+@{
yyparse ("input");
yyparse ("input");
return 0;
-}
-@end verbatim
+@}
+@end group
+@end example
@noindent
If the file @file{input} contains
-@verbatim
+@example
input:1: Hello,
input:2: World!
-@end verbatim
+@end example
@noindent
then instead of getting the first line twice, you get:
@node Strings are Destroyed
@section Strings are Destroyed
-@display
+@quotation
My parser seems to destroy old strings, or maybe it loses track of
them. Instead of reporting @samp{"foo", "bar"}, it reports
@samp{"bar", "bar"}, or even @samp{"foo\nbar", "bar"}.
-@end display
+@end quotation
This error is probably the single most frequent ``bug report'' sent to
Bison lists, but is only concerned with a misunderstanding of the role
of the scanner. Consider the following Lex code:
-@verbatim
-%{
+@example
+@group
+%@{
#include <stdio.h>
char *yylval = NULL;
-%}
+%@}
+@end group
+@group
%%
.* yylval = yytext; return 1;
\n /* IGNORE */
%%
+@end group
+@group
int
main ()
-{
+@{
/* Similar to using $1, $2 in a Bison action. */
char *fst = (yylex (), yylval);
char *snd = (yylex (), yylval);
printf ("\"%s\", \"%s\"\n", fst, snd);
return 0;
-}
-@end verbatim
+@}
+@end group
+@end example
If you compile and run this code, you get:
@node Implementing Gotos/Loops
@section Implementing Gotos/Loops
-@display
+@quotation
My simple calculator supports variables, assignments, and functions,
but how can I implement gotos, or loops?
-@end display
+@end quotation
Although very pedagogical, the examples included in the document blur
the distinction to make between the parser---whose job is to recover
@node Multiple start-symbols
@section Multiple start-symbols
-@display
+@quotation
I have several closely related grammars, and I would like to share their
implementations. In fact, I could use a single grammar but with
multiple entry points.
-@end display
+@end quotation
Bison does not support multiple start-symbols, but there is a very
simple means to simulate them. If @code{foo} and @code{bar} are the two
@example
%token START_FOO START_BAR;
%start start;
-start: START_FOO foo
- | START_BAR bar;
+start:
+ START_FOO foo
+| START_BAR bar;
@end example
These tokens prevents the introduction of new conflicts. As far as the
@node Secure? Conform?
@section Secure? Conform?
-@display
+@quotation
Is Bison secure? Does it conform to POSIX?
-@end display
+@end quotation
If you're looking for a guarantee or certification, we don't provide it.
However, Bison is intended to be a reliable program that conforms to the
@node I can't build Bison
@section I can't build Bison
-@display
+@quotation
I can't build Bison because @command{make} complains that
@code{msgfmt} is not found.
What should I do?
-@end display
+@end quotation
Like most GNU packages with internationalization support, that feature
is turned on by default. If you have problems building in the @file{po}
@node Where can I find help?
@section Where can I find help?
-@display
+@quotation
I'm having trouble using Bison. Where can I find help?
-@end display
+@end quotation
First, read this fine manual. Beyond that, you can send mail to
@email{help-bison@@gnu.org}. This mailing list is intended to be
@node Bug Reports
@section Bug Reports
-@display
+@quotation
I found a bug. What should I include in the bug report?
-@end display
+@end quotation
Before you send a bug report, make sure you are using the latest
version. Check @url{ftp://ftp.gnu.org/pub/gnu/bison/} or one of its
send additional files as well (such as `config.h' or `config.cache').
Patches are most welcome, but not required. That is, do not hesitate to
-send a bug report just because you can not provide a fix.
+send a bug report just because you cannot provide a fix.
Send bug reports to @email{bug-bison@@gnu.org}.
@node More Languages
@section More Languages
-@display
+@quotation
Will Bison ever have C++ and Java support? How about @var{insert your
favorite language here}?
-@end display
+@end quotation
C++ and Java support is there now, and is documented. We'd love to add other
languages; contributions are welcome.
@node Beta Testing
@section Beta Testing
-@display
+@quotation
What is involved in being a beta tester?
-@end display
+@end quotation
It's not terribly involved. Basically, you would download a test
release, compile it, and use it to build and run a parser or two. After
@node Mailing Lists
@section Mailing Lists
-@display
+@quotation
How do I join the help-bison and bug-bison mailing lists?
-@end display
+@end quotation
See @url{http://lists.gnu.org/}.
@deffn {Variable} @@$
In an action, the location of the left-hand side of the rule.
-@xref{Locations, , Locations Overview}.
+@xref{Tracking Locations}.
@end deffn
@deffn {Variable} @@@var{n}
-In an action, the location of the @var{n}-th symbol of the right-hand
-side of the rule. @xref{Locations, , Locations Overview}.
+In an action, the location of the @var{n}-th symbol of the right-hand side
+of the rule. @xref{Tracking Locations}.
@end deffn
@deffn {Variable} @@@var{name}
-In an action, the location of a symbol addressed by name.
-@xref{Locations, , Locations Overview}.
+In an action, the location of a symbol addressed by name. @xref{Tracking
+Locations}.
@end deffn
@deffn {Variable} @@[@var{name}]
-In an action, the location of a symbol addressed by name.
-@xref{Locations, , Locations Overview}.
+In an action, the location of a symbol addressed by name. @xref{Tracking
+Locations}.
@end deffn
@deffn {Variable} $$
@item LALR(1)
The class of context-free grammars that Bison (like most other parser
generators) can handle by default; a subset of LR(1).
-@xref{Mystery Conflicts, ,Mysterious Reduce/Reduce Conflicts}.
+@xref{Mysterious Conflicts}.
@item LR(1)
The class of context-free grammars in which at most one token of
@c LocalWords: NUM exp subsubsection kbd Ctrl ctype EOF getchar isdigit nonfree
@c LocalWords: ungetc stdin scanf sc calc ulator ls lm cc NEG prec yyerrok rr
@c LocalWords: longjmp fprintf stderr yylloc YYLTYPE cos ln Stallman Destructor
-@c LocalWords: smallexample symrec val tptr FNCT fnctptr func struct sym enum
+@c LocalWords: symrec val tptr FNCT fnctptr func struct sym enum
@c LocalWords: fnct putsym getsym fname arith fncts atan ptr malloc sizeof Lex
@c LocalWords: strlen strcpy fctn strcmp isalpha symbuf realloc isalnum DOTDOT
@c LocalWords: ptypes itype YYPRINT trigraphs yytname expseq vindex dtype Unary
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