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Add %precedence support.

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Unfortunately it is not possible to reuse the %prec directive.  This
is because to please POSIX, we do not require to end the rules with a
semicolon.  As a result,

foo: bar %prec baz

is ambiguous: either a rule which precedence is that of baz, or a rule,
and then a declaration of the precedence of the token baz.

	* doc/bison.texinfo: Document %precedence.
	(Precedence Only): New.
	* src/assoc.h, src/assoc.c (precedence_assoc): New.
	* src/conflicts.c (resolve_sr_conflict): Support it.
	* src/scan-gram.l, src/parse-gram.y (%precedence): New token.
	Parse it.
	* tests/calc.at: Use %precedence for NEG.
	* tests/conflicts.at (%precedence does not suffice)
	(%precedence suffices): New tests.
This commit is contained in:
Akim Demaille
2008-08-07 23:15:34 +02:00
parent c85be41a07
commit d78f0ac9d8
9 changed files with 195 additions and 40 deletions
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22
ChangeLog
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@@ -1,3 +1,25 @@
2008-11-10 Akim Demaille <demaille@gostai.com>
Add %precedence support.
Unfortunately it is not possible to reuse the %prec directive. This
is because to please POSIX, we do not require to end the rules with a
semicolon. As a result,
foo: bar %prec baz
is ambiguous: either a rule which precedence is that of baz, or a rule,
and then a declaration of the precedence of the token baz.
* doc/bison.texinfo: Document %precedence.
(Precedence Only): New.
* src/assoc.h, src/assoc.c (precedence_assoc): New.
* src/conflicts.c (resolve_sr_conflict): Support it.
* src/scan-gram.l, src/parse-gram.y (%precedence): New token.
Parse it.
* tests/calc.at: Use %precedence for NEG.
* tests/conflicts.at (%precedence does not suffice)
(%precedence suffices): New tests.
2008-11-09 Akim Demaille <demaille@gostai.com>
Make benches in a sub dirs.

105
doc/bison.texinfo
View File

@@ -264,7 +264,8 @@ The Bison Parser Algorithm
Operator Precedence
* Why Precedence:: An example showing why precedence is needed.
* Using Precedence:: How to specify precedence in Bison grammars.
* Using Precedence:: How to specify precedence and associativity.
* Precedence Only:: How to specify precedence only.
* Precedence Examples:: How these features are used in the previous example.
* How Precedence:: How they work.
@@ -1858,8 +1859,8 @@ parentheses nested to arbitrary depth. Here is the Bison code for
%token NUM
%left '-' '+'
%left '*' '/'
%left NEG /* negation--unary minus */
%right '^' /* exponentiation */
%precedence NEG /* negation--unary minus */
%right '^' /* exponentiation */
%% /* The grammar follows. */
input: /* empty */
@@ -1892,15 +1893,16 @@ In the second section (Bison declarations), @code{%left} declares token
types and says they are left-associative operators. The declarations
@code{%left} and @code{%right} (right associativity) take the place of
@code{%token} which is used to declare a token type name without
associativity. (These tokens are single-character literals, which
associativity/precedence. (These tokens are single-character literals, which
ordinarily don't need to be declared. We declare them here to specify
the associativity.)
the associativity/precedence.)
Operator precedence is determined by the line ordering of the
declarations; the higher the line number of the declaration (lower on
the page or screen), the higher the precedence. Hence, exponentiation
has the highest precedence, unary minus (@code{NEG}) is next, followed
by @samp{*} and @samp{/}, and so on. @xref{Precedence, ,Operator
by @samp{*} and @samp{/}, and so on. Unary minus is not associative,
only precedence matters (@code{%precedence}. @xref{Precedence, ,Operator
Precedence}.
The other important new feature is the @code{%prec} in the grammar
@@ -2003,7 +2005,7 @@ the same as the declarations for the infix notation calculator.
%left '-' '+'
%left '*' '/'
%left NEG
%precedence NEG
%right '^'
%% /* The grammar follows. */
@@ -2251,8 +2253,8 @@ Here are the C and Bison declarations for the multi-function calculator.
%right '='
%left '-' '+'
%left '*' '/'
%left NEG /* negation--unary minus */
%right '^' /* exponentiation */
%precedence NEG /* negation--unary minus */
%right '^' /* exponentiation */
@end group
%% /* The grammar follows. */
@end smallexample
@@ -4019,7 +4021,8 @@ Bison will convert this into a @code{#define} directive in
the parser, so that the function @code{yylex} (if it is in this file)
can use the name @var{name} to stand for this token type's code.
Alternatively, you can use @code{%left}, @code{%right}, or
Alternatively, you can use @code{%left}, @code{%right},
@code{%precedence}, or
@code{%nonassoc} instead of @code{%token}, if you wish to specify
associativity and precedence. @xref{Precedence Decl, ,Operator
Precedence}.
@@ -4095,7 +4098,8 @@ of ``$end'':
@cindex declaring operator precedence
@cindex operator precedence, declaring
Use the @code{%left}, @code{%right} or @code{%nonassoc} declaration to
Use the @code{%left}, @code{%right}, @code{%nonassoc}, or
@code{%precedence} declaration to
declare a token and specify its precedence and associativity, all at
once. These are called @dfn{precedence declarations}.
@xref{Precedence, ,Operator Precedence}, for general information on
@@ -4131,6 +4135,10 @@ left-associativity (grouping @var{x} with @var{y} first) and
means that @samp{@var{x} @var{op} @var{y} @var{op} @var{z}} is
considered a syntax error.
@code{%precedence} gives only precedence to the @var{symbols}, and
defines no associativity at all. Use this to define precedence only,
and leave any potential conflict due to associativity enabled.
@item
The precedence of an operator determines how it nests with other operators.
All the tokens declared in a single precedence declaration have equal
@@ -6221,7 +6229,8 @@ shift and when to reduce.
@menu
* Why Precedence:: An example showing why precedence is needed.
* Using Precedence:: How to specify precedence in Bison grammars.
* Using Precedence:: How to specify precedence and associativity.
* Precedence Only:: How to specify precedence only.
* Precedence Examples:: How these features are used in the previous example.
* How Precedence:: How they work.
@end menu
@@ -6276,8 +6285,9 @@ makes right-associativity.
@node Using Precedence
@subsection Specifying Operator Precedence
@findex %left
@findex %right
@findex %nonassoc
@findex %precedence
@findex %right
Bison allows you to specify these choices with the operator precedence
declarations @code{%left} and @code{%right}. Each such declaration
@@ -6287,13 +6297,63 @@ those operators left-associative and the @code{%right} declaration makes
them right-associative. A third alternative is @code{%nonassoc}, which
declares that it is a syntax error to find the same operator twice ``in a
row''.
The last alternative, @code{%precedence}, allows to define only
precedence and no associativity at all. As a result, any
associativity-related conflict that remains will be reported as an
compile-time error. The directive @code{%nonassoc} creates run-time
error: using the operator in a associative way is a syntax error. The
directive @code{%precedence} creates compile-time errors: an operator
@emph{can} be involved in an associativity-related conflict, contrary to
what expected the grammar author.
The relative precedence of different operators is controlled by the
order in which they are declared. The first @code{%left} or
@code{%right} declaration in the file declares the operators whose
order in which they are declared. The first precedence/associativity
declaration in the file declares the operators whose
precedence is lowest, the next such declaration declares the operators
whose precedence is a little higher, and so on.
@node Precedence Only
@subsection Specifying Precedence Only
@findex %precedence
Since @acronym{POSIX} Yacc defines only @code{%left}, @code{%right}, and
@code{%nonassoc}, which all defines precedence and associativity, little
attention is paid to the fact that precedence cannot be defined without
defining associativity. Yet, sometimes, when trying to solve a
conflict, precedence suffices. In such a case, using @code{%left},
@code{%right}, or @code{%nonassoc} might hide future (associativity
related) conflicts that would remain hidden.
The dangling @code{else} ambiguity (@pxref{Shift/Reduce, , Shift/Reduce
Conflicts}) can be solved explictly. This shift/reduce conflicts occurs
in the following situation, where the period denotes the current parsing
state:
@example
if @var{e1} then if @var{e2} then @var{s1} . else @var{s2}
@end example
The conflict involves the reduction of the rule @samp{IF expr THEN
stmt}, which precedence is by default that of its last token
(@code{THEN}), and the shifting of the token @code{ELSE}. The usual
disambiguation (attach the @code{else} to the closest @code{if}),
shifting must be preferred, i.e., the precedence of @code{ELSE} must be
higher than that of @code{THEN}. But neither is expected to be involved
in an associativity related conflict, which can be specified as follows.
@example
%precedence THEN
%precedence ELSE
@end example
The unary-minus is another typical example where associativity is
usually over-specified, see @ref{Infix Calc, , Infix Notation
Calculator: @code{calc}}. The @code{%left} directive is traditionaly
used to declare the precedence of @code{NEG}, which is more than needed
since it also defines its associativity. While this is harmless in the
traditional example, who knows how @code{NEG} might be used in future
evolutions of the grammar@dots{}
@node Precedence Examples
@subsection Precedence Examples
@@ -6355,8 +6415,8 @@ outlandish at first, but it is really very common. For example, a minus
sign typically has a very high precedence as a unary operator, and a
somewhat lower precedence (lower than multiplication) as a binary operator.
The Bison precedence declarations, @code{%left}, @code{%right} and
@code{%nonassoc}, can only be used once for a given token; so a token has
The Bison precedence declarations
can only be used once for a given token; so a token has
only one precedence declared in this way. For context-dependent
precedence, you need to use an additional mechanism: the @code{%prec}
modifier for rules.
@@ -9865,7 +9925,7 @@ Specify the programming language for the generated parser.
@end deffn
@deffn {Directive} %left
Bison declaration to assign left associativity to token(s).
Bison declaration to assign precedence and left associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
@end deffn
@@ -9900,7 +9960,7 @@ parser file. @xref{Decl Summary}.
@end deffn
@deffn {Directive} %nonassoc
Bison declaration to assign nonassociativity to token(s).
Bison declaration to assign precedence and nonassociativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
@end deffn
@@ -9920,6 +9980,11 @@ Bison declaration to assign a precedence to a specific rule.
@xref{Contextual Precedence, ,Context-Dependent Precedence}.
@end deffn
@deffn {Directive} %precedence
Bison declaration to assign precedence to token(s), but no associativity
@xref{Precedence Decl, ,Operator Precedence}.
@end deffn
@deffn {Directive} %pure-parser
Deprecated version of @code{%define api.pure} (@pxref{Decl Summary, ,%define}),
for which Bison is more careful to warn about unreasonable usage.
@@ -9931,7 +9996,7 @@ Require a Version of Bison}.
@end deffn
@deffn {Directive} %right
Bison declaration to assign right associativity to token(s).
Bison declaration to assign precedence and right associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
@end deffn

5
src/assoc.c
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@@ -1,5 +1,5 @@
/* Associativity information.
Copyright (C) 2002, 2005, 2006 Free Software Foundation, Inc.
Copyright (C) 2002, 2005, 2006, 2008 Free Software Foundation, Inc.
This file is part of Bison, the GNU Compiler Compiler.
@@ -41,5 +41,8 @@ assoc_to_string (assoc a)
case non_assoc:
return "%nonassoc";
case precedence_assoc:
return "%precedence";
}
}

11
src/assoc.h
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@@ -1,5 +1,5 @@
/* Associativity information.
Copyright (C) 2002, 2006 Free Software Foundation, Inc.
Copyright (C) 2002, 2006, 2008 Free Software Foundation, Inc.
This file is part of Bison, the GNU Compiler Compiler.
@@ -22,10 +22,11 @@
/* Associativity values for tokens and rules. */
typedef enum
{
undef_assoc,
right_assoc,
left_assoc,
non_assoc
undef_assoc, /** Not defined. */
right_assoc, /** %right */
left_assoc, /** %left */
non_assoc, /** %nonassoc */
precedence_assoc /** %precedence */
} assoc;
char const *assoc_to_string (assoc a);

17
src/conflicts.c
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@@ -1,7 +1,7 @@
/* Find and resolve or report lookahead conflicts for bison,
Copyright (C) 1984, 1989, 1992, 2000, 2001, 2002, 2003, 2004, 2005, 2006,
2007 Free Software Foundation, Inc.
2007, 2008 Free Software Foundation, Inc.
This file is part of Bison, the GNU Compiler Compiler.
@@ -285,16 +285,21 @@ resolve_sr_conflict (state *s, int ruleno, symbol **errors, int *nerrs)
flush_reduce (lookahead_tokens, i);
}
else
/* Matching precedence levels.
For left association, keep only the reduction.
For right association, keep only the shift.
For nonassociation, keep neither. */
/* Matching precedence levels.
For non-defined associativity, keep both: unexpected
associativity conflict.
For left associativity, keep only the reduction.
For right associativity, keep only the shift.
For nonassociativity, keep neither. */
switch (symbols[i]->assoc)
{
default:
case undef_assoc:
abort ();
case precedence_assoc:
break;
case right_assoc:
log_resolution (redrule, i, right_resolution);
flush_reduce (lookahead_tokens, i);

8
src/parse-gram.y
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@@ -115,6 +115,7 @@ static int current_prec = 0;
%token PERCENT_LEFT "%left"
%token PERCENT_RIGHT "%right"
%token PERCENT_NONASSOC "%nonassoc"
%token PERCENT_PRECEDENCE "%precedence"
%token PERCENT_PREC "%prec"
%token PERCENT_DPREC "%dprec"
@@ -412,9 +413,10 @@ precedence_declaration:
;
precedence_declarator:
"%left" { $$ = left_assoc; }
| "%right" { $$ = right_assoc; }
| "%nonassoc" { $$ = non_assoc; }
"%left" { $$ = left_assoc; }
| "%right" { $$ = right_assoc; }
| "%nonassoc" { $$ = non_assoc; }
| "%precedence" { $$ = precedence_assoc; }
;
type.opt:

1
src/scan-gram.l
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@@ -183,6 +183,7 @@ splice (\\[ \f\t\v]*\n)*
"%output" return PERCENT_OUTPUT;
"%parse-param" return PERCENT_PARSE_PARAM;
"%prec" return PERCENT_PREC;
"%precedence" return PERCENT_PRECEDENCE;
"%printer" return PERCENT_PRINTER;
"%pure"[-_]"parser" return PERCENT_PURE_PARSER;
"%require" return PERCENT_REQUIRE;

6
tests/calc.at
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@@ -106,11 +106,11 @@ static void unget_char (]AT_LEX_PRE_FORMALS[ int c);
%token <ival> NUM "number"
%type <ival> exp
%nonassoc '=' /* comparison */
%nonassoc '=' /* comparison */
%left '-' '+'
%left '*' '/'
%left NEG /* negation--unary minus */
%right '^' /* exponentiation */
%precedence NEG /* negation--unary minus */
%right '^' /* exponentiation */
/* Grammar follows */
%%

60
tests/conflicts.at
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@@ -1,6 +1,6 @@
# Exercising Bison on conflicts. -*- Autotest -*-
# Copyright (C) 2002, 2003, 2004, 2005, 2007 Free Software Foundation, Inc.
# Copyright (C) 2002, 2003, 2004, 2005, 2007, 2008 Free Software Foundation, Inc.
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
@@ -326,6 +326,62 @@ state 5
AT_CLEANUP
## ---------------------- ##
## %precedence suffices. ##
## ---------------------- ##
AT_SETUP([%precedence suffices])
AT_DATA([input.y],
[[%precedence "then"
%precedence "else"
%%
stmt:
"if" cond "then" stmt
| "if" cond "then" stmt "else" stmt
| "stmt"
;
cond:
"exp"
;
]])
AT_BISON_CHECK([-o input.c input.y])
AT_CLEANUP
## ------------------------------ ##
## %precedence does not suffice. ##
## ------------------------------ ##
AT_SETUP([%precedence does not suffice])
AT_DATA([input.y],
[[%precedence "then"
%precedence "else"
%%
stmt:
"if" cond "then" stmt
| "if" cond "then" stmt "else" stmt
| "stmt"
;
cond:
"exp"
| cond "then" cond
;
]])
AT_BISON_CHECK([-o input.c input.y], 0, [],
[[input.y: conflicts: 1 shift/reduce
input.y:12.3-18: warning: rule useless in parser due to conflicts: cond: cond "then" cond
]])
AT_CLEANUP
## -------------------------------- ##
## Defaulted Conflicted Reduction. ##
## -------------------------------- ##
@@ -878,7 +934,7 @@ AT_CHECK([[cat input.output | sed -n '/^state 0$/,/^state 1$/p']], 0,
13 empty_c3: . ['c']
'b' shift, and go to state 1
'c' reduce using rule 13 (empty_c3)
$default reduce using rule 9 (empty_a)