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* doc/bison.texinfo: Fix some typos.

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(GLR Semantic Actions): New subsection discussing special
considerations because GLR semantic actions might be deferred.
(Actions): Mention look-ahead usage of yylval.
(Actions and Locations): Mention look-ahead usage of yylloc.
(Special Features for Use in Actions): Add YYEOF entry and mention it
in the yychar entry.
In the yychar entry, remove mention of the local yychar case (pure
parser) since this is irrelevant information when writing semantic
actions and since it's already discussed in `Bison Symbols' where
yychar is otherwise described as an external variable.
In the yychar entry, don't call it the `current' look-ahead since it
isn't when semantic actions are deferred.
In the yychar and yyclearin entries, add note about deferred semantic
actions.
Add yylloc and yylval entries discussing look-ahead usage.
(Look-Ahead Tokens): When discussing yychar, don't call it the
`current' look-ahead, and do mention yylval and yylloc.
(Error Recovery): Cross-reference `Action Features' when mentioning
yyclearin.
(Bison Symbols): In the yychar entry, don't call it the `current'
look-ahead.
In the yylloc and yylval entries, mention look-ahead usage.
This commit is contained in:
Joel E. Denny
2006-01-11 22:39:27 +00:00
parent de366a2f6b
commit 32c292920e
2 changed files with 143 additions and 19 deletions
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26
ChangeLog
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@@ -1,3 +1,29 @@
2006-01-11 Joel E. Denny <jdenny@ces.clemson.edu>
* doc/bison.texinfo: Fix some typos.
(GLR Semantic Actions): New subsection discussing special
considerations because GLR semantic actions might be deferred.
(Actions): Mention look-ahead usage of yylval.
(Actions and Locations): Mention look-ahead usage of yylloc.
(Special Features for Use in Actions): Add YYEOF entry and mention it
in the yychar entry.
In the yychar entry, remove mention of the local yychar case (pure
parser) since this is irrelevant information when writing semantic
actions and since it's already discussed in `Bison Symbols' where
yychar is otherwise described as an external variable.
In the yychar entry, don't call it the `current' look-ahead since it
isn't when semantic actions are deferred.
In the yychar and yyclearin entries, add note about deferred semantic
actions.
Add yylloc and yylval entries discussing look-ahead usage.
(Look-Ahead Tokens): When discussing yychar, don't call it the
`current' look-ahead, and do mention yylval and yylloc.
(Error Recovery): Cross-reference `Action Features' when mentioning
yyclearin.
(Bison Symbols): In the yychar entry, don't call it the `current'
look-ahead.
In the yylloc and yylval entries, mention look-ahead usage.
2006-01-08 Joel E. Denny <jdenny@ces.clemson.edu>
* tests/glr-regression.at: Update copyright year to 2006.

136
doc/bison.texinfo
View File

@@ -44,7 +44,7 @@ This manual is for @acronym{GNU} Bison (version @value{VERSION},
@value{UPDATED}), the @acronym{GNU} parser generator.
Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998,
1999, 2000, 2001, 2002, 2003, 2004, 2005 Free Software Foundation, Inc.
1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
@quotation
Permission is granted to copy, distribute and/or modify this document
@@ -145,9 +145,10 @@ The Concepts of Bison
Writing @acronym{GLR} Parsers
* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars
* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities
* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler
* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars.
* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities.
* GLR Semantic Actions:: Deferred semantic actions have special concerns.
* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler.
Examples
@@ -733,9 +734,10 @@ user-defined function on the resulting values to produce an arbitrary
merged result.
@menu
* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars
* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities
* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler
* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars.
* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities.
* GLR Semantic Actions:: Deferred semantic actions have special concerns.
* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler.
@end menu
@node Simple GLR Parsers
@@ -1094,6 +1096,52 @@ productions that participate in any particular merge have identical
and the parser will report an error during any parse that results in
the offending merge.
@node GLR Semantic Actions
@subsection GLR Semantic Actions
@cindex deferred semantic actions
By definition, a deferred semantic action is not performed at the same time as
the associated reduction.
This raises caveats for several Bison features you might use in a semantic
action in a @acronym{GLR} parser.
@vindex yychar
@cindex @acronym{GLR} parsers and @code{yychar}
@vindex yylval
@cindex @acronym{GLR} parsers and @code{yylval}
@vindex yylloc
@cindex @acronym{GLR} parsers and @code{yylloc}
In any semantic action, you can examine @code{yychar} to determine the type of
the look-ahead token present at the time of the associated reduction.
After checking that @code{yychar} is not set to @code{YYEMPTY} or @code{YYEOF},
you can then examine @code{yylval} and @code{yylloc} to determine the
look-ahead token's semantic value and location, if any.
In a nondeferred semantic action, you can also modify any of these variables to
influence syntax analysis.
@xref{Look-Ahead, ,Look-Ahead Tokens}.
@findex yyclearin
@cindex @acronym{GLR} parsers and @code{yyclearin}
In a deferred semantic action, it's too late to influence syntax analysis.
In this case, @code{yychar}, @code{yylval}, and @code{yylloc} are set to
shallow copies of the values they had at the time of the associated reduction.
For this reason alone, modifying them is dangerous.
Moreover, the result of modifying them is undefined and subject to change with
future versions of Bison.
For example, if a semantic action might be deferred, you should never write it
to invoke @code{yyclearin} (@pxref{Action Features}) or to attempt to free
memory referenced by @code{yylval}.
@findex YYERROR
@cindex @acronym{GLR} parsers and @code{YYERROR}
Another Bison feature requiring special consideration is @code{YYERROR}
(@pxref{Action Features}), which you can invoke in any semantic action to
initiate error recovery.
During deterministic @acronym{GLR} operation, the effect of @code{YYERROR} is
the same as its effect in an @acronym{LALR}(1) parser.
In a deferred semantic action, its effect is undefined.
@c The effect is probably a syntax error at the split point.
@node Compiler Requirements
@subsection Considerations when Compiling @acronym{GLR} Parsers
@cindex @code{inline}
@@ -3140,6 +3188,12 @@ As long as @code{bar} is used only in the fashion shown here, @code{$0}
always refers to the @code{expr} which precedes @code{bar} in the
definition of @code{foo}.
@vindex yylval
It is also possible to access the semantic value of the look-ahead token, if
any, from a semantic action.
This semantic value is stored in @code{yylval}.
@xref{Action Features, ,Special Features for Use in Actions}.
@node Action Types
@subsection Data Types of Values in Actions
@cindex action data types
@@ -3457,6 +3511,12 @@ exp: @dots{}
@end group
@end example
@vindex yylloc
It is also possible to access the location of the look-ahead token, if any,
from a semantic action.
This location is stored in @code{yylloc}.
@xref{Action Features, ,Special Features for Use in Actions}.
@node Location Default Action
@subsection Default Action for Locations
@vindex YYLLOC_DEFAULT
@@ -4763,6 +4823,12 @@ In either case, the rest of the action is not executed.
Value stored in @code{yychar} when there is no look-ahead token.
@end deffn
@deffn {Macro} YYEOF
@vindex YYEOF
Value stored in @code{yychar} when the look-ahead is the end of the input
stream.
@end deffn
@deffn {Macro} YYERROR;
@findex YYERROR
Cause an immediate syntax error. This statement initiates error
@@ -4779,15 +4845,20 @@ is recovering from a syntax error, and 0 the rest of the time.
@end deffn
@deffn {Variable} yychar
Variable containing the current look-ahead token. (In a pure parser,
this is actually a local variable within @code{yyparse}.) When there is
no look-ahead token, the value @code{YYEMPTY} is stored in the variable.
Variable containing either the look-ahead token, or @code{YYEOF} when the
look-ahead is the end of the input stream, or @code{YYEMPTY} when no look-ahead
has been performed so the next token is not yet known.
Do not modify @code{yychar} in a deferred semantic action (@pxref{GLR Semantic
Actions}).
@xref{Look-Ahead, ,Look-Ahead Tokens}.
@end deffn
@deffn {Macro} yyclearin;
Discard the current look-ahead token. This is useful primarily in
error rules. @xref{Error Recovery}.
error rules.
Do not invoke @code{yyclearin} in a deferred semantic action (@pxref{GLR
Semantic Actions}).
@xref{Error Recovery}.
@end deffn
@deffn {Macro} yyerrok;
@@ -4796,6 +4867,22 @@ errors. This is useful primarily in error rules.
@xref{Error Recovery}.
@end deffn
@deffn {Variable} yylloc
Variable containing the look-ahead token location when @code{yychar} is not set
to @code{YYEMPTY} or @code{YYEOF}.
Do not modify @code{yylloc} in a deferred semantic action (@pxref{GLR Semantic
Actions}).
@xref{Actions and Locations, ,Actions and Locations}.
@end deffn
@deffn {Variable} yylval
Variable containing the look-ahead token semantic value when @code{yychar} is
not set to @code{YYEMPTY} or @code{YYEOF}.
Do not modify @code{yylval} in a deferred semantic action (@pxref{GLR Semantic
Actions}).
@xref{Actions, ,Actions}.
@end deffn
@deffn {Value} @@$
@findex @@$
Acts like a structure variable containing information on the textual location
@@ -5033,7 +5120,11 @@ doing so would produce on the stack the sequence of symbols @code{expr
'!'}. No rule allows that sequence.
@vindex yychar
The current look-ahead token is stored in the variable @code{yychar}.
@vindex yylval
@vindex yylloc
The look-ahead token is stored in the variable @code{yychar}.
Its semantic value and location, if any, are stored in the variables
@code{yylval} and @code{yylloc}.
@xref{Action Features, ,Special Features for Use in Actions}.
@node Shift/Reduce
@@ -5850,6 +5941,7 @@ The previous look-ahead token is reanalyzed immediately after an error. If
this is unacceptable, then the macro @code{yyclearin} may be used to clear
this token. Write the statement @samp{yyclearin;} in the error rule's
action.
@xref{Action Features, ,Special Features for Use in Actions}.
For example, suppose that on a syntax error, an error handling routine is
called that advances the input stream to some point where parsing should
@@ -8039,7 +8131,7 @@ token. @xref{Action Features, ,Special Features for Use in Actions}.
@end deffn
@deffn {Variable} yychar
External integer variable that contains the integer value of the current
External integer variable that contains the integer value of the
look-ahead token. (In a pure parser, it is a local variable within
@code{yyparse}.) Error-recovery rule actions may examine this variable.
@xref{Action Features, ,Special Features for Use in Actions}.
@@ -8100,7 +8192,7 @@ the next token. @xref{Lexical, ,The Lexical Analyzer Function
@deffn {Macro} YYLEX_PARAM
An obsolete macro for specifying an extra argument (or list of extra
arguments) for @code{yyparse} to pass to @code{yylex}. he use of this
arguments) for @code{yyparse} to pass to @code{yylex}. The use of this
macro is deprecated, and is supported only for Yacc like parsers.
@xref{Pure Calling,, Calling Conventions for Pure Parsers}.
@end deffn
@@ -8109,9 +8201,12 @@ macro is deprecated, and is supported only for Yacc like parsers.
External variable in which @code{yylex} should place the line and column
numbers associated with a token. (In a pure parser, it is a local
variable within @code{yyparse}, and its address is passed to
@code{yylex}.) You can ignore this variable if you don't use the
@samp{@@} feature in the grammar actions. @xref{Token Locations,
,Textual Locations of Tokens}.
@code{yylex}.)
You can ignore this variable if you don't use the @samp{@@} feature in the
grammar actions.
@xref{Token Locations, ,Textual Locations of Tokens}.
In semantic actions, it stores the location of the look-ahead token.
@xref{Actions and Locations, ,Actions and Locations}.
@end deffn
@deffn {Type} YYLTYPE
@@ -8123,7 +8218,10 @@ members. @xref{Location Type, , Data Types of Locations}.
External variable in which @code{yylex} should place the semantic
value associated with a token. (In a pure parser, it is a local
variable within @code{yyparse}, and its address is passed to
@code{yylex}.) @xref{Token Values, ,Semantic Values of Tokens}.
@code{yylex}.)
@xref{Token Values, ,Semantic Values of Tokens}.
In semantic actions, it stores the semantic value of the look-ahead token.
@xref{Actions, ,Actions}.
@end deffn
@deffn {Macro} YYMAXDEPTH
@@ -8384,7 +8482,7 @@ grammatically indivisible. The piece of text it represents is a token.
@c LocalWords: yychar yydebug msg YYNTOKENS YYNNTS YYNRULES YYNSTATES
@c LocalWords: cparse clex deftypefun NE defmac YYACCEPT YYABORT param
@c LocalWords: strncmp intval tindex lvalp locp llocp typealt YYBACKUP
@c LocalWords: YYEMPTY YYRECOVERING yyclearin GE def UMINUS maybeword
@c LocalWords: YYEMPTY YYEOF YYRECOVERING yyclearin GE def UMINUS maybeword
@c LocalWords: Johnstone Shamsa Sadaf Hussain Tomita TR uref YYMAXDEPTH
@c LocalWords: YYINITDEPTH stmnts ref stmnt initdcl maybeasm VCG notype
@c LocalWords: hexflag STR exdent itemset asis DYYDEBUG YYFPRINTF args