use of the %error-verbose directive is deprecated in favor of
%define parse.error "verbose".
+** Semantic predicates
+
+ The new, experimental, semantic-predicate feature allows actions of
+ the form %?{ BOOLEAN-EXPRESSION }, which cause syntax errors (as for
+ YYERROR) if the expression evaluates to 0, and are evaluated immediately
+ in GLR parsers, rather than being deferred. The result is that they
+ allow the programmer to prune possible parses based on the values of
+ runtime expressions.
+
* Changes in version 2.5 (????-??-??):
-** Named References Support
+** Grammar symbol names can now contain non-initial dashes:
+
+ Consistently with directives (such as %error-verbose) and with
+ %define variables (e.g. push-pull), grammar symbol names may contain
+ dashes in any position except the beginning. This is a GNU
+ extension over POSIX Yacc. Thus, use of this extension is reported
+ by -Wyacc and rejected in Yacc mode (--yacc).
+
+** Named references:
Historically, Yacc and Bison have supported positional references
($n, $$) to allow access to symbol values from inside of semantic
These features are experimental in this version. More user feedback
will help to stabilize them.
-** IELR(1) and Canonical LR(1) Support
+** IELR(1) and canonical LR(1):
IELR(1) is a minimal LR(1) parser table generation algorithm. That
is, given any context-free grammar, IELR(1) generates parser tables
with the full language recognition power of canonical LR(1) but with
- nearly the same number of parser states as LALR(1). This reduction in
- parser states is often an order of magnitude. More importantly,
+ nearly the same number of parser states as LALR(1). This reduction
+ in parser states is often an order of magnitude. More importantly,
because canonical LR(1)'s extra parser states may contain duplicate
conflicts in the case of non-LR(1) grammars, the number of conflicts
for IELR(1) is often an order of magnitude less as well. This can
%define lr.type ielr
%define lr.type canonical-lr
- The default reduction optimization in the parser tables can also be
- adjusted using `%define lr.default-reductions'. See the documentation
- for `%define lr.type' and `%define lr.default-reductions' in the
- section `Bison Declaration Summary' in the Bison manual for the
- details.
+ The default-reduction optimization in the parser tables can also be
+ adjusted using `%define lr.default-reductions'. For details on both
+ of these features, see the new section `Tuning LR' in the Bison
+ manual.
These features are experimental. More user feedback will help to
stabilize them.
-** Unrecognized %code qualifiers are now an error not a warning.
+** LAC (Lookahead Correction) for syntax error handling:
-** %define improvements.
+ Canonical LR, IELR, and LALR can suffer from a couple of problems
+ upon encountering a syntax error. First, the parser might perform
+ additional parser stack reductions before discovering the syntax
+ error. Such reductions can perform user semantic actions that are
+ unexpected because they are based on an invalid token, and they
+ cause error recovery to begin in a different syntactic context than
+ the one in which the invalid token was encountered. Second, when
+ verbose error messages are enabled (with %error-verbose or the
+ obsolete `#define YYERROR_VERBOSE'), the expected token list in the
+ syntax error message can both contain invalid tokens and omit valid
+ tokens.
-*** Unrecognized variables are now an error not a warning.
+ The culprits for the above problems are %nonassoc, default
+ reductions in inconsistent states, and parser state merging. Thus,
+ IELR and LALR suffer the most. Canonical LR can suffer only if
+ %nonassoc is used or if default reductions are enabled for
+ inconsistent states.
-*** Multiple invocations for any variable is now an error not a warning.
+ LAC is a new mechanism within the parsing algorithm that solves
+ these problems for canonical LR, IELR, and LALR without sacrificing
+ %nonassoc, default reductions, or state merging. When LAC is in
+ use, canonical LR and IELR behave almost exactly the same for both
+ syntactically acceptable and syntactically unacceptable input.
+ While LALR still does not support the full language-recognition
+ power of canonical LR and IELR, LAC at least enables LALR's syntax
+ error handling to correctly reflect LALR's language-recognition
+ power.
-*** Can now be invoked via the command line.
+ Currently, LAC is only supported for deterministic parsers in C.
+ You can enable LAC with the following directive:
+
+ %define parse.lac full
+
+ See the new section `LAC' in the Bison manual for additional
+ details including a few caveats.
+
+ LAC is an experimental feature. More user feedback will help to
+ stabilize it.
+
+** %define improvements:
+
+*** Can now be invoked via the command line:
Each of these command-line options
quietly override %define, but -D and --define do not. For further
details, see the section "Bison Options" in the Bison manual.
-*** Variables renamed.
+*** Variables renamed:
The following %define variables
The old names are now deprecated but will be maintained indefinitely
for backward compatibility.
-*** Values no longer need to be quoted in grammar file.
+*** Values no longer need to be quoted in grammar file:
If a %define value is an identifier, it no longer needs to be placed
within quotations marks. For example,
%define api.push-pull push
-** Symbol names.
-
- Consistently with directives (such as %error-verbose) and variables
- (e.g. push-pull), symbol names may include dashes in any position,
- similarly to periods and underscores. This is GNU extension over
- POSIX Yacc whose use is reported by -Wyacc, and rejected in Yacc
- mode (--yacc).
-
-** YYFAIL now produces warnings and Java parsers no longer implement it.
-
- YYFAIL has existed for many years as an undocumented feature of
- deterministic parsers in C generated by Bison. More recently, it was
- a documented feature of Bison's experimental Java parsers. As
- promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
- semantic action now produces a deprecation warning, and Java parsers
- no longer implement YYFAIL at all. For further details, including a
- discussion of how to suppress C preprocessor warnings about YYFAIL
- being unused, see the Bison 2.4.2 NEWS entry.
-
-** Temporary hack for adding a semicolon to the user action.
-
- Previously, Bison appended a semicolon to every user action for
- reductions when the output language defaulted to C (specifically, when
- neither %yacc, %language, %skeleton, or equivalent command-line
- options were specified). This allowed actions such as
-
- exp: exp "+" exp { $$ = $1 + $3 };
-
- instead of
+*** Unrecognized variables are now errors not warnings.
- exp: exp "+" exp { $$ = $1 + $3; };
+*** Multiple invocations for any variable is now an error not a warning.
- As a first step in removing this misfeature, Bison now issues a
- warning when it appends a semicolon. Moreover, in cases where Bison
- cannot easily determine whether a semicolon is needed (for example, an
- action ending with a cpp directive or a braced compound initializer),
- it no longer appends one. Thus, the C compiler might now complain
- about a missing semicolon where it did not before. Future releases of
- Bison will cease to append semicolons entirely.
+** Unrecognized %code qualifiers are now errors not warnings.
-** Character literals not of length one.
+** Character literals not of length one:
Previously, Bison quietly converted all character literals to length
one. For example, without warning, Bison interpreted the operators in
;
Bison now warns when a character literal is not of length one. In
- some future release, Bison will report an error instead.
+ some future release, Bison will start reporting an error instead.
-** Verbose error messages fixed for nonassociative tokens.
-
- When %error-verbose is specified, syntax error messages produced by
- the generated parser include the unexpected token as well as a list of
- expected tokens. Previously, this list erroneously included tokens
- that would actually induce a syntax error because conflicts for them
- were resolved with %nonassoc. Such tokens are now properly omitted
- from the list.
-
-** Destructor calls fixed for lookaheads altered in semantic actions.
+** Destructor calls fixed for lookaheads altered in semantic actions:
Previously for deterministic parsers in C, if a user semantic action
altered yychar, the parser in some cases used the old yychar value to
determine which destructor to call for the lookahead upon a syntax
error or upon parser return. This bug has been fixed.
-** C++ parsers use YYRHSLOC
+** C++ parsers use YYRHSLOC:
Similarly to the C parsers, the C++ parsers now define the YYRHSLOC
macro and use it in the default YYLLOC_DEFAULT. You are encouraged
} \
while (false)
-** YYLLOC_DEFAULT in C++
+** YYLLOC_DEFAULT in C++:
The default implementation of YYLLOC_DEFAULT used to be issued in
the header file. It is now output in the implementation file, after
the user %code sections so that its #ifndef guard does not try to
override the user's YYLLOC_DEFAULT if provided.
-* Changes in version 2.4.3 (????-??-??):
+** YYFAIL now produces warnings and Java parsers no longer implement it:
+
+ YYFAIL has existed for many years as an undocumented feature of
+ deterministic parsers in C generated by Bison. More recently, it was
+ a documented feature of Bison's experimental Java parsers. As
+ promised in Bison 2.4.2's NEWS entry, any appearance of YYFAIL in a
+ semantic action now produces a deprecation warning, and Java parsers
+ no longer implement YYFAIL at all. For further details, including a
+ discussion of how to suppress C preprocessor warnings about YYFAIL
+ being unused, see the Bison 2.4.2 NEWS entry.
+
+** Temporary hack for adding a semicolon to the user action:
+
+ Previously, Bison appended a semicolon to every user action for
+ reductions when the output language defaulted to C (specifically, when
+ neither %yacc, %language, %skeleton, or equivalent command-line
+ options were specified). This allowed actions such as
+
+ exp: exp "+" exp { $$ = $1 + $3 };
+
+ instead of
+
+ exp: exp "+" exp { $$ = $1 + $3; };
+
+ As a first step in removing this misfeature, Bison now issues a
+ warning when it appends a semicolon. Moreover, in cases where Bison
+ cannot easily determine whether a semicolon is needed (for example, an
+ action ending with a cpp directive or a braced compound initializer),
+ it no longer appends one. Thus, the C compiler might now complain
+ about a missing semicolon where it did not before. Future releases of
+ Bison will cease to append semicolons entirely.
+
+** Verbose syntax error message fixes:
+
+ When %error-verbose or the obsolete `#define YYERROR_VERBOSE' is
+ specified, syntax error messages produced by the generated parser
+ include the unexpected token as well as a list of expected tokens.
+ The effect of %nonassoc on these verbose messages has been corrected
+ in two ways, but a more complete fix requires LAC, described above:
+
+*** When %nonassoc is used, there can exist parser states that accept no
+ tokens, and so the parser does not always require a lookahead token
+ in order to detect a syntax error. Because no unexpected token or
+ expected tokens can then be reported, the verbose syntax error
+ message described above is suppressed, and the parser instead
+ reports the simpler message, "syntax error". Previously, this
+ suppression was sometimes erroneously triggered by %nonassoc when a
+ lookahead was actually required. Now verbose messages are
+ suppressed only when all previous lookaheads have already been
+ shifted or discarded.
+
+*** Previously, the list of expected tokens erroneously included tokens
+ that would actually induce a syntax error because conflicts for them
+ were resolved with %nonassoc in the current parser state. Such
+ tokens are now properly omitted from the list.
+
+*** Expected token lists are still often wrong due to state merging
+ (from LALR or IELR) and default reductions, which can both add
+ invalid tokens and subtract valid tokens. Canonical LR almost
+ completely fixes this problem by eliminating state merging and
+ default reductions. However, there is one minor problem left even
+ when using canonical LR and even after the fixes above. That is,
+ if the resolution of a conflict with %nonassoc appears in a later
+ parser state than the one at which some syntax error is
+ discovered, the conflicted token is still erroneously included in
+ the expected token list. Bison's new LAC implementation,
+ described above, eliminates this problem and the need for
+ canonical LR. However, LAC is still experimental and is disabled
+ by default.
+
+** Java skeleton fixes:
+
+*** A location handling bug has been fixed.
+
+*** The top element of each of the value stack and location stack is now
+ cleared when popped so that it can be garbage collected.
+
+*** Parser traces now print the top element of the stack.
+
+** -W/--warnings fixes:
+
+*** Bison now properly recognizes the "no-" versions of categories:
+
+ For example, given the following command line, Bison now enables all
+ warnings except warnings for incompatibilities with POSIX Yacc:
+
+ bison -Wall,no-yacc gram.y
+
+*** The "none" category no longer disables a preceding "error":
+
+ For example, for the following command line, Bison now reports
+ errors instead of warnings for incompatibilities with POSIX Yacc:
+
+ bison -Werror,none,yacc gram.y
+
+*** The "none" category now disables all Bison warnings.
+
+ Previously, the "none" category disabled only Bison warnings for
+ which there existed a specific -W/--warning category. However,
+ given the following command line, Bison is now guaranteed to
+ suppress all warnings:
+
+ bison -Wnone gram.y
+
+* Changes in version 2.4.3 (2010-08-05):
+
+** Bison now obeys -Werror and --warnings=error for warnings about
+ grammar rules that are useless in the parser due to conflicts.
** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
been fixed.
errors in Bison 2.5. They will remain warnings, which should be
sufficient for POSIX while avoiding backward compatibility issues.
+** Minor documentation fixes.
+
* Changes in version 2.4.2 (2010-03-20):
** Some portability problems that resulted in failures and livelocks
-----
-Copyright (C) 1995-2010 Free Software Foundation, Inc.
+Copyright (C) 1995-2011 Free Software Foundation, Inc.
This file is part of Bison, the GNU Parser Generator.
projects / bison.git / blobdiff