Bison News
----------
-Changes in version 2.3+:
+Changes in version 2.3a+ (????-??-??):
+
+* The quotes around NAME that used to be required in the following directive
+ are now deprecated:
+
+ %define NAME "VALUE"
+
+* The directive `%pure-parser' is now deprecated in favor of:
+
+ %define api.pure
+
+ which has the same effect except that Bison is more careful to warn about
+ unreasonable usage in the latter case.
+
+* Push Parsing
+
+ Bison can now generate an LALR(1) parser in C with a push interface. That
+ is, instead of invoking yyparse, which pulls tokens from `yylex', you can
+ push one token at a time to the parser using `yypush_parse', which will
+ return to the caller after processing each token. By default, the push
+ interface is disabled. Either of the following directives will enable it:
+
+ %define api.push_pull "push" // Just push; does not require yylex.
+ %define api.push_pull "both" // Push and pull; requires yylex.
+
+ See the new section `A Push Parser' in the Bison manual for details.
+
+* The -g and --graph options now output graphs in Graphviz DOT format,
+ not VCG format.
+
+* An experimental directive %language specifies the language of the
+ generated parser, which can be C (the default) or C++. This
+ directive affects the skeleton used, and the names of the generated
+ files if the grammar file's name ends in ".y".
+
+* The grammar file may now specify the name of the parser header file using
+ %defines. For example:
+
+ %defines "parser.h"
+
+* Unreachable State Removal
+
+ Previously, Bison sometimes generated parser tables containing unreachable
+ states. A state can become unreachable during conflict resolution if Bison
+ disables a shift action leading to it from a predecessor state. Bison now:
+
+ 1. Removes unreachable states.
+
+ 2. Does not report any conflicts that appeared in unreachable states.
+ WARNING: As a result, you may need to update %expect and %expect-rr
+ directives in existing grammar files.
+
+ 3. For any rule used only in such states, Bison now reports the rule as
+ "never reduced because of conflicts".
+
+ This feature can be disabled with the following directive:
+
+ %define lr.keep_unreachable_states
+
+ See the %define entry in the `Bison Declaration Summary' in the Bison manual
+ for further discussion.
+
+* Lookahead Set Correction in the `.output' Report
+
+ When instructed to generate a `.output' file including lookahead sets
+ (using `--report=lookahead', for example), Bison now prints each reduction's
+ lookahead set only next to the associated state's one item that (1) is
+ associated with the same rule as the reduction and (2) has its dot at the end
+ of its RHS. Previously, Bison also erroneously printed the lookahead set
+ next to all of the state's other items associated with the same rule. This
+ bug affected only the `.output' file and not the generated parser source
+ code.
+
+* The `=' that used to be required in the following directives is now
+ deprecated:
+
+ %file-prefix "parser"
+ %name-prefix "c_"
+ %output "parser.c"
+
+* An Alternative to `%{...%}' -- `%code QUALIFIER {CODE}'
+
+ Bison 2.3a provided a new set of directives as a more flexible alternative to
+ the traditional Yacc prologue blocks. Those have now been consolidated into
+ a single %code directive with an optional qualifier field, which identifies
+ the purpose of the code and thus the location(s) where Bison should generate
+ it:
+
+ 1. `%code {CODE}' replaces `%after-header {CODE}'
+ 2. `%code requires {CODE}' replaces `%start-header {CODE}'
+ 3. `%code provides {CODE}' replaces `%end-header {CODE}'
+ 4. `%code top {CODE}' replaces `%before-header {CODE}'
+
+ See the %code entries in section `Bison Declaration Summary' in the Bison
+ manual for a summary of the new functionality. See the new section `Prologue
+ Alternatives' for a detailed discussion including the advantages of %code
+ over the traditional Yacc prologues.
+
+ The prologue alternatives are experimental. More user feedback will help to
+ determine whether they should become permanent features.
+
+* Revised warning: unset or unused mid-rule values
+
+ Since Bison 2.2, Bison has warned about mid-rule values that are set but not
+ used within any of the actions of the parent rule. For example, Bison warns
+ about unused $2 in:
+
+ exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
+
+ Now, Bison also warns about mid-rule values that are used but not set. For
+ example, Bison warns about unset $$ in the mid-rule action in:
+
+ exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
+
+ However, Bison now disables both of these warnings by default since they
+ sometimes prove to be false alarms in existing grammars employing the Yacc
+ constructs $0 or $-N (where N is some positive integer).
+
+ To enable these warnings, specify the flag `--warnings=midrule-values' or
+ `-W', which is a synonym for `--warnings=all'.
+
+* Default %destructor or %printer with `<*>' or `<>'
+
+ Bison now recognizes two separate kinds of default %destructor's and
+ %printer's:
+
+ 1. Place `<*>' in a %destructor/%printer symbol list to define a default
+ %destructor/%printer for all grammar symbols for which you have formally
+ declared semantic type tags.
+
+ 2. Place `<>' in a %destructor/%printer symbol list to define a default
+ %destructor/%printer for all grammar symbols without declared semantic
+ type tags.
+
+ Bison no longer supports the `%symbol-default' notation from Bison 2.3a.
+ `<*>' and `<>' combined achieve the same effect with one exception: Bison no
+ longer applies any %destructor to a mid-rule value if that mid-rule value is
+ not actually ever referenced using either $$ or $n in a semantic action.
+
+ The default %destructor's and %printer's are experimental. More user
+ feedback will help to determine whether they should become permanent
+ features.
+
+ See the section `Freeing Discarded Symbols' in the Bison manual for further
+ details.
+
+Changes in version 2.3a, 2006-09-13:
+
+* Instead of %union, you can define and use your own union type
+ YYSTYPE if your grammar contains at least one <type> tag.
+ Your YYSTYPE need not be a macro; it can be a typedef.
+ This change is for compatibility with other Yacc implementations,
+ and is required by POSIX.
+
+* Locations columns and lines start at 1.
+ In accordance with the GNU Coding Standards and Emacs.
+
+* You may now declare per-type and default %destructor's and %printer's:
+
+ For example:
+
+ %union { char *string; }
+ %token <string> STRING1
+ %token <string> STRING2
+ %type <string> string1
+ %type <string> string2
+ %union { char character; }
+ %token <character> CHR
+ %type <character> chr
+ %destructor { free ($$); } %symbol-default
+ %destructor { free ($$); printf ("%d", @$.first_line); } STRING1 string1
+ %destructor { } <character>
+
+ guarantees that, when the parser discards any user-defined symbol that has a
+ semantic type tag other than `<character>', it passes its semantic value to
+ `free'. However, when the parser discards a `STRING1' or a `string1', it
+ also prints its line number to `stdout'. It performs only the second
+ `%destructor' in this case, so it invokes `free' only once.
+
+ [Although we failed to mention this here in the 2.3a release, the default
+ %destructor's and %printer's were experimental, and they were rewritten in
+ future versions.]
* Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with `-y',
`--yacc', or `%yacc'), Bison no longer generates #define statements for
helps to sanitize the global namespace during preprocessing, but POSIX Yacc
requires them. Bison still generates an enum for token names in all cases.
-* Handling of prologue blocks is now more consistent but potentially backward
- incompatible.
+* Handling of traditional Yacc prologue blocks is now more consistent but
+ potentially incompatible with previous releases of Bison.
As before, you declare prologue blocks in your grammar file with the
`%{ ... %}' syntax. To generate the pre-prologue, Bison concatenates all
- prologue blocks that you've declared before any %union. If you've declared a
- %union, Bison concatenates all prologue blocks that you've declared after it
- to generate the post-prologue. (The new %before-definitions and
- %after-definitions have a similar effect as %union on the prologues. See
- below.)
+ prologue blocks that you've declared before the first %union. To generate
+ the post-prologue, Bison concatenates all prologue blocks that you've
+ declared after the first %union.
- Previous versions of Bison inserted the pre-prologue into both the header
+ Previous releases of Bison inserted the pre-prologue into both the header
file and the code file in all cases except for LALR(1) parsers in C. In the
latter case, Bison inserted it only into the code file. For parsers in C++,
the point of insertion was before any token definitions (which associate
Now, Bison never inserts the pre-prologue into the header file. In the code
file, it always inserts it before the token definitions.
-* Bison now provides the %before-definitions and %after-definitions directives.
-
- For example, in your grammar file:
-
- %{
- /* A pre-prologue block. For Yacc portability, Bison no longer puts this
- * in the header file. In the code file, Bison inserts it before any
- * %before-definitions blocks. */
- %}
- %before-definitions {
- /* Bison inserts this into both the header file and code file. In both
- * files, the point of insertion is before any Bison-generated token,
- * semantic type, location type, and class definitions. This is a good
- * place to define %union dependencies, for example. */
+* Bison now provides a more flexible alternative to the traditional Yacc
+ prologue blocks: %before-header, %start-header, %end-header, and
+ %after-header.
+
+ For example, the following declaration order in the grammar file reflects the
+ order in which Bison will output these code blocks. However, you are free to
+ declare these code blocks in your grammar file in whatever order is most
+ convenient for you:
+
+ %before-header {
+ /* Bison treats this block like a pre-prologue block: it inserts it into
+ * the code file before the contents of the header file. It does *not*
+ * insert it into the header file. This is a good place to put
+ * #include's that you want at the top of your code file. A common
+ * example is `#include "system.h"'. */
+ }
+ %start-header {
+ /* Bison inserts this block into both the header file and the code file.
+ * In both files, the point of insertion is before any Bison-generated
+ * token, semantic type, location type, and class definitions. This is a
+ * good place to define %union dependencies, for example. */
}
%union {
- /* With previous versions of Bison, the first %union in your grammar file
- * separated the pre-prologue blocks from the post-prologue blocks. Now,
- * the first %union, %before-definitions, or %after-definitions does
- * that. */
+ /* Unlike the traditional Yacc prologue blocks, the output order for the
+ * new %*-header blocks is not affected by their declaration position
+ * relative to any %union in the grammar file. */
}
- %after-definitions {
- /* If you want something in the header file and in the code file and it
- * depends on any of the Bison-generated definitions in the header file,
- * put it here. */
+ %end-header {
+ /* Bison inserts this block into both the header file and the code file.
+ * In both files, the point of insertion is after the Bison-generated
+ * definitions. This is a good place to declare or define public
+ * functions or data structures that depend on the Bison-generated
+ * definitions. */
}
- %{
- /* A post-prologue block. If you want something in the code file but not
- * in the header file and it depends on Bison-generated definitions, put
- * it here. In the code file, Bison inserts it after any
- * %after-definitions blocks. */
- %}
+ %after-header {
+ /* Bison treats this block like a post-prologue block: it inserts it into
+ * the code file after the contents of the header file. It does *not*
+ * insert it into the header file. This is a good place to declare or
+ * define internal functions or data structures that depend on the
+ * Bison-generated definitions. */
+ }
+
+ If you have multiple occurrences of any one of the above declarations, Bison
+ will concatenate the contents in declaration order.
+
+ [Although we failed to mention this here in the 2.3a release, the prologue
+ alternatives were experimental, and they were rewritten in future versions.]
* The option `--report=look-ahead' has been changed to `--report=lookahead'.
The old spelling still works, but is not documented and may be removed
-----
Copyright (C) 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003,
-2004, 2005, 2006 Free Software Foundation, Inc.
+2004, 2005, 2006, 2007 Free Software Foundation, Inc.
This file is part of Bison, the GNU Compiler Compiler.
-Bison is free software; you can redistribute it and/or modify
+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
-the Free Software Foundation; either version 2, or (at your option)
-any later version.
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
-Bison is distributed in the hope that it will be useful,
+This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
-along with autoconf; see the file COPYING. If not, write to
-the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
-Boston, MA 02110-1301, USA.
+along with this program. If not, see <http://www.gnu.org/licenses/>.
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