+Changes in version 2.3a+ (????-??-??):
+
+* The -g and --graph options now output graphs in Graphviz DOT format,
+ not VCG format.
+
+* The grammar file may now specify the name of the parser header file using
+ %defines. For example:
+
+ %defines "parser.h"
+
+* The `=' that used to be required in the following declarations is now
+ deprecated:
+
+ %file-prefix "parser"
+ %name-prefix "c_"
+ %output "parser.c"
+
+* 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'.
+
+* 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.
+
+* The Yacc prologue alternatives from Bison 2.3a have been rewritten as the
+ following directives:
+
+ 1. %code {CODE}
+
+ Other than semantic actions, this is probably the most common place you
+ should write verbatim code for the parser implementation. It replaces
+ the traditional Yacc prologue, `%{CODE%}', for most purposes. Compare
+ with:
+
+ - `%{CODE%}' appearing after the first `%union {CODE}' in a grammar
+ file. While Bison will continue to support `%{CODE%}' for backward
+ compatibility, `%code {CODE}' is cleaner as its functionality does
+ not depend on its position in the grammar file relative to any
+ `%union {CODE}'. Specifically, `%code {CODE}' always inserts your
+ CODE into the parser code file after the usual contents of the
+ parser header file.
+ - `%after-header {CODE}', which only Bison 2.3a supported.
+
+ 2. %requires {CODE}
+
+ This is the right place to write dependency code for externally exposed
+ definitions required by Bison. Such exposed definitions are those
+ usually appearing in the parser header file. Thus, this is the right
+ place to define types referenced in `%union {CODE}' directives, and it
+ is the right place to override Bison's default YYSTYPE and YYLTYPE
+ definitions. Compare with:
+
+ - `%{CODE%}' appearing before the first `%union {CODE}' in a grammar
+ file. Unlike `%{CODE%}', `%requires {CODE}' inserts your CODE both
+ into the parser code file and into the parser header file since
+ Bison's required definitions should depend on it in both places.
+ - `%start-header {CODE}', which only Bison 2.3a supported.
+
+ 3. %provides {CODE}
+
+ This is the right place to write additional definitions you would like
+ Bison to expose externally. That is, this directive inserts your CODE
+ both into the parser header file and into the parser code file after
+ Bison's required definitions. Compare with:
+
+ - `%end-header {CODE}', which only Bison 2.3a supported.
+
+ 4. %code-top {CODE}
+
+ Occasionally it is desirable to insert code near the top of the parser
+ code file. For example:
+
+ %code-top {
+ #define _GNU_SOURCE
+ #include <stdio.h>
+ }
+
+ Compare with:
+
+ - `%{CODE%}' appearing before the first `%union {CODE}' in a grammar
+ file. `%code-top {CODE}' is cleaner as its functionality does not
+ depend on its position in the grammar file relative to any
+ `%union {CODE}'.
+ - `%before-header {CODE}', which only Bison 2.3a supported.
+
+ If you have multiple occurrences of any one of the above four directives,
+ Bison will concatenate the contents in the order they appear in the grammar
+ file.
+
+ The prologue alternatives are experimental. More user feedback will help to
+ determine whether they should become permanent features.
+
+ Also see the new section `Prologue Alternatives' in the Bison manual.
+
+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
+ associating token numbers with token names. Removing the #define statements
+ 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 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 the first %union. To generate
+ the post-prologue, Bison concatenates all prologue blocks that you've
+ declared after the first %union.
+
+ 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
+ token numbers with names). For parsers in C, the point of insertion was
+ after the token definitions.
+
+ 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 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 {
+ /* 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. */
+ }
+ %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. */
+ }
+ %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
+ in a future release.
+
+Changes in version 2.3, 2006-06-05:
+
+* GLR grammars should now use `YYRECOVERING ()' instead of `YYRECOVERING',
+ for compatibility with LALR(1) grammars.
+
+* It is now documented that any definition of YYSTYPE or YYLTYPE should
+ be to a type name that does not contain parentheses or brackets.
+
+Changes in version 2.2, 2006-05-19:
+
+* The distribution terms for all Bison-generated parsers now permit
+ using the parsers in nonfree programs. Previously, this permission
+ was granted only for Bison-generated LALR(1) parsers in C.
+
+* %name-prefix changes the namespace name in C++ outputs.
+
+* The C++ parsers export their token_type.
+
+* Bison now allows multiple %union declarations, and concatenates
+ their contents together.
+
+* New warning: unused values
+ Right-hand side symbols whose values are not used are reported,
+ if the symbols have destructors. For instance:
+
+ exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
+ | exp "+" exp
+ ;
+
+ will trigger a warning about $$ and $5 in the first rule, and $3 in
+ the second ($1 is copied to $$ by the default rule). This example
+ most likely contains three errors, and could be rewritten as:
+
+ exp: exp "?" exp ":" exp
+ { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
+ | exp "+" exp
+ { $$ = $1 ? $1 : $3; if ($1) free ($3); }
+ ;
+
+ However, if the original actions were really intended, memory leaks
+ and all, the warnings can be suppressed by letting Bison believe the
+ values are used, e.g.:
+
+ exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
+ | exp "+" exp { $$ = $1; (void) $3; }
+ ;
+
+ If there are mid-rule actions, the warning is issued if no action
+ uses it. The following triggers no warning: $1 and $3 are used.
+
+ exp: exp { push ($1); } '+' exp { push ($3); sum (); };
+
+ The warning is intended to help catching lost values and memory leaks.
+ If a value is ignored, its associated memory typically is not reclaimed.
+
+* %destructor vs. YYABORT, YYACCEPT, and YYERROR.
+ Destructors are now called when user code invokes YYABORT, YYACCEPT,
+ and YYERROR, for all objects on the stack, other than objects
+ corresponding to the right-hand side of the current rule.
+
+* %expect, %expect-rr
+ Incorrect numbers of expected conflicts are now actual errors,
+ instead of warnings.
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