Bison version 2.4 is now available.  Relative to 2.3, this version
contains many bug fixes and many exciting new features that need
more user feedback.  Some of the more notable features are:

  1. XML or Graphviz DOT format for the LALR(1) automaton.
  2. Push parsers.
  3. Java-based parsers.
  4. %code, a flexible alternative to traditional Yacc prologue blocks.
  5. Default %destructor and %printer directives.

See "NEWS" below for further details.

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This release was bootstrapped with the following tools:
  Autoconf 2.62
  Automake 1.10.1
  Flex 2.5.35
  Gnulib v0.0-1332-g2ec9ae2


Changes relative to test version 2.3b:

* %language is an experimental feature.

  We first introduced this feature in test release 2.3b as a cleaner
  alternative to %skeleton.  Since then, we have discussed the possibility of
  modifying its effect on Bison's output file names.  Thus, in this release,
  we consider %language to be an experimental feature that will likely evolve
  in future releases.

* Forward compatibility with GNU M4 has been improved.

* Several bugs in the C++ skeleton and the experimental Java skeleton have been

Other changes relative to version 2.3:

* 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 current push parsing interface is experimental and may evolve.  More user
  feedback will help to stabilize it.

* The -g and --graph options now output graphs in Graphviz DOT format,
  not VCG format.  Like --graph, -g now also takes an optional FILE argument
  and thus cannot be bundled with other short options.

* Java

  Bison can now generate an LALR(1) parser in Java.  The skeleton is
  `data/'.  Consider using the new %language directive instead of
  %skeleton to select it.

  See the new section `Java Parsers' in the Bison manual for details.

  The current Java interface is experimental and may evolve.  More user
  feedback will help to stabilize it.

* %language

  This new directive specifies the programming language of the generated
  parser, which can be C (the default), C++, or Java.  Besides the skeleton
  that Bison uses, the directive affects the names of the generated files if
  the grammar file's name ends in ".y".  However, %language is experimental
  and its effect on generated file names will likely evolve.

* XML Automaton Report

  Bison can now generate an XML report of the LALR(1) automaton using the new
  `--xml' option.  The current XML schema is experimental and may evolve.  More
  user feedback will help to stabilize it.

* The grammar file may now specify the name of the parser header file using
  %defines.  For example:

    %defines "parser.h"

* When reporting useless rules, useless nonterminals, and unused terminals,
  Bison now employs the terms "useless in grammar" instead of "useless",
  "useless in parser" instead of "never reduced", and "unused in grammar"
  instead of "unused".

* 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
       "useless in parser due to 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

* --report-file=FILE is a new option to override the default `.output' file

* The `=' that used to be required in the following directives is now

    %file-prefix "parser"
    %name-prefix "c_"
    %output "parser.c"

* An Alternative to `%{...%}' -- `%code QUALIFIER {CODE}'

  Bison now provides %code, a more flexible alternative to the traditional Yacc
  prologue blocks.  The %code directive takes an optional qualifier field,
  which identifies the purpose of the code and thus the location(s) where Bison
  should generate it.  For example, the following forms are available in the
  case of C/C++:

    1. `%code          {CODE}' -- The default for the parser source code file.
    2. `%code requires {CODE}' -- Dependencies of YYSTYPE and YYLTYPE.
    3. `%code provides {CODE}' -- Other definitions for the parser header file.
    4. `%code top      {CODE}' -- Near the top of the parser source code file.

  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 option `--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

    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.

  The default %destructor's and %printer's are experimental.  More user
  feedback will help to determine whether they should become permanent

  See the section `Freeing Discarded Symbols' in the Bison manual for further

* %left, %right, and %nonassoc can now declare token numbers.  This is required
  by POSIX.  However, see the end of section `Operator Precedence' in the Bison
  manual for a caveat concerning the treatment of literal strings.

* The nonfunctional --no-parser, -n, and %no-parser options have been
  completely removed from Bison.

* 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.

* 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.
  To insert code at other locations, try the new %code directive mentioned

* 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.

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