doc: don't use @acronym.

[bison.git] / NEWS

diff --git a/NEWS b/NEWS
index 2a0592a1329a683b34f7f16dae941836f83a7de9..593807c7987c6552ce16bc7d5c59eca72c306c0b 100644 (file)
--- a/NEWS
+++ b/NEWS
@@ -6,7 +6,7 @@ Bison News
 ** Additional yylex/yyparse arguments
 
   The new directive %param declare additional argument to both yylex
 ** Additional yylex/yyparse arguments
 
   The new directive %param declare additional argument to both yylex

-  and yyparse.  The %lex-param, %parse-param, and %param directive
+  and yyparse.  The %lex-param, %parse-param, and %param directives

   support one or more arguments.  Instead of
 
       %lex-param   {arg1_type *arg1}
   support one or more arguments.  Instead of
 
       %lex-param   {arg1_type *arg1}


@@ -51,9 +51,18 @@ Bison News
   use of the %error-verbose directive is deprecated in favor of
   %define parse.error "verbose".
 
   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 (????-??-??):
 
 * Changes in version 2.5 (????-??-??):
 

-** Named References Support
+** Named references:

 
   Historically, Yacc and Bison have supported positional references
   ($n, $$) to allow access to symbol values from inside of semantic
 
   Historically, Yacc and Bison have supported positional references
   ($n, $$) to allow access to symbol values from inside of semantic


@@ -78,13 +87,13 @@ Bison News
   These features are experimental in this version.  More user feedback
   will help to stabilize them.
 
   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
 
   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
   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


@@ -108,15 +117,57 @@ Bison News
   These features are experimental.  More user feedback will help to
   stabilize them.
 
   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 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 `#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 completely
+  solves these problems for canonical LR, IELR, and LALR without
+  sacrificing %nonassoc, default reductions, or state mering.  When
+  LAC is in use, canonical LR and IELR behave 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.
+
+  Currently, LAC is only supported for deterministic parsers in C.
+  You can enable LAC with the following directive:

 
 

-*** Can now be invoked via the command line.
+    %define parse.lac full
+
+  See the documentation for `%define parse.lac' in the section `Bison
+  Declaration Summary' in the Bison manual for additional details.
+
+  LAC is an experimental feature.  More user feedback will help to
+  stabilize it.
+
+** Grammar symbol names can now contain dashes:
+
+  Consistently with directives (such as %error-verbose) and variables
+  (e.g. push-pull), grammar 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).
+
+** %define improvements:
+
+*** Can now be invoked via the command line:

 
   Each of these command-line options
 
 
   Each of these command-line options
 


@@ -135,7 +186,7 @@ Bison News
   quietly override %define, but -D and --define do not.  For further
   details, see the section "Bison Options" in the Bison manual.
 
   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 following %define variables
 


@@ -150,7 +201,7 @@ Bison News
   The old names are now deprecated but will be maintained indefinitely
   for backward compatibility.
 
   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,
 
   If a %define value is an identifier, it no longer needs to be placed
   within quotations marks.  For example,


@@ -161,15 +212,75 @@ Bison News
 
     %define api.push-pull push
 
 
     %define api.push-pull push
 

-** Symbol names.
+*** Unrecognized variables are now errors not warnings.

 
 

-  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).
+*** Multiple invocations for any variable is now an error not a warning.
+
+** Unrecognized %code qualifiers are now errors not warnings.
+
+** 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
+  the following grammar to be the same token:
+
+    exp: exp '++'
+       | exp '+' exp
+       ;
+
+  Bison now warns when a character literal is not of length one.  In
+  some future release, Bison will start reporting an error instead.
+
+** 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.

 
 

-** YYFAIL now produces warnings and Java parsers no longer implement it.
+** 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
+  to use it.  If, for instance, your location structure has "first"
+  and "last" members, instead of
+
+      # define YYLLOC_DEFAULT(Current, Rhs, N)                             \
+        do                                                                 \
+          if (N)                                                           \
+            {                                                              \
+              (Current).first = (Rhs)[1].location.first;                   \
+              (Current).last  = (Rhs)[N].location.last;                    \
+            }                                                              \
+          else                                                             \
+            {                                                              \
+              (Current).first = (Current).last = (Rhs)[0].location.last;   \
+            }                                                              \
+        while (false)
+
+  use:
+
+      # define YYLLOC_DEFAULT(Current, Rhs, N)                             \
+        do                                                                 \
+          if (N)                                                           \
+            {                                                              \
+              (Current).first = YYRHSLOC (Rhs, 1).first;                   \
+              (Current).last  = YYRHSLOC (Rhs, N).last;                    \
+            }                                                              \
+          else                                                             \
+            {                                                              \
+              (Current).first = (Current).last = YYRHSLOC (Rhs, 0).last;   \
+            }                                                              \
+        while (false)
+
+** 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.
+
+** 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
 
   YYFAIL has existed for many years as an undocumented feature of
   deterministic parsers in C generated by Bison.  More recently, it was


@@ -180,13 +291,7 @@ Bison News
   discussion of how to suppress C preprocessor warnings about YYFAIL
   being unused, see the Bison 2.4.2 NEWS entry.
 
   discussion of how to suppress C preprocessor warnings about YYFAIL
   being unused, see the Bison 2.4.2 NEWS entry.
 

-** `%prec IDENTIFIER' requires IDENTIFIER to be defined separately.
-
-  As promised in Bison 2.4.2's NEWS entry, it is now an error if a token
-  that appears after a %prec directive is not defined by %token, %left,
-  %right, or %nonassoc.  This is required by POSIX.
-
-** Temporary hack for adding a semicolon to the user action.
+** 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
 
   Previously, Bison appended a semicolon to every user action for
   reductions when the output language defaulted to C (specifically, when


@@ -207,42 +312,64 @@ Bison News
   about a missing semicolon where it did not before.  Future releases of
   Bison will cease to append semicolons entirely.
 
   about a missing semicolon where it did not before.  Future releases of
   Bison will cease to append semicolons entirely.
 

-** 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
-  the following grammar to be the same token:
-
-    exp: exp '++'
-       | exp '+' exp
-       ;
-
-  Bison now warns when a character literal is not of length one.  In
-  some future release, Bison will report 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.
-
-  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.
-
-* Changes in version 2.4.3 (????-??-??):
+** Verbose syntax error message fixes:
+
+  When %error-verbose or `#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 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.
+
+* 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.
 
 ** Failures in the test suite for GCC 4.5 have been fixed.
 
 
 ** Problems with spawning M4 on at least FreeBSD 8 and FreeBSD 9 have
    been fixed.
 
 ** Failures in the test suite for GCC 4.5 have been fixed.
 

+** Failures in the test suite for some versions of Sun Studio C++ have
+   been fixed.
+
+** Contrary to Bison 2.4.2's NEWS entry, it has been decided that
+   warnings about undefined %prec identifiers will not be converted to
+   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
 * Changes in version 2.4.2 (2010-03-20):
 
 ** Some portability problems that resulted in failures and livelocks


@@ -260,6 +387,8 @@ Bison News
   %prec directive.  It is now restored.  However, for backward
   compatibility with recent Bison releases, it is only a warning for
   now.  In Bison 2.5 and later, it will return to being an error.
   %prec directive.  It is now restored.  However, for backward
   compatibility with recent Bison releases, it is only a warning for
   now.  In Bison 2.5 and later, it will return to being an error.

+  [Between the 2.4.2 and 2.4.3 releases, it was decided that this
+  warning will not be converted to an error in Bison 2.5.]

 
 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
 
 
 ** Detection of GNU M4 1.4.6 or newer during configure is improved.
 


@@ -1382,7 +1511,7 @@ End:
 
 -----
 
 
 -----
 

-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.
 
 
 This file is part of Bison, the GNU Parser Generator.