parse.lac: document.

[bison.git] / NEWS

diff --git a/NEWS b/NEWS
index 61b14b08f937930f7b09e0f06241055ffea7a759..e3510938b62d1a2c941c6df46d309728381d98a0 100644 (file)
--- a/NEWS
+++ b/NEWS
@@ -58,6 +58,46 @@ 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.
 

+** LAC (lookahead correction) for syntax error handling:
+
+  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.
+
+  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.
+
+  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:
+
+    %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.
+

 ** Unrecognized %code qualifiers are now an error not a warning.
 
 ** %define improvements.
 ** Unrecognized %code qualifiers are now an error not a warning.
 
 ** %define improvements.


@@ -164,14 +204,43 @@ Bison News
   Bison now warns when a character literal is not of length one.  In
   some future release, Bison will report an error instead.
 
   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.
+** 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.

 
 ** Destructor calls fixed for lookaheads altered in semantic actions.
 
 
 ** Destructor calls fixed for lookaheads altered in semantic actions.
 


@@ -222,7 +291,7 @@ Bison News
   the user %code sections so that its #ifndef guard does not try to
   override the user's YYLLOC_DEFAULT if provided.
 
   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 (????-??-??):
+* 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.
 
 ** Bison now obeys -Werror and --warnings=error for warnings about
    grammar rules that are useless in the parser due to conflicts.


@@ -240,6 +309,8 @@ Bison News
    errors in Bison 2.5.  They will remain warnings, which should be
    sufficient for POSIX while avoiding backward compatibility issues.
 
    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