accessed -> addressed (fix typo in changelog entry)

[bison.git] / NEWS

diff --git a/NEWS b/NEWS
index ec0a8dec3d434a1295a4d8b19fc65f5ceeba26f4..784efeb7f389625a742ff18f44e00e102683c24a 100644 (file)
--- a/NEWS
+++ b/NEWS
@@ -3,6 +3,48 @@ Bison News
 
 Changes in version 2.1a:
 
+* New warning: unused values
+  Typed right-hand side symbols whose value are not used are reported.
+  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 should be rewritten as:
+
+     exp: exp "?" exp ":" exp { $$ = $1 ? $3 : $5; }
+       | exp "+" exp         { $$ = $1 + $3; }
+       ;
+
+  However, if the original actions were really intended, 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 (); };
+
+  Mid-rule actions that use $$ cause the corresponding value to be
+  set, therefore the following action must use it.  The following rule
+  triggers a warning about $2.
+
+     exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
+
+  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.
@@ -18,8 +60,8 @@ Changes in version 2.1a:
   Bison version VERSION or higher.
 
 * lalr1.cc: The token and value types are now class members.
-  The tokens where defined as free form enums and cpp macros.  YYSTYPE
-  was defined as a free form union.  Both are now class members:
+  The tokens were defined as free form enums and cpp macros.  YYSTYPE
+  was defined as a free form union.  They are now class members:
   tokens are enumerations of the `yy::parser::token' struct, and the
   semantic values have the `yy::parser::semantic_type' type.
 
@@ -35,6 +77,8 @@ Changes in version 2.1a:
 
 Changes in version 2.1, 2005-09-16:
 
+* The C++ lalr1.cc skeleton supports %lex-param.
+
 * Bison-generated parsers now support the translation of diagnostics like
   "syntax error" into languages other than English.  The default
   language is still English.  For details, please see the new