Re: %destructor feedback

[Paul Eggert]

Thanks for doing all that.  The example in NEWS was a bit confusing,
since one normally wouldn't want to ignore $3 in 'exp "+" exp', so I
tried to improve it by installing the following patch.

2005-12-22  Paul Eggert  <address@hidden>

        * NEWS: Improve wording for unused-value warnings.

--- NEWS        22 Dec 2005 11:40:05 -0000      1.137
+++ NEWS        23 Dec 2005 00:13:11 -0000      1.138
@@ -5,21 +5,29 @@ Changes in version 2.1a:
 
 * New warning: unused values
   Typed right-hand side symbols whose value are not used are reported.
-  For instance
+  For instance:
 
-       exp: exp "?" exp ":" exp { $$ = $1 + $3; }
-          | exp "+" exp
-         ;
+     exp: exp "?" exp ":" exp { $$ = $1 ? $1 : $3; }
+       | exp "+" exp
+       ;
 
   will trigger a warning about $5 of the first rule, and $3 in the
-  second ($1 is copied to $$ by the default rule).  To avoid this
-  warning, let Bison believe the value is used, e.g.
+  second ($1 is copied to $$ by the default rule).  This example
+  most likely contains two errors, and should be rewritten as:
 
-       exp: exp "?" exp ":" exp { $$ = $1 + $3; $5; }
-          | exp "+" exp         { $$ = $1; $3; }
+     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; }
+       ;
 
-  This helps catching lost values and memory leaks: if a value is
-  ignored, its associated memory will never be reclaimed.
+  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,