X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/affac6132ae6ee354e1497e456b5c81a0c84334f..02d7cce6e21c97f4245f333eaab7e76b38fe373a:/NEWS

diff --git a/NEWS b/NEWS
index 94e9cc17..784efeb7 100644
--- a/NEWS
+++ b/NEWS
@@ -5,21 +5,40 @@ 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 ? $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 "?" exp ":" exp { $$ = $1 + $3; }
-          | exp "+" exp
-	  ;
+     exp: exp { push ($1); } '+' exp { push ($3); sum (); };
 
-  will trigger a warning about $5 of the first rule, and $3 in the
-  second ($1 is copied to $$ by the default rule).  To avoid this
-  warning, let Bison believe the value is used, e.g.
+  Mid-rule actions that use $$ cause the corresponding value to be
+  set, therefore the following action must use it.  The following rule
+  triggers a warning about $2.
 
-       exp: exp "?" exp ":" exp { $$ = $1 + $3; $5; }
-          | exp "+" exp         { $$ = $1; $3; }
+     exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
 
-  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,