From: Joel E. Denny <jdenny@ces.clemson.edu>
Date: Tue, 21 Feb 2006 21:01:57 +0000 (+0000)
Subject: * doc/bison.texinfo (Mid-Rule Actions): Explain how to bury a
X-Git-Tag: v2.3b~444
X-Git-Url: https://git.saurik.com/bison.git/commitdiff_plain/841a773708453d8e46d69d0727654fbdba6e063e?ds=inline

* doc/bison.texinfo (Mid-Rule Actions): Explain how to bury a
mid-rule action inside a nonterminal symbol in order to declare a
destructor for its semantic value.
---

diff --git a/ChangeLog b/ChangeLog
index c8d3e981..d56eb26e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2006-02-21  Joel E. Denny  <jdenny@ces.clemson.edu>
+
+	* doc/bison.texinfo (Mid-Rule Actions): Explain how to bury a
+	mid-rule action inside a nonterminal symbol in order to declare a
+	destructor for its semantic value.
+
 2006-02-16  Paul Eggert  <eggert@cs.ucla.edu>
 
 	* data/yacc.c [(! defined yyoverflow || YYERROR_VERBOSE) && !
diff --git a/doc/bison.texinfo b/doc/bison.texinfo
index 35b978af..eb36845f 100644
--- a/doc/bison.texinfo
+++ b/doc/bison.texinfo
@@ -3317,6 +3317,46 @@ earlier action is used to restore the prior list of variables.  This
 removes the temporary @code{let}-variable from the list so that it won't
 appear to exist while the rest of the program is parsed.
 
+@findex %destructor
+@cindex discarded symbols, mid-rule actions
+@cindex error recovery, mid-rule actions
+In the above example, if the parser initiates error recovery (@pxref{Error
+Recovery}) while parsing the tokens in the embedded statement @code{stmt},
+it might discard the previous semantic context @code{$<context>5} without
+restoring it.
+Thus, @code{$<context>5} needs a destructor (@pxref{Destructor Decl, , Freeing
+Discarded Symbols}).
+However, Bison currently provides no means to declare a destructor for a
+mid-rule action's semantic value.
+
+One solution is to bury the mid-rule action inside a nonterminal symbol and to
+declare a destructor for that symbol:
+
+@example
+@group
+%type <context> let
+%destructor @{ pop_context ($$); @} let
+
+%%
+
+stmt:  let stmt
+               @{ $$ = $2;
+                 pop_context ($1); @}
+       ;
+
+let:   LET '(' var ')'
+               @{ $$ = push_context ();
+                 declare_variable ($3); @}
+       ;
+
+@end group
+@end example
+
+@noindent
+Note that the action is now at the end of its rule.
+Any mid-rule action can be converted to an end-of-rule action in this way, and
+this is what Bison actually does to implement mid-rule actions.
+
 Taking action before a rule is completely recognized often leads to
 conflicts since the parser must commit to a parse in order to execute the
 action.  For example, the following two rules, without mid-rule actions,
@@ -3410,10 +3450,7 @@ compound: subroutine
 
 @noindent
 Now Bison can execute the action in the rule for @code{subroutine} without
-deciding which rule for @code{compound} it will eventually use.  Note that
-the action is now at the end of its rule.  Any mid-rule action can be
-converted to an end-of-rule action in this way, and this is what Bison
-actually does to implement mid-rule actions.
+deciding which rule for @code{compound} it will eventually use.
 
 @node Locations
 @section Tracking Locations