Re: Reductions during Bison error handling

[Paul Eggert]

> Date: Thu, 23 May 2002 16:17:58 +0200
> From: Hans Aberg <address@hidden>
> 
> In math, typically the semantically more significant (to the human
> reader's cognition) quantity should be on the LHS.

In math, one normally sees expressions like '0 < i < n'.  The closest
equivalent in C is '0 < i && i < n'.  That is what I was using here.
I find this more readable than 'i > 0 && i < n'.  Once you get used
to the "<"-oriented style I hope you'll agree that it has its advantages.

> One should be aware of that long statements in the "for" and "while" heads
> decreases legibility.

Yes, on second thought that part of the change was a bit overboard.  I
installed the following patch instead.  It goes back to something that
is more like the original Bison code than what I proposed, or even
that Paul H. proposed.  If RMS finds a problem with this approach we
can undo the patch.

2002-05-24  Paul Hilfinger  <address@hidden>
        and Paul Eggert  <address@hidden>
        
        * data/bison.simple (yyparse): Correct error handling to conform to
        POSIX and yacc.  Specifically, after syntax error is discovered,
        do not reduce further before shifting the error token.
        Clean up the code a bit by removing the labels yyerrdefault,
        yyerrhandle, yyerrpop.
        * NEWS: Document the above.

--- NEWS.~1.43.~        2002-05-05 04:55:45.000000000 -0700
+++ NEWS        2002-05-24 05:37:48.258881000 -0700
@@ -17,6 +17,15 @@ Changes in version 1.49b:
   user specified that one of her tokens is numbered 256, then error
   will be mapped onto another number.
 
+* Error recovery now conforms to documentation and to POSIX
+  When a Bison-generated parser encounters a syntax error, it now pops
+  the stack until it finds a state that allows shifting the error
+  token.  Formerly, it popped the stack until it found a state that
+  allowed some non-error action other than a default reduction on the
+  error token.  The new behavior has long been the documented behavior,
+  and has long been required by POSIX.  For more details, please see
+  <http://mail.gnu.org/pipermail/bug-bison/2002-May/001452.html>.
+
 * Large grammars
   Large grammars are now supported (large token numbers, large grammar
   size (= sum of the LHS and RHS lengths), large LALR tables).
--- data/bison.simple.~1.26.~   2002-05-20 15:56:53.000000000 -0700
+++ data/bison.simple   2002-05-24 04:42:03.257880000 -0700
@@ -1120,71 +1120,40 @@ yyerrlab1:
 
   yyerrstatus = 3;     /* Each real token shifted decrements this.  */
 
-  goto yyerrhandle;
-
-
-/*-------------------------------------------------------------------.
-| yyerrdefault -- current state does not do anything special for the |
-| error token.                                                       |
-`-------------------------------------------------------------------*/
-yyerrdefault:
-#if 0
-  /* This is wrong; only states that explicitly want error tokens
-     should shift them.  */
-
-  /* If its default is to accept any token, ok.  Otherwise pop it.  */
-  yyn = yydefact[yystate];
-  if (yyn)
-    goto yydefault;
-#endif
-
-
-/*---------------------------------------------------------------.
-| yyerrpop -- pop the current state because it cannot handle the |
-| error token.                                                   |
-`---------------------------------------------------------------*/
-yyerrpop:
-  if (yyssp == yyss)
-    YYABORT;
-  yyvsp--;
-  yystate = *--yyssp;
+  for (;;)
+    {
+      yyn = yypact[yystate];
+      if (yyn != YYFLAG)
+       {
+         yyn += YYTERROR;
+         if (0 <= yyn && yyn <= YYLAST && yycheck[yyn] == YYTERROR)
+           {
+             yyn = yytable[yyn];
+             if (0 < yyn)
+               break;
+           }
+       }
+
+      /* Pop the current state because it cannot handle the error token.  */
+      if (yyssp == yyss)
+       YYABORT;
+      yyvsp--;
+      yystate = *--yyssp;
 #if YYLSP_NEEDED
-  yylsp--;
+      yylsp--;
 #endif
 
 #if YYDEBUG
-  if (yydebug)
-    {
-      short *yyssp1 = yyss - 1;
-      YYFPRINTF (stderr, "Error: state stack now");
-      while (yyssp1 != yyssp)
-       YYFPRINTF (stderr, " %d", *++yyssp1);
-      YYFPRINTF (stderr, "\n");
-    }
+      if (yydebug)
+       {
+         short *yyssp1 = yyss - 1;
+         YYFPRINTF (stderr, "Error: state stack now");
+         while (yyssp1 != yyssp)
+           YYFPRINTF (stderr, " %d", *++yyssp1);
+         YYFPRINTF (stderr, "\n");
+       }
 #endif
-
-/*--------------.
-| yyerrhandle.  |
-`--------------*/
-yyerrhandle:
-  yyn = yypact[yystate];
-  if (yyn == YYFLAG)
-    goto yyerrdefault;
-
-  yyn += YYTERROR;
-  if (yyn < 0 || yyn > YYLAST || yycheck[yyn] != YYTERROR)
-    goto yyerrdefault;
-
-  yyn = yytable[yyn];
-  if (yyn < 0)
-    {
-      if (yyn == YYFLAG)
-       goto yyerrpop;
-      yyn = -yyn;
-      goto yyreduce;
     }
-  else if (yyn == 0)
-    goto yyerrpop;
 
   if (yyn == YYFINAL)
     YYACCEPT;