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Re: Two problems with the GLR parsers


From: Paul Eggert
Subject: Re: Two problems with the GLR parsers
Date: Fri, 15 Nov 2002 23:40:55 -0800 (PST)

> From: Paul Hilfinger <address@hidden>
> Date: Thu, 14 Nov 2002 16:01:00 -0800
> 
> Why would that give 50 as the location of error, which was pushed
> while at location 30?  

Good question.  This raises another question: should something like
YYLLOC_DEFAULT apply to error-token location computation too?  (And I
have another question for Akim: why does src/parse-gram.y override
YYLLOC_DEFAULT?  Perhaps that should be commented?)

> As the starred lines indicate, the error token is shifted 4 times!
> Fortunately, the reduction is taken only once.  I'm not sure what to
> make of this, so I'll ask around before I try to make glr duplicate
> the behavior (or, of course, change input.at).

Surely the user shouldn't care how the Bison-generated parser
internally arrives at the correct actions, so long as the actions are
correct.  But perhaps I am missing some subtleties about why the user
would care.

The POSIX spec for how a Yacc-generated parser is supposed to handle
the error token can be found in the "Error Handling" section of
<http://www.opengroup.org/onlinepubs/007904975/utilities/yacc.html>.
It's a bit more complicated than what the Bison manual says, e.g. it
talks about what to do at end of input.  (Perhaps the Bison manual
should be updated?)  Anyway, I installed the following new test case
to check that the POSIX-required error handling is being implemented
properly in this particular grammar.

2002-11-15  Paul Eggert  <address@hidden>

        * tests/actions.at (Actions after errors): New test case.

Index: actions.at
===================================================================
RCS file: /cvsroot/bison/bison/tests/actions.at,v
retrieving revision 1.21
retrieving revision 1.22
diff -p -u -r1.21 -r1.22
--- actions.at  15 Nov 2002 20:32:21 -0000      1.21
+++ actions.at  16 Nov 2002 07:31:35 -0000      1.22
@@ -82,6 +82,153 @@ AT_CLEANUP
 
 
 
+## ---------------------- ##
+## Actions after errors.  ##
+## ---------------------- ##
+
+AT_SETUP([Actions after errors])
+
+
+
+AT_DATA_GRAMMAR([[input.y]],
+[[%{
+#include <stdio.h>
+#include <stdlib.h>
+
+static int yylex (void);
+static void yyerror (char const *);
+
+#define YYDEBUG 1
+%}
+%union { int ival; }
+%type <ival> 'x' ';' thing line input
+
+%%
+input:
+  /* Nothing. */
+    {
+      $$ = 0;
+      printf ("input(%d): /* Nothing */\n", $$);
+    }
+| line input /* Right recursive to load the stack so that popping at
+               EOF can be exercised.  */
+    {
+      $$ = 2;
+      printf ("input(%d): line(%d) input(%d)\n", $$, $1, $2);
+    }
+;
+
+line:
+  thing thing thing ';'
+    {
+      $$ = $1;
+      printf ("line(%d): thing(%d) thing(%d) thing(%d) ';'(%d)\n",
+             $$, $1, $2, $3, $4);
+    }
+| thing thing ';'
+    {
+      $$ = $1;
+      printf ("line(%d): thing(%d) thing(%d) ';'(%d)\n", $$, $1, $2, $3);
+    }
+| thing ';'
+    {
+      $$ = $1;
+      printf ("line(%d): thing(%d) ';'(%d)\n", $$, $1, $2);
+    }
+| error ';'
+    {
+      $$ = -1;
+      printf ("line(%d): error ';'(%d)\n", $$, $2);
+    }
+;
+
+thing:
+  'x'
+    {
+      $$ = $1;
+      printf ("thing(%d): 'x'(%d)\n", $$, $1);
+    }
+;
+%%
+static size_t counter;
+
+static int
+yylex (void)
+{
+  static char const input[] =
+    {
+      /* Exericise the discarding of stack top and input until `error'
+         can be reduced.  */
+      'x', 'x', 'x', 'x', 'x', 'x', ';',
+
+      /* Load the stack and provoke an error that cannot be caught by
+         the grammar, to check that the stack is cleared. */
+      'x', 'x', ';',
+      'x', ';',
+      'y'
+    };
+
+  if (counter < sizeof input)
+    {
+       yylval.ival = counter;
+       printf ("sending: '%c' (value = %d)\n", input[counter], yylval.ival);
+       return input[counter++];
+    }
+  else
+    {
+      printf ("sending: EOF\n");
+      return EOF;
+    }
+}
+
+static void
+yyerror (char const *msg)
+{
+  printf ("%lu: %s\n", (unsigned long int) counter, msg);
+}
+
+int
+main (void)
+{
+  yydebug = !!getenv ("YYDEBUG");
+  return yyparse ();
+}
+]])
+
+AT_CHECK([bison -o input.c input.y])
+AT_COMPILE([input])
+AT_PARSER_CHECK([./input], 1,
+[[sending: 'x' (value = 0)
+thing(0): 'x'(0)
+sending: 'x' (value = 1)
+thing(1): 'x'(1)
+sending: 'x' (value = 2)
+thing(2): 'x'(2)
+sending: 'x' (value = 3)
+4: syntax error
+sending: 'x' (value = 4)
+sending: 'x' (value = 5)
+sending: ';' (value = 6)
+line(-1): error ';'(6)
+sending: 'x' (value = 7)
+thing(7): 'x'(7)
+sending: 'x' (value = 8)
+thing(8): 'x'(8)
+sending: ';' (value = 9)
+line(7): thing(7) thing(8) ';'(9)
+sending: 'x' (value = 10)
+thing(10): 'x'(10)
+sending: ';' (value = 11)
+line(10): thing(10) ';'(11)
+sending: 'y' (value = 12)
+13: syntax error
+sending: EOF
+]])
+
+AT_CLEANUP
+
+
+
 ## ---------------- ##
 ## Exotic Dollars.  ##
 ## ---------------- ##




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