cazz has asked for the wisdom of the Perl Monks concerning the following question:

I'm working on a grammer that I would like to do some tests that I can't quite figure out how to do with the normal P::RD grammer tests.

Yes, I know I should figure out how to implement my tests with a more detailed grammer... until I get around to that, I'm hoping I can figure out how to modify the error message from the action block.

I know how to fail parsing, just set $return to undef. However, I'm having a difficult time modifying the error message produced by P::RD.

Getting to the error production from inside the action isn't difficult since the error production is the last production in the rule: 

$thisrule->{'prods'}[ $#{ $thisrule->{'prods'} } ]

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Which, in my example is: 
$VAR1 = bless( {
                 'number' => '1',
                 'strcount' => 0,
                 'dircount' => 1,
                 'uncommit' => 0,
                 'error' => 1,
                 'patcount' => 0,
                 'actcount' => 0,
                 'items' => [
                              bless( {
                                       'msg' => ' my message here',
                                       'hashname' => '__DIRECTIVE1__',
                                       'commitonly' => '',
                                       'lookahead' => 0,
                                       'line' => 82
                                     }, 'Parse::RecDescent::Error' )
                            ],
                 'line' => 82
               }, 'Parse::RecDescent::Production' );

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I should be able to (though not very nicely) get to msg via this:
        $thisrule->{'prods'}[ $#{ $thisrule->{'prods'} } ]->{'items'}-
+>[0]->{'msg'}

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Setting that to be a different value, then triggering the specific error I'm looking for causes the error message to be changed, in the data structure (so says DD) however, I still get the default error message.

Is there a better way than this ugly hack I'm struggling through? If not, what am I doing wrong in my ugly hack?

Replies are listed 'Best First'.
Re: changing P::RD error message from within an action
by ikegami (Patriarch) on May 31, 2005 at 22:21 UTC

    Looking at the code (grammar.pm) generated by

    use Parse::RecDescent;

my $grammar = <<'__EOI__';
parse: <error:FOO!>
__EOI__

Parse::RecDescent->Precompile($grammar, "grammar")
or die("Bad grammar\n");

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    reveals (rearranged to avoid forced line breaks)

    $_tok = do {
   if (1) {
      do {
         push @{$thisparser->{errors}}, [qq{FOO!},$thisline];
      } unless  $_noactions;
      undef
   } else {
      0
   }
};

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    Let's test it:

    use Parse::RecDescent;

my $grammar = <<'__EOI__';
parse: {
   # Create error message.
   my $msg = 'FOO!';

   # Create error.
   push(@{$thisparser->{errors}}, [ $msg, $thisline ])
      unless $_noactions;

   # Fail the action.
   undef;
}
__EOI__

my $parser = Parse::RecDescent->new($grammar)
or die("Bad grammar\n");

$parser->parse('');

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    Output:

           ERROR (line 1): FOO!

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    bingo!

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