On Friday, Jun 13, 2003 Richard Jelinek said:
> Hi Descendants od Rec.
>
> Subrule/production ordering. I just don't get it.
>
> given this grammar
>
> return new Parse::RecDescent (q{
> meaning: proplist
> { if(length($text)) {
> print ">>$text<< remains unparsed.\n";
> return undef;
> }
> 1; }
> | { return undef; }
>
> property: phrase '(' proplist ')'
> | '(' proplist ')'
> | '~' property
> | phrase
>
>
> proplist: property xor_prop(s?)
> | property and_prop(s?)
>
> xor_prop: '|' property
> and_prop: ',' property
>
>
> phrase: /[^\(\)\=\,\|\>\~]+/
>
> });
>
> when I try to parse 'SYN(d(e|f))'
>
> everything goes well. If the string is 'SYN(d(e,f))', a syntax error
> is spilled out. If I swap the first two subrules/productions of
> proplist, the situation is vice versa.
Ordering is how you establish priority in ambiguous situations. If you turn
on $::RD_TRACE=100 you will see what the parser is doing. If you follow the
second SYN(d(e,f)) case you will see that the parser is correct, the grammar
you describe does not parse that input. Its not a problem with the order,
its a fundamental problem with your grammar. So its not that you do not
understand the ordering rules, its that you do not understand your grammar.
This successfully parses both, but I'm, not sure its what you want.
$::RD_TRACE=100;
$it= new Parse::RecDescent (q{
meaning: proplist
{ if(length($text)) {
print ">>$text<< remains unparsed.\n";
return undef;
}
1; }
| { return undef; }
property: phrase '(' proplist ')'
| '(' proplist ')'
| '~' property
| phrase
proplist: property xorand_prop(s?)
xorand_prop: xor_prop|and_prop
xor_prop: '|' property
and_prop: ',' property
phrase: /[^\(\)\=\,\|\>\~]+/
});
$it->meaning('SYN(d(e|f))');
$it->meaning('SYN(d(e,f))');
>
> I found nowhere in the Parse::RecDescent docs, that the ordering of
> productions does matter. But it seems it does. If there isn't
> something blatantly evident I've overseen this makes writing and
> maintenance of these grammars harder than it could be.
>
> --
> best regards,
>
> Dipl.-Inf. Richard Jelinek
>
> - The PetaMem Group - Prague/Nuremberg - www.petamem.com -
> -= 2325182 Mind Units =-
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