RE: regular expression operators in CF grammars

Kannapinn to Clark:


> After all, we attach semantics to the detailed grammar structure
> of an EBNF when targeting compiler construction, don't we? If,
> for example, I write down a simple EBNF rule
>
> S := 'if' E 'then' S ['else' S 'end']
>
> I want to attach semantic code depending on whether the
> optional part has actually been seen by the parser or not.


One way to do that in Meta-S:


S ::= "if" E "then" S [$x("else") S "end"];


Reduction code:


if($MATCHED()) // since reductions are fired on failure to match as
well
{
        if($CAST("x").GetLength()) // non empty; will == "else" if that
portion matched
        {
// we now know that the else portion matched, and know
                        // that the appropriate tree nodes (the second S) are
// available for traversal
        }
        else
        {
// we have the simple form
        }
}


Since $x(expr) is a local cast, and since x is not referenced in S,
this is a safe bind that doesn't
change the semantics of G.


--
Quinn Tyler Jackson
http://members.shaw.ca/qjackson/

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