RE: regular expression operators in CF grammars

"Quinn Tyler Jackson" <qjackson@shaw.ca>
14 Sep 2002 16:35:59 -0400

          From comp.compilers

Related articles
[8 earlier articles]
Re: regular expression operators in CF grammars nospam@snafu.de (Sönke Kannapinn) (2002-08-10)
Re: regular expression operators in CF grammars cfc@shell01.TheWorld.com (Chris F Clark) (2002-08-23)
Re: regular expression operators in CF grammars tbandrow@unitedsoftworks.com (tj bandrowsky) (2002-09-03)
Re: regular expression operators in CF grammars cfc@shell01.TheWorld.com (Chris F Clark) (2002-09-12)
Re: regular expression operators in CF grammars cfc@TheWorld.com (Chris F Clark) (2002-09-14)
RE: regular expression operators in CF grammars qjackson@shaw.ca (Quinn Tyler Jackson) (2002-09-14)
RE: regular expression operators in CF grammars qjackson@shaw.ca (Quinn Tyler Jackson) (2002-09-14)
| List of all articles for this month |

From: "Quinn Tyler Jackson" <qjackson@shaw.ca>
Newsgroups: comp.compilers
Date: 14 Sep 2002 16:35:59 -0400
Organization: Compilers Central
References: 02-05-142 02-05-151 02-06-024 02-06-056 02-06-090 02-07-010 02-07-025 02-07-043 02-08-035 02-08-078 02-09-018 02-09-070 <20020912232254.48B8712D0@0lsen.net> 02-09-094
Keywords: parse
Posted-Date: 14 Sep 2002 16:35:59 EDT

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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