Re: Expressiveness of RDF as Rule Conclusion Language (was Re: W hat is an RDF Query? ) from Drew McDermott on 2001-10-09 (www-rdf-rules@w3.org from October 2001)

From: Drew McDermott <drew.mcdermott@yale.edu>
Date: Tue, 9 Oct 2001 14:25:28 -0400 (EDT)
To: www-rdf-rules@w3.org
Message-Id: <200110091825.f99IPSA09751@pantheon-po03.its.yale.edu>

   [me]
   >I can think of two
   >responses:
   >
   >a) Don't require it to be a legal RDF graph.  Make it an "RDF graph
   >schema," that is, something containing variables that becomes an RDF
   >graph when nodes are substituted for the variables.

   [Pat Hayes]
   That *is* a legal RDF graph. RDF graphs have blank nodes which have 
   exactly the semantics of existential variables. 

   [me]
   [This idea] leave[s] the variables without explicit scope, as in
   >Prolog.  If you want explicit scope, things get more interesting.

   True; the implied scope is the graph. This makes things much easier.

Okay, if you change Sandro's "document scope" to "graph scope," you're
right.  But then a rule like 

 (R1 ?x ?y) |-   (R2 ?y ?x)

becomes unstatable, because the ?x and ?y in the graph on the left are
different variables from the ?x and ?y in the graph on the right.

Unless I'm missing something.

                                             -- Drew McDermott

Received on Tuesday, 9 October 2001 14:25:35 UTC