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Re: Expressiveness of RDF as Rule Conclusion Language (was Re: W hat is an RDF Query? )

From: Pat Hayes <phayes@ai.uwf.edu>
Date: Fri, 12 Oct 2001 12:05:30 -0500
Message-Id: <p05101004b7ecd1e24272@[205.160.76.193]>
To: "Seth Russell" <seth@robustai.net>
Cc: www-rdf-rules@w3.org
>  Drew McDermott said:
>
>     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.
>
>Well, If I understand RDF graphs, I think you are.  Don't forget every term
>(variable or not) has to be a URI in a RDF graph.

No! Blank nodes and literals are not URIs in an RDF grpah. The 
distinction is important for just this reason. bNode identifiers in 
an N-triples document are local to the document and do not have 
global scope.

>  But if we scope formulas
>to a context (a collection of statements) the ?x ?y in the left hand of the
>formula become *the same* as the ?x ?y on the right .... that's the way RDF
>works .... doesn't it ?

Nope, because RDF doesn't have contexts. (There are things like this 
in N3, but then N3 goes beyond RDF.)

The graph-merging rules described in section 3 of the RDF MT document 
should make this clear: if you merge two RDF graphs then you *must* 
merge nodes with the same URI, but you *must not* merge blank nodes.

Pat
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Received on Friday, 12 October 2001 13:05:33 GMT

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