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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 23:49:38 -0500
Message-Id: <p05101020b7ed775b0b85@[]>
To: "Seth Russell" <seth@robustai.net>
Cc: www-rdf-rules@w3.org
>From: "Pat Hayes" <phayes@ai.uwf.edu>
>>  >  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
>>  >(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.
>Which is why I used the word 'term' ... I guess I should have used the word
>'vocabulary' directly from the MT.  One practical way to make a schema that
>extended RDF to express Drew's formula, above, would be to invent URIs for
>the variables.

Oh, I see. But URI's aren't like variables, because they have global 
scope. Blank nodes are more like variables, which is why I jumped on 
you. That is, RDf graphs do have things in them that are *exactly* 
like existential variables in logic, but they are not URIs, and 
mustn't be made into URIs.

>  My only point was that those URI would force the left ?x to
>be the same identical node as the right ?x.
>The mentograph shows that clearly:
>http://robustai.net/mentography/entailsIdentity.gif   Incidentally in
>mentographs blank  nodes (unnamed nodes) show up as boxes with nothing
>inside the box where the name of the box goes. I love simple things.
>>  >  But if we scope formulas
>>  >to a context (a collection of statements) the ?x ?y in the left hand of
>>  >formula become *the same* as the ?x ?y on the right .... that's the way
>>  >works .... doesn't it ?
>>  Nope, because RDF doesn't have contexts. (There are things like this
>>  in N3, but then N3 goes beyond RDF.)
>Alas ... but I'm sure it will someday ... aren't you?
>Actually I think a case could be made that we can do contexts in the current
>RDF since it  allows bags of statements:

Bags, but not of statements (unless you reify; but then you aren't 
asserting the things in the bag).

>substitute the word bag for the
>word context and use the MT to interpret a bag of statements as a set of
>statements.  Now I'm sure I didn't say that correctly, but if you blur your
>eyes a bit, maybe you will see what I meant.  I think our abilities to use
>RDF for logical inferences is severely diminished without contexts ... don't

Yup. But then RDF is very good for what it does, so lets leave it to 
do that and use something else to do other things, right?

>  How do we say this set of statements is monotonic and this set is not?
>>  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.
>Why would (must?) one interpret a variable as an anonymous node?

See the MT document, section 2.

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Received on Saturday, 13 October 2001 00:49:47 UTC

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