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Re: Rules WG -- draft charter -- NAF

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Thu, 20 Nov 2003 15:09:10 -0500 (EST)
Message-Id: <20031120.150910.63459445.pfps@research.bell-labs.com>
To: stefan@ISI.EDU
Cc: www-rdf-rules@w3.org

From: Stefan Decker <stefan@ISI.EDU>
Subject: Re: Rules WG -- draft charter -- NAF
Date: Thu, 20 Nov 2003 19:19:56 +0000

> Peter,
> 
> >Well, I am still confused as to how this would work.  How would one
> >interpret a rule on an RDF graph without reference to the RDF (or some
> >other) semantics?
> 
> 
> It should take the RDF semantics into account.
> But  compared to the OWL the semantics of RDF semantics is minimal.
> But maybe I should first try to define what I mean by (extended) semantics 
> of an RDF graph.
> The following definition could serve as a starter:
> 
> Set T be the set of all triples.
> An extended semantics esem is a mapping from pow(T) -> pow(T) such that
>        sem(R) \subseteq R, R \in Pow(T).
	 ^esem     ^p

> So every extended semantics is itself monotonic :-)
> E.g., RDF Schema semantics applied to an RDF graph delivers
> only additional triples (e.g., additional subClassOf triples),
> but would never remove anything.

Well, not necessarily.  Nonmonotonic here roughly means that if R1 is
smaller than R2 then esem(R1) is smaller than esem(R2), which you certainly
don't get as a consequence of this definition.

I any case, what does this have to do with the RDF semantics at all?

> Then a way to define CWA to extended semantics is by simply 
> doing  CWA(rdfschema(R));
> I think this works at least for some semantics - I don't think it works for 
> all, but it would be interesting
> to investigate where it works.

What is the CWA relationship here?  It certainly isn't a relationship
between triple sets!

> > > I was just talking about CWA on a graph.
> >
> >Hmm.
> >
> >What would the CWA make of the following graph?
> >
> >rdf:type rdf:type rdf:Predicate .
> >rdf:type rdfs:domain rdfs:Resource .
> >ex:john rdf:type ex:Person .
> >Would it require that there be no domain element for, for example,
> >rdfs:Class?
> given a simple interpretation, yes - no domain element in RDFS:Class.
> Given an RDF interpretation, the answer is still "no domain element", since
> all that an RDF interpretation adds are the "axiomatic triples" -
> (the RDF semantic conditions don't add anything).
> Is this agreeable?

I don't see how this can be suitable.  

> If yes, one could try to go one step higher on the semantics ladder and
> investigate what an RDFS interpretation would add.
> Taking the definition from above, one would try to compute the extended 
> semantics
> and then apply CWA to it. Do you see obvious problems with that?

Yes, certainly.  First, how can you determine whether to do the RDF or the
RDFS (or the OWL) CWA?  Second, the RDF and RDFS CWA of any graph are going
to be OWL-inconsistent.

> 
> >What would the CWA make of the folowing graph?
> >
> >ex:Student rdfs:subClassOf _:x .
> >_:x rdf:type owl:Restriction .
> >_:x owl:onProperty ex:sid .
> >_:x owl:allValuesFrom xsd:integer .
> Interpreting _:x as a skolem constant
> and the rest as simple triple data I don't see the problem?

Well, first, how does the CWA work on _:x.  Suppose, for example, there are
two blank nodes that look the same.  Is the CWA going to force them to
denote different domain elements?  (For that matter, what about two URI
refs that have the same information attached to them?)

> >In particular, would it require that there be no domain element for _:y in
> >the following.
> >
> >_:y rdf:type owl:Restriction .
> >_:y owl:onProperty ex:sid .
> >_:y owl:allValuesFrom rdfs:Literal .
> Again, just using an RDF interpretation from the model theory I don't see 
> the problem.
> Of course if I apply an OWL interpretation things look differently.
> But maybe one should first start simple.

Well, simple starting points that prevent future growth are certainly not
preferred, and this is what I am seeing here.

> Am I missing anything?

Well, your proposal is certainly missing something.  I don't see how it can
work well, if at all.

> >I forsee many similar problems in applying a CWA without use of semantics.
> 
> Could you explain them?

Well, without a semantics I don't see that you have any connection to RDF
at all.  The (beginnings of a) semantics that you provide above still
doesn't have much connection (if any) to RDF.  How do you reconcile how
your CWA works with how the RDF semantics works?  I've given a couple of
examples, but there are many more.  It appears to me that you will have to
mirror all the RDF (or RDFS or OWL) inferences to make your scheme work,
and even then I don't see how you actually get a CWA out of your scheme at
all.

> Best,
>          Stefan

peter
Received on Thursday, 20 November 2003 15:09:22 UTC

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