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Re: Rules WG -- draft charter -- NAF

From: Stefan Decker <stefan@ISI.EDU>
Date: Fri, 21 Nov 2003 16:51:05 +0000
Message-Id: <5.2.1.1.2.20031120201650.05dbe8f8@nitro.isi.edu>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
Cc: www-rdf-rules@w3.org

At 08:09 PM 11/20/2003, Peter F. Patel-Schneider wrote:
>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
> >        esem(R) \subseteq R, R \in pow(T)
>
> > 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),

Yes.

>which you certainly
>don't get as a consequence of this definition.

Actually, you do.
Every esem is a continuous function on a complete lattice.
Every continuous function on a complete lattice is monotonic.


>I any case, what does this have to do with the RDF semantics at all?
I try to explain what I means by a semantics - like RDF Schema and what it 
could mean
to apply the CWA on a semantics of a datamodel.
In a way I would like to clearify the confusion around CWA applied to RDF, 
RDF Schema, etc.
Let me know if this is of no interest to you.


> > 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 had Ray Reiters definition in mind, modified to the RDF case

CWA(A) =  A \union {\neg t \mid t is a triple and is not contained in A))

//since RDF Interpretations add triples we need to take these additional 
triples into account as well -
// so there is a sloppyness here, which is not a big problem.


> > > > 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.

Arguments?


> > 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.

Good question. One way is simply to say: the user knows which semantics to 
apply.
another possibility is that RDF Graphs carry metadata which indicates which 
semantics applies.


> >
> > >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?)
These two questions are not related - there can be an equality theory 
applied before CWA
(as another esem).
But yes, blank nodes are interesting.
I think a Herbrand interpretation would be useful - that would mean, yes 
CWA would
generate different domain elements. Where could something go wrong with this?
You think otherwise?


> > >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.
Why?



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

> > >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.
See the definition of CWA above - together with the definition of an esem
it seems to me to provide a framework to discuss multiple semantics and
still are able to use the CWA.
For RDF semantics itselfs this works by just adding the axiomatic triples 
to the model
For RDF Schema you add the additional triples coming from the RDF Schema 
semantics.
The CWA can be applied the the graph be queried.

Best,
         Stefan




> > Best,
> >          Stefan
>
>peter



--
http://www.isi.edu/~stefan
Received on Friday, 21 November 2003 11:52:50 UTC

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