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Re: RDF and OWL compatibility

From: Dave Reynolds <der@hplb.hpl.hp.com>
Date: Thu, 05 Jan 2006 14:51:44 +0000
Message-ID: <43BD3280.7000004@hplb.hpl.hp.com>
To: Jos de Bruijn <jos.debruijn@deri.org>
CC: public-rif-wg@w3.org

Jos de Bruijn wrote:

> I think it is important to distinguish here between (a) rules with RDF
> in the antecedent (body) and (b) rules with RDF in the consequent
> (head). 
> 
> Rules of type (a) are relatively easy to deal with, both in a classical
> setting and an LP setting.
> 
> Rules of type (b) could become a bit tricky, if we were to allow bnodes
> and RDF vocabulary (as pointed out in the wiki).

Some of our use cases involve use of rules for RDF transformation and so 
need RDF in the head, including RDF vocabulary (at least things like 
rdf:type). So simply outlawing such rules doesn't seem like a viable option.

On the question of RDF vocabulary in the head, the only description on the 
wiki of problems with that seems to be:

[[[
However, here is the first case where the first treatment of RDF 
meta-modelling becomes truely problematic. Consider, for example,

       p(c,a) .
       q(c,b) .
       rdfs:domain(X,Y) <- p(c,X), q(c,Y) .
       a(d,e) .

where the two uses of a (as a constant and as a property) are interrelated. 
It is possible to use a different treatment, employing a "holds" predicate, 
but this is not particularly appealing.
]]]

That difficulty doesn't seem to arise with a triple(s,p,o) representation 
or if a higher order syntax is available for expressing the RDFS closure 
rules. Perhaps you/Peter could elaborate on the "not particularly 
appealing" comment.

On the question of bNodes in the head, I hear the argument that it is not 
sufficient to just treat these as new Skolem constants but my intuitive 
understanding of the issue is too weak. It would be really helpful if 
someone could construct a test case which demonstrates the difference in 
results that arise between correct treatment of bNodes in the head versus 
treatment as Skolem constants. In the concrete cases I've seen where bNodes 
are used in the head of rules they seem to be intended as a form of 
anonymous gensym - so the Skolem constant semantics may be the more 
practically useful interpretation.

> Additionally, as became apparent during the discussion on OWL
> compatibility in San Francisco, we will most likely need two different
> semantics for our language in phase 1. One semantics based on (1)
> classical models and one semantics (2) based on minimal models.
> This might require two different (but I guess similar) RDF mappings.

Possibly but I'm not sure that the choice of semantics correlates so 
strongly with the choice of RDF mapping. The mapping seems to correlate 
more strongly with the availability of a higher order syntax.

Dave
Received on Thursday, 5 January 2006 14:53:06 GMT

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