W3C home > Mailing lists > Public > www-rdf-rules@w3.org > June 2005

Re: Web Rule Language - WRL vs SWRL

From: Michael Kifer <kifer@cs.sunysb.edu>
Date: Wed, 29 Jun 2005 23:11:33 -0400
To: Ian Horrocks <horrocks@cs.man.ac.uk>
Cc: Bijan Parsia <bparsia@isr.umd.edu>, Jim Hendler <hendler@cs.umd.edu>, www-rdf-rules@w3.org, dreer@fh-furtwangen.de, Jos de Bruijn <jos.debruijn@deri.org>
Message-Id: <20050630031133.319C6CB5D3@kiferserv.kiferhome.com>

Ian Horrocks wrote:
> On 29 Jun 2005, at 22:37, Michael Kifer wrote:
> [snip]
> >>>>> N3 is essentially a different syntax for F-logic and its extensions
> >>>>> (but
> >>>>> N3's semantics is defined by use cases ;-). As far as I can tell,
> >>>>> with
> >>>>> each
> >>>>> new presentation that I hear N3 is moving in the direction of LP.
> >>>>
> >>>> I think that we should stick to discussing how things actually *are*
> >>>> rather than directions in which you hope/believe they might be 
> >>>> moving.
> >>>
> >>> I am discussing things as they already are. N3 now has a form of 
> >>> SNAF.
> >>
> >> Maybe, but this doesn't make it LP - we have long since known how to
> >> support a form of SNAF in DL using the so-called K operator.
> >
> > What makes something an LP language?
> You tell me - you are the one who is claiming that things are "moving 
> in the direction of LP".

Let me remind you that 5 lines above you said
"this doesn't make it (N3) LP". So, by CWA (albeit not FOL) you must have a
definition of LP in mind.
Now, 6 lines above the current line I asked you what is your definition?
Instead of answering you ask me back. We are having a hell of a
conversation here.

> >> So, an LP language would find an entailment that is *not* supported by
> >> RDF semantics (which would allow for models in which John had other
> >> children). Ergo, LP is semantically incompatible with RDF.
> >
> > You misunderstood my reply.
> > I meant that I acknowledged your point about more expressive languages
> > not giving the same answers when they are both applied to the same 
> > dataset.
> >
> > RDF by itself doesn't have any queries defined over it, so it is
> > meaningless to claim that a query language L over RDF syntax is
> > incompatible with RDF.
> This is not what is being claimed. What is being claimed is that an RDF 
> triple of the form <x,P,y> is *not* semantically equivalent to the LP 
> rule P(x,y). This is the equivalence that is implicit in the layering 
> that we have been discussing (i.e., LP on top of DLP on top of RDF), 
> and which leads to LP entailments that are not valid w.r.t. the 
> semantics of RDF (or DLP).

Yes, they are equivalent within the RDF subset.*
Withing this subset, the only queries that you can ask is whether a set of
triples (bodyless rules) imply another triple (bodyless rule).

*Modulo the blank nodes. The post-facto RDF semantics treats blank nodes as
 head-existential, which is outside of LP. But there is another, LP-style
 semantics for blank nodes.


PS. By saying that your point is well-taken, I acknowledged consistency of
    your argument about two language extensions being incompatible with
    each other.
    But I was talking about a different definition, that of one language
    being an extension of another. This is a standard definition and
    according to it LP is an extension of DLP, and DLP is an extension of
    RDF (modulo the blanks).
Received on Thursday, 30 June 2005 03:13:27 UTC

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