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Re: Web Rule Language - WRL vs SWRL

From: Michael Kifer <kifer@cs.sunysb.edu>
Date: Thu, 30 Jun 2005 21:51:15 -0400
To: Ian Horrocks <horrocks@cs.man.ac.uk>
Cc: www-rdf-rules@w3.org
Message-Id: <20050701015116.28BF0CB5D3@kiferserv.kiferhome.com>


Ian Horrocks wrote:
>
> One reason seems to be the following (potentially infinite) sequence of 
> argumentation:
> 
> Ian: DLP (with FO semantics as per the paper) and DLP-Datalog (i.e., 
> with Datalog semantics) are not equivalent (they have different 
> models), but are indistinguishable w.r.t. entailment of ground DLP 
> atoms. The can, however, be distinguished if we consider other kinds of 
> entailment (i.e., richer query languages).
> 
> **
> 
> Michael:  DLP and DLP-Datalog are equivalent if we restrict our 
> attention to entailment of ground DLP atoms.
> 
> Ian: Yes, they are *indistinguishable* if we restrict our attention to 
> entailment of ground DLP atoms, but not if we consider other kinds of 
> entailment (i.e., richer query languages).
> 
> Repeat from **

Good. At least we agree on where the cycle is.

> > 2. The claim is that both stacks extend DLP *as languages* -- 
> > semantically
> >    and syntactically (the latter after some mapping, of course).
> >    We use the standard definition of language extension here (which I
> >    have spelled out in a previous message).
> 
> Your "standard definition" assumes a common underlying semantic 
> framework, which is not the case here.

Good. So now we agree that LP does extend DLP modulo the "common underlying
semantic framework"; see below.

> E.g., consider DLP, OWL-Lite, 
> OWL-DL, SWRL and FOL. All share the same set of models; the only 
> difference is the richness of the language that is available for 
> restricting the set of admissible models. A DLP theory (set of rules) 
> would have the same set of modes if treated as OWL, SWRL or FOL. As we 
> have seen, this is *not* the case for DLP and Datalog.

But you agree that there are embedding homomorphisms (both over models and
syntax), which form a commutative diagram. This is good enough for me.
I did explain this in my email where I defined what we mean by extension.

As far as I am concerned, this difference is a trivial matter. Embedded
objects are commonly identified with their images in mathematics.
If you think that this embedding is a big deal, you are entitled to disagree.


> > 3. The interoperability part is only alluded to in the diagram under 
> > the name
> >    "logical framework". (I wonder who proposed he term "logical 
> > framework"
> >    -- this term is much better than what we had in the original 
> > diagram.)
> >
> >    The interoperability framework uses the black box architecture 
> > similar
> >    to AL-Log, Eiter et al., etc.
> 
> We all seem to agree on the desirability of maximising interoperability 
> and developing a suitable logical framework. Some of us argue that we 
> should do this *first*, i.e., we should establish some planning 
> regulations before sanctioning a free for all on tower construction.

What we don't agree about is that this should precede the development of an
LP language for the web.


> >>> People think databases but use OWL.
> >>> Now, this is a real semantic mismatch: people mean (and want!) one
> >>> thing,
> >>> but get a completely different thing.
> >>
> >> You need to get out more. I meet and interact with many users who are
> >> building large and complex ontologies, and who have a very good grasp
> >> of the semantics of OWL. Hopefully you caught the presentation from
> >> Christine Golbreich at the rules workshop and have been keeping up 
> >> with
> >> the work being done in the SWBP working group. You should also look at
> >> what is being done by NCI, SNOMED and the Gene Ontology Consortium
> >> amongst many others.
> >
> > Thanks for the advise.  I am trying to get out whenever I can. And you
> > should also try to visit places outside of your immediate neighborhood.
> >
> > I am not involved in any of the aforesaid great projects, but 
> > occasionally
> > I do run into interesting articles
> >     http://ontology.buffalo.edu/medo/NCIT.pdf
> >     
> > http://www.ipsi.fraunhofer.de/orion/pubFulltexts/NCIReview18Feb04.pdf
> > which raise questions about the use of OWL for NCIT.
> 
> Of course there will always be questions and criticism aimed at any 
> large project of this kind, but the fact is that many people *are* 
> using OWL, and that many of them have a very good understanding of the 
> semantics of OWL. It is condescending, not to say insulting, to suggest 
> otherwise.

I didn't realize that those papers are insulting. I thought that they are
quite thoughtful and point to real problems. What those papers say is that
people do make mistakes even if they understand the semantics. It is just
too hard to state certain things correctly, and some things are possibly
missing.  All this is even more telling given that the NCIT project enjoys
the benefit of substantial involvement of outside KR experts.

When I "go out," I sometimes run into people who don't enjoy those benefits
and see that they are grappling with much more basic things (typically
thinking and wanting database-style semantics---read CWA---while using OWL).
Cardinality restrictions is usually the misunderstanding #1.


> >>> SWRL is complex in a wrong way. How many people use it in a way
> >>> where programs come out substantially non-Datalog? (I.e., where one
> >>> can't accomplish the same thing in an LP-style language in a simpler 
> >>> way?)
> >>
> >> See my comment above - more than are drempt of in your philosophy.
> >
> > We are talking about SWRL here, not OWL, are we?
> > Can you please point me to the use of SWRL in NCI, SNOMED, etc., which
> > requires the complexity of SWRL? (Something that goes beyond, say, the
> > simple AL-Log?)
> 
> Now it is *my* turn to suggest that *you* haven't been paying 
> attention. 


Appreciate your help with keeping my attention focused.


> The presentation by Christine Golbreich at the rules 
> workshop included several such examples. In medicine (and in many other 
> domains where complex structures must be modelled) it is often 
> important to capture situations such as "a fracture of the shaft of the 
> femur is a kind of a fracture of the femur". This can be expressed in 
> SWRL (using of axioms asserting complex relationships between binary 
> predicates) but not in OWL.

I didn't say "show me the benefit of SWRL with respect to OWL."  I said,
show me a use case "where one can't accomplish the same thing in an
LP-style language in a simpler way" and "point me to the use of SWRL ...
which requires the complexity of SWRL? (Admittedly, by mentioning AL-Log I
was aiming too low.)

With no intention to detract from Christine's excellent project, I must say
that the use of the rules there is LP-trivial. My aforesaid request was
to see a realistic use case where people would need to use the truly
complex parts of SWRL, such as descriptions in the head.


	--michael 
Received on Friday, 1 July 2005 01:51:36 GMT

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