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Re: Web Rule Language - WRL vs SWRL

From: Ian Horrocks <horrocks@cs.man.ac.uk>
Date: Thu, 30 Jun 2005 11:59:59 +0100
Message-Id: <194a91ef664deaf74dca1b645d49a170@cs.man.ac.uk>
Cc: www-rdf-rules@w3.org
To: Michael Kifer <kifer@cs.sunysb.edu>

On 30 Jun 2005, at 06:35, Michael Kifer wrote:

>> So it is inappropriate to show LP and FO language towers
>> interoperating via a common DLP base - they would sink in and topple
>> over.
> Ian,
> We are going back and forth on this.


>  I suppose that this is so because you
> were traveling and didn't read all the messages.

But I don't agree that this is the cause of our going back and forth. 
One reason seems to be the following (potentially infinite) sequence of 

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 **

> Let me summarize briefly
> 1. I am not aware of anyone (among the people who proposed the 
> 2-stacks on
>    top of DLP) claiming that the two stacks *interoperate* through DLP.
>    I believe that this is your own extrapolation from nothing more than
>    just the appearance of the diagram (and maybe some vaguely 
> remembered
>    accompanying words--certainly not my words).

See my other email regarding the two stacks. Appearances can be 
deceptive, but they are non the less important - otherwise why bother 
with diagrams at all. The appearance of this diagram would be less 
deceptive if the two towers were separated at both the DLP and RDF 

> 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. 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.

> 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.

>>> 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 

>>> Second, DLP is not the only interoperability option. A more general
>>> one (and the one that I happen to like) treats the different 
>>> languages
>>> as black boxes, which can be queries according to the native 
>>> semantics
>>> of those boxes.
>>> I was actually surprised that you referenced Eiter et al. paper, 
>>> because in
>>> my view they argue in favor of the 2-stack architecture with a 
>>> defined
>>> interoperability layer based on the black box idea.
>> Now we are getting somewhere! Developing a framework that allows for
>> multiple language types to co-exist in the semantic web while
>> maximising interoperability is exactly what myself (and others) have
>> been arguing for! I believe that the Eiter work, as well as other work
>> by Motik, Rosati, Franconi, and others, provides a promising basis for
>> such a framework, but that some of the details still need to be worked
>> out.
> Nice that we appear to agree on something.
>>> 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. 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.


> 	--michael
Received on Thursday, 30 June 2005 11:00:05 UTC

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