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Re: Web Rule Language - WRL vs SWRL

From: Michael Kifer <kifer@cs.sunysb.edu>
Date: Tue, 28 Jun 2005 15:09:41 -0400
To: Ian Horrocks <horrocks@cs.man.ac.uk>
Cc: Jos de Bruijn <jos.debruijn@deri.org>, Holger Wache <holger@cs.vu.nl>, Jim Hendler <hendler@cs.umd.edu>, dreer@fh-furtwangen.de, public-sws-ig@w3.org, www-rdf-rules@w3.org, Bijan Parsia <bparsia@isr.umd.edu>
Message-Id: <20050628190941.CA7E8CB5D3@kiferserv.kiferhome.com>


> 
> Sorry to be late joining the party (I take a vacation and world war 
> three breaks out!), but I feel compelled to add my "twopennyworth" (as 
> we say). Much of this has been said before (by Jim, Bijan and Holger), 
> but I will try to summarise...

Hi Ian,
welcome to the party :-)


> Regarding DLP under Horn (equivalently DL/FOL) and LP semantics, it is 
> true that *under certain assumptions* the two semantics are 
> indistinguishable, but this does *not* mean that they are equivalent. 
> The assumption being made in this case (as Bijan has pointed out) is 
> that only ground entailment will be considered. This is a rather strong 
> assumption: it is common to compensate for weak representation 
> languages (such as DLP) by providing much more expressive query 
> languages (e.g., the case of SQL), and with more expressive languages 
> we can easily distinguish two different semantics (as illustrated in 
> [1]).
> 
> This is particularly important in a layered architecture, where the 
> increased expressive power of higher layers will naturally allow the 
> two different semantics to be distinguished. This was the main point of 
> [1], i.e., that we should not fool ourselves about the degree of 
> interoperability that would be provided by the architecture proposed in 
> the so-called "updated layer cake". Such interoperability only exists 
> so long as neither tower extends above the DLP level *and* so long as 
> we only allow for very weak query languages; DLP does not, therefore, 
> provide a suitable foundation for a layered architecture, as higher 
> layers (or more expressive query languages) might make false 
> assumptions about the intended semantics of DLP ontologies.

This wasn't clear from the paper.
In any case, the claimed interoperability doesn't extend to the more
powerful languages. 

Your argument is well-taken. However, if you are querying an ontology with
an LP language then you expect that CWA will be applied in the current
state.
That is, if your DLP ontology says that John has one child, Bill, and
nothing else, then your query "get all people who have exactly one child"
(a query like the one in your paper) is expected to return John, since in
the current state he is not known to have more children.

There will be certain amount of semantic mismatch, but this most likely
won't matter. From talking to people I get the impression that
they build very simple ontologies using OWL. Most of them are in DLP with
some use of cardinality restrictions. However, when I press, it turns out that
people don't understand what cardinality restrictions mean in OWL. They
think they are like cardinality constraints in databases.

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.


> Regarding the existing layered architecture of OWL, and the proposed 
> extension to SWRL, nobody (well not me anyway) claims that it is 
> perfect, and there will (as Holger has pointed out) no doubt be some 
> compatibility issues between different tools, but the situation is 
> hardly comparable to the one that is being proposed in the "updated 
> layer cake": as we have seen, even the very limited degree of 
> interoperability suggested by this diagram turns out to be a chimera. 
> In contrast, RDF, OWL and SWRL share a common semantic framework, and 
> allow for a relatively clean layering: OWL-Lite, OWL-DL and SWRL are 
> layered on a subset of RDF, but share the same semantics (and it can be 
> syntactically determined when RDF ontologies are within this subset).

First of all, I disagree that RDF has the same semantics. It is just a
language of facts. It all depends on the query language that you use on top of
RDF.  (E.g., the relational model can be queried with SQL or first-order
logic---two languages with very different semantics.)

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.


> Regarding the complexity of SWRL, yes it is a complex language, but as 
> seen from presentations at the rules workshop (and as pointed out by 
> Holger), some users *need* this level of complexity. For them, it is 
> essential to have a more expressive language that layers on top of OWL. 
> Should we thus forge ahead with the development of *two* fundamentally 
> incompatible rules languages? Surely (as Jim has pointed out) we can do 
> better than that, even if it requires a little more research effort!

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?)


	--michael  



> Regards,
> 
> Ian
> 
> [1] 
> http://www.cs.man.ac.uk/~horrocks/Publications/download/2005/HPPH05.pdf
> 
> 
> On 23 Jun 2005, at 14:28, Bijan Parsia wrote:
> 
> >
> > On Jun 23, 2005, at 3:06 AM, Jos de Bruijn wrote:
> >
> >> -----BEGIN PGP SIGNED MESSAGE-----
> >> Hash: SHA1
> >>
> >> Bijan Parsia wrote:
> > [snip]
> >>> Key is the slipping in of "ground".
> >>
> >> No. For query answering only ground entailment is relevant.\
> >
> > Interesting presumption.
> >
> >>>> entailments for both
> >>>> semantics are *equivalent* and thus the queries would return the 
> >>>> same
> >>>> result.
> >>>
> >>>
> >>> Of course, RDF entailment includes existential generalization, so
> >>> that's not quite right. There seems to be more work that you need to
> >>> do to get what you wanted (e.g., you need to look at the semantics
> >>> of the query language; is the query "not" classical? how would that
> >>> classical not interact with the LP semantics?)
> >>
> >> The RDF language contains existentials and I'm not claiming that this
> >> can be done by a rule language.
> >> We are talking about Horn Logic and Horn Logic does not have
> >> existentials! I never claimed this!
> >> You claimed
> >
> > Please point to where I claimed this.
> >
> >>  that a Horn formula under FOL semantics has other ground
> >> entailments than a Horn formula under LP semantics and this is simply
> >> not true.
> >> I think this can conclude our discussion on this topic.
> > [snip]
> > Oh, I *quite* agree. But perhaps not for the same reasons.
> >
> > Cheers,
> > Bijan.
> >
> >
> 
> 
> 
Received on Tuesday, 28 June 2005 19:11:32 GMT

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