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Re: Proposed change to the OWL-2 Direct Semantics entailment regime

From: Guido Vetere <gvetere@it.ibm.com>
Date: Mon, 6 Dec 2010 21:58:47 +0100
To: Bijan Parsia <bparsia@cs.man.ac.uk>
Cc: public-rdf-dawg@w3.org
Message-ID: <OFDF2E85D7.C7B85F3F-ONC12577F1.006B8A2C-C12577F1.00733FBA@it.ibm.com>
> Bijan Parsia <bparsia@cs.man.ac.uk> 

> > We don't use SPARQL as a query language (we adopt a datalog-style 
> > syntax instead) but we might support (some) SPARQL as a front-end in
> > the future, as long as it does not misses relevant features. I 
> > cannot tell if making all variable distinguished would definitely 
> > prevent form covering some relevant use case.
> 
> That's unfortunate. It would be really helpful to find some cases 
> where users would *notice* the difference. Best is when they would 
> rely on non-distinguished variables.
 
Maybe the average user would hardly understand the difference, but we know 
about that difference, we know that it may show up in some cases and we 
should understand (in advance) if it matters for customers. I cannot 
honestly tell about concrete cases we experimented with, because we didn't 
consider how our queries would have been answered without the features we 
actually support. But we can try to go through some of the use case we've 
run and make some simulation. In this case, we'll be back to you. 
 
> I hope I don't give offense by asking the following clarificatory 
> question: Do you really mean variables which range over unnamed 
> individuals, or do you just mean variables which are projected away 
> (in the Datalog world, these coincide as there are no unnamed 
> individuals; hence my question)?

We don't have unnamed individuals, we may have generated names. 

> > provided that supporting this feature would be up to implementers.
> > After all, not all SPARQL features are going be supported by all 
> > implementers, I guess. 

> The problem is that nondistinguished variables get prohibitively 
> harder as you raise the expressiviety of the logic. Ideally, we 
> would like to make it easy for a user to port a query from an RDF 
> engine to a OWL QL engine to an OWL DL engine and get compatible 
> results (if the engines all support all of SPARQL). Nondistinguished
> varibles makes that impossible.

I understand your point. Maybe this in naive, but why not consider a sort 
of SPARQL layering, much like OWL does? 

> On the flip side, would you find it extremely burdensome to add 
> nondistinguished variables as an extension? I see you already depart
> from the OWL spec by imposing the UNA, would this departure 
> discourage you from implementing SPARQL at all?

Not at all, the point is that we would miss a feature that we would 
otherwise support.
 
> Thanks very much for your time.

It's a pleasure.

    
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Received on Monday, 6 December 2010 20:59:22 GMT

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