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RE: A slight issue with datatypes in OWL 2 RL

From: Boris Motik <boris.motik@comlab.ox.ac.uk>
Date: Fri, 23 Jan 2009 09:40:39 -0000
To: "'Ivan Herman'" <ivan@w3.org>
Cc: "'W3C OWL Working Group'" <public-owl-wg@w3.org>
Message-ID: <2ACE712A78EA4E8E80DF9FA00DDEF33A@wolf>


I agree that the implementation of datatypes is nontrivial in OWL 2 RL. This, however, is already the case for the existing
datatypes. I really cannot see how the datatypes that we left out would make the implementation any harder.



> -----Original Message-----
> From: Ivan Herman [mailto:ivan@w3.org]
> Sent: 23 January 2009 08:51
> To: Boris Motik
> Cc: 'W3C OWL Working Group'
> Subject: Re: A slight issue with datatypes in OWL 2 RL
> Boris,
> before we do this... let me just raise this issue: just because we _can_
> does not meant that we necessarily _should_.
> In my view, one of the goals of RL is a possibility for an easy
> implementation, too. With my limited implementation experience the
> datatype handling of RL is by far the most complex part of an
> implementation. Sure, if one goes for a very efficient implementation
> then taking care of things like owl:sameAs becomes also more complex,
> but that is not 100% necessary for a compliant thing. Datatype handling
> is. (I actually did not even have the time to implement it, I just rely
> on the underlying RDF/Python environment and do whatever it can do. I
> can see many implementations doing just that.) Oracle has already
> indicated that they are not really in favour of an owl:rational
> inclusion in OWL RL, and I think their reaction reflects the same concerns.
> Based on this I actually do _not_ believe that this is just an editorial
>  comment but would definitely warrant a new LC round because it would
> significantly add to the complexity of implementations. My personal
> interpretation (maybe wrong!) of that comment in the document is that
> some datatypes (like rational) may actually be dropped from the list and
> not add all other datatypes blindly...
> My 2 cents...:-)
> Ivan
> Boris Motik wrote:
> > Hello,
> >
> > Here is a Last Call comment about datatypes in OWL 2 RL. This issue was pointed out by Jos de
> Bruijn during the RIF integration
> > meeting, and I remembered it today after a private discussion about datatypes with Zhe. Thanks to
> both of them!
> >
> > Currently, OWL 2 RL disallows certain datatypes on the grounds that reasoning with them would not
> be polynomial. Now we could
> > actually relax this restriction and allow all OWL 2 datatypes to occur in OWL 2 RL ontologies.
> >
> > This is actually an oversight of mine, caused by the following technical issue. OWL 2 EL and OWL 2
> QL have existential quantifiers;
> > hence, you can state existence of concrete objects whose values is not known precisely. But then,
> if you allow combinations of
> > datatypes such that the intersection of possibly negated datatypes is finite, you really do get
> into problems: your reasoning
> > suddenly becomes NP-hard because you need to start guessing the appropriate value of existentially
> implied object. To prevent this
> > from occurring, I selected the set of allowed datatypes in OWL 2 EL such that each intersection of
> possibly negated datatypes is
> > either empty or infinite; then, I merely copied this set to all the profiles.
> >
> > As Jos rightly pointed out at the RIF integration meeting, however, OWL 2 RL *does not* have
> existential quantifiers; consequently,
> > the value of each concrete object is fully known. But then, there is no need to actually restrict
> the set of datatypes: to support a
> > datatype, you just need a procedure that recognizes whether some literal is in the range of a
> particular datatype (which is easy to
> > do for all of OWL 2 datatypes).
> >
> >
> > The fix to this comment would be to revise the datatypes section for OWL 2 RL and allow all OWL 2
> datatypes to occur in OWL 2 RL
> > ontologies. Since we already have a note saying that the set of supported datatypes might change, I
> believe that this change would
> > not warrant another Last Call round.
> >
> > I'm really sorry about this oversight!
> >
> > Regards,
> >
> > 	Boris
> >
> >
> --
> Ivan Herman, W3C Semantic Web Activity Lead
> Home: http://www.w3.org/People/Ivan/
> mobile: +31-641044153
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Received on Friday, 23 January 2009 09:41:30 GMT

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