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Re: A slight issue with datatypes in OWL 2 RL

From: Zhe Wu <alan.wu@oracle.com>
Date: Fri, 23 Jan 2009 12:13:42 -0500
Message-ID: <4979FAC6.7080704@oracle.com>
To: Ivan Herman <ivan@w3.org>
CC: Boris Motik <boris.motik@comlab.ox.ac.uk>, "'W3C OWL Working Group'" <public-owl-wg@w3.org>

Hi,

I agree with Ivan on this. Actually, in Oracle's comments to the LC, 
there is one item asking to remove owl:Rational.
Easier implementation (or ideally easy implementation :)) is important 
to implementers.

Thanks,

Zhe

Ivan Herman wrote:
> Each new datatype has to be implemented somehow (interpretation of the
> lexical space rules, relationships to other, etc, etc). The
> implementation of any of those can be non-trivial. So we are just adding
> new and new non-trivial things.
>
> Ivan
>
> Boris Motik wrote:
>   
>> Hello,
>>
>> 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.
>>
>> Regards,
>>
>> 	Boris
>>
>>     
>>> -----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
>>> PGP Key: http://www.ivan-herman.net/pgpkey.html
>>> FOAF: http://www.ivan-herman.net/foaf.rdf
>>>       
>
>   
Received on Friday, 23 January 2009 17:14:33 GMT

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