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A slight issue with datatypes in OWL 2 RL

From: Boris Motik <boris.motik@comlab.ox.ac.uk>
Date: Thu, 22 Jan 2009 17:32:02 -0000
To: "'W3C OWL Working Group'" <public-owl-wg@w3.org>
Message-ID: <6381F3D061344FCB92523C40BF2A83FF@wolf>


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!


Received on Thursday, 22 January 2009 17:32:51 UTC

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