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Re: Issue-130 (conformance)

From: Sandro Hawke <sandro@w3.org>
Date: Wed, 03 Sep 2008 20:49:36 -0400
To: Ian Horrocks <ian.horrocks@comlab.ox.ac.uk>
cc: W3C OWL Working Group <public-owl-wg@w3.org>
Message-ID: <20903.1220489376@ubuhebe>

> The idea we came up with on the teleconf was to say that  
> implementations MUST issue a warning when they can't be sure about a  
> False answer, and to add that implementations not wanting to check  
> the conditions could simply return Unknown instead of False. On  
> closer examination, this doesn't make total sense and/or seems  
> unnecessarily complicated.
> In the first place, it is strange to say that implementations MUST do  
> something, and then say that they can do something else instead. In  
> the second place, it doesn't seem to make much sense to return False  
> with a warning that this answer can't be trusted -- surely it would  
> be much more sensible to return Unknown under these circumstances.  
> This would allow the conformance conditions on OWL RL checkers to be  
> greatly simplified, i.e., to say that they can only return False when  
> the entailment doesn't hold (just like for the other checkers). We  
> could then add a note for implementers pointing out that Theorem 1  
> tells them just when it is safe for rule-based implementations to  
> return False (when the conditions are satisfied and the entailment  
> isn't found by the rules). The note can also say that if they don't  
> want to implement the check they can simply avoid returning False.
> To make this clearer/more concrete, I implemented this in the  
> Conformance document [1].
> [1] http://www.w3.org/2007/OWL/wiki/Conformance

I agree with all this.  But I see there's no longer a difference between
OWL Full and OWL RL entailment checking.   Hmmm.

Maybe the difference is that OWL 2 RL entailment checkers SHOULD return
True if FO(O1) <union> R entails FO(O2)?    I guess that's what I'd
expect them to do.

> Still to do: add something about what this would mean in the case of  
> query answering, to the effect that a warning MAY/SHOULD/MUST be  
> issued if "Unknown" would be the answer to any relevant entailment  
> problem.

To phrase it more positively, maybe: 

    Systems which do query answering using OWL SHOULD return an
    indication of whether the performed OWL reasoning was complete or

Is this practical?    I'm not sure.  

I personally like query interfaces to provide this information, but I
realize they often don't.   (It comes up without inference when you use
something like a LIMIT clause.)  

In SPARQL (which doesn't provide this information), we can do an in-band
query for reasoner metadata like this:

   SELECT ?completeness [...your vars...]   {
        ?g1 { [...your pattern...] } 
        { ?g1 owlq:completenessLevel ?completeness } 

I think this works (thanks to Eric Prud'hommeaux for working it out with
me), but it would require standardizing one RDF predicate (written here
as owlq:completenessLevel).  It's kind of an outlier -- it's certainly
not part of the OWL language -- but we could be defined in some normative

     -- Sandro
Received on Thursday, 4 September 2008 00:51:24 UTC

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