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From: Pat Hayes <phayes@ihmc.us>
Date: Tue, 12 Sep 2006 17:19:24 -0500
Message-Id: <p06230906c12cdfbab857@[]>
To: Bijan Parsia <bparsia@cs.man.ac.uk>
Cc: RDF Data Access Working Group <public-rdf-dawg@w3.org>

>I'm raising this mostly on behalf of Jorge 
>Pérez, Axel Polleres, and myself, though we all 
>have slightly different perspectives. I
>"""One may test that a graph pattern is not 
>expressed by specifying an optional graph 
>pattern that introduces a variable and testing 
>to see that the variable is not bound. This is 
>called Negation as Failure in logic 
>The main concern is that if we are going to allow NAF (which we currently do)

Well, wait a minute. I don't think that we do. 
And I would prefer that the spec not mention NAF 
this explicitly. Just checking that something is 
not present in the KB does not, in itself, 
constitute NAF reasoning. What makes it NAF is to 
infer from its absence that it is false, and 
*that* isn't sanctioned either by the RDF or 
SPARQL semantics, although to be fair its also 
not explicitly ruled out by them either. It seems 
to me that being able to detect non-binding - 
which I agree is too useful for us not to provide 
it - is what might be called NAF-neutral: it 
allows someone to implement NAF if they want to, 
but it does not of itself sanction it 
semantically. Which, I suggest, is exactly the 
stance we should adopt, since RDF is also neutral 
on the matter. It is possible to have an RDF 
semantic extension which would make NAF valid, as 
well as those like OWL that do not.

>, then we should allow for it in a more 
>convenient form, e.g., a not or \+ operator. I 
>think if we stick with it in this limited form, 
>then we should call it out better as it's a 
>pretty fundamental (though very useful) shift in 
>SemWeb philosophy at the W3C.

I don't think that what we have now amounts to a 
shift of policy. I agree we should state it 
differently, and more carefully, than we 
currently do, however.


>We might also want to coordinate with the RIF 
>about it as they will almost certainly be 
>defining NAFy operators.
>The other option is to kill bound and unbound. 
>Jorge claims that bound is never useful, i.e., 
>never alters results; I haven't worked that out 
>myself yet. Jorge also claims "some of the hard 
>results of complexity are heaviily involved with 
>the use of !bound", which isn't *too* surprising 
>as  nonmon typically raises complexity.
>I welcome pointers to past discussions.

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Received on Tuesday, 12 September 2006 22:19:42 UTC

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