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Re: Changes to make S&AS consistent with RDF Semantics document

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Thu, 04 Dec 2003 12:06:23 -0500 (EST)
Message-Id: <20031204.120623.41920433.pfps@research.bell-labs.com>
To: herman.ter.horst@philips.com
Cc: horrocks@cs.man.ac.uk, www-webont-wg@w3.org

From: herman.ter.horst@philips.com
Subject: Re: Changes to make S&AS consistent with RDF Semantics document
Date: Thu, 4 Dec 2003 17:42:20 +0100

[...]

> However, there still remain questions.
> Whether you like it or not, rdf:XMLLiteral is part of OWL, in the sense
> that each OWL DL consistency checker or OWL Full consistency checker that
> is worthy of the name should recognize that the RDF graph G that I 
> mentioned,
>      v p l
>      p rdfs:range rdfs:Literal
> where l is an ill-typed XML literal, is inconsistent. 

I dispute this claim.  I have an OWL DL reasoner that does not perform this
check.

> Whether XMLLiteral is in or out of the datatype map D, OWL DL entailment 
> as well as OWL Full entailment now always incorporates the meaning of 
> rdf:XMLLiteral, inherited from the RDFS Semantics. 

Sure, but so what?

> In the light of this, what do you mean with your statement above:
> >... rdf:XMLLiteral is not a required part of OWL.

The OWL documents imply that rdf:XMLLiteral is not a required part of OWL,
as indicated by the definition of OWL datatype maps.

> By leaving out of Section 3 the assumption about XMLLiteral, in the 
> current way, the abstract syntax-based, direct semantics is not 
> just another side of the same coin as the OWL DL semantics.
> It looks like the current editor's version of S&AS has three inequivalent
> versions of OWL semantics: OWL Full, OWL DL, and a third version that 
> might
> be described as 'abstract syntax OWL without XMLLiteral'.
> Each of these versions is labeled normative in S&AS.

Yes, to some extent.

> Doesn't this contradict the WG decision about semantic layering, to have
> two semantics versions, now called OWL Full and OWL DL?

Not really, at least in my opinion.

> One way to solve this would be to restrict the definition now in 
> Section 5.4 of OWL DL entailment to datatype maps with XMLLiteral,
> and to state another definition of OWL DL entailment for datatype maps 
> without XMLLiteral, in terms of the direct semantics and the mapping T,
> as this is a known case of conflict between the direct semantics
> and OWL DL semantics.
> This would realize the desire that XMLLiteral is not required in
> OWL DL.
> 
> Another, simpler, way to rule out the third version of the semantics 
> would be to adopt the suggestion (*) that I give above, in Section 3 
> of S&AS.

Yes, this *would* be possible.  However, it would be a *change* in the
definition of OWL.

> This is not to say that the changes should then be undone to the 
> formulations in Section 5 of the entailment definitions and the 
> correspondence theorem etc., I like them to be (and actually asked 
> to make them) completely explicit, in the current way.
> 
> ===
> 
> 
> >> 
> >> (Section 3.1: second bulleted condition:)
> >> >-It is now assumed that LV contains each Unicode string
> >> >and each pair of two Unicode strings.
> >
> >This should actually be weakened to pairs of Unicode strings and language
> >tags, or whatever the RDF model theory says.  Again this is something 
> that
> >has undergone recent change in the RDF model theory.
> >
> >> >For the correspondence with Section 5, it would be
> >> >sufficient to assume only that plain literals in
> >> >V (and L) are contained in LV.
> >> On further reflection, it seems that not only the assumption
> >> about plain literals but also the assumption about
> >> typed literals could be weakened.
> >> The condition could be rephrased, for example, as follows:
> >> "LV, the literal values of I, is a subset of R that
> >> contains the values of plain literals in V, and,
> >> for each datatype d in D and well-typed literal
> >> "v"^^d in V, the value L2V(d)(v)."
> >
> >Part of this would work, except that it would have to refer to the RDF
> >model theory to pick up the ``value'' for literals. 
> 
> This was indeed a slip.  Ian and I corrected it to become 
> essentially the following condition:
> (**) "LV, the literal values of I, is a subset of R that
> contains the values of plain literals in V, and, for each
> datatype d in D and v in  L(d) intersect V,  the value
> L2V(d)(v)."
> 
> After discussion with me, Ian adopted this definition in the 
> "PR version of S&AS" of Tuesday (with a corresponding phrase 
> in the beginning of Section 5).
> Yesterday, Peter sent a message to webont stating that this
> would change the OWL Full semantics and without further 
> discussion, within one hour, Peter, from outside the WG, 
> put the earlier condition back in, which was subsequently 
> confirmed by Ian.

As requested by Jim Hendler, I produced a revised version of S&AS.  I fixed
up the proofs in Appendix A and removed changes that changed the definition
of OWL.  

> That this would change the semantics, is indeed an objective
> reason for undoing the change.
> I can live with the current version of this definition. 
> It does not seem to be inconsistent with the RDF Semantics 
> document.
> (However, nobody has done a complete review of the proof
> appendix of S&AS, which might uncover such an inconsistency.)
> 
> The reason for proposing (**) was that a similar condition
> in the RDFS semantics led to a "great simplification" [1]
> of the proof theory, although this occurred in a situation
> without datatypes, apart from XMLLiteral.

I do not believe that the Web Ontology Working Group should produce a worse
semantics just because there were some perceived technical advantages to
the approach in the RDF Core Working Group.  First, there is no analogous
proof theory for OWL.  Second, OWL is much more powerful than RDF and thus
can uncover mistakes in the RDF model theory.  

> [...]
> 
> Herman ter Horst
> 
> [1] http://lists.w3.org/Archives/Public/www-rdf-comments/2003OctDec/0161.html

[...]

peter
Received on Thursday, 4 December 2003 12:06:57 GMT

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