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Re: RDF Semantics: corrections

From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
Date: Tue, 13 Jan 2004 19:03:47 -0500 (EST)
Message-Id: <20040113.190347.112804303.pfps@research.bell-labs.com>
To: phayes@ihmc.us
Cc: herman.ter.horst@philips.com, jjc@hpl.hp.com, hendler@cs.umd.edu, schreiber@swi.psy.uva.nl, connolly@w3.org, sandro@w3.org, www-rdf-comments@w3.org, bwm@hplb.hpl.hp.com

From: pat hayes <phayes@ihmc.us>
Subject: Re: RDF Semantics: corrections
Date: Tue, 13 Jan 2004 17:37:33 -0600

> >From: pat hayes <phayes@ihmc.us>
> >Subject: Re: RDF Semantics: corrections
> >Date: Tue, 13 Jan 2004 15:36:29 -0600
> >
> >[...]
> >
> >
> >>  For clarification, my question to you was concerned with the effects
> >>  on OWL rather than on RDF as such. I was not meaning to ask your
> >>  opinion on the change in a general sense: as indicated, I will take
> >>  responsibility for that. I was asking you only to give an opinion on
> >>  whether the suggestion made by Herman for a 'protective' modification
> >>  to the *text* of the OWL document is indeed sufficient to ensure that
> >>  the RDF and OWL specs, with these changes, will still be compatible.
> >>  The intention is that this will make no change to OWL itself.
> >
> >Well, you are really asking for us to approve both the change to RDF and
> >related change to OWL to insulate OWL from the effect of the change to
> >RDF.  From what I see about the changes I would vote against both.
> 
> No, any changes or otherwise to RDF will be the result of decisions 
> made by the RDF WG.  What I was really asking your opinion of was the 
> insulation device proposed by Herman.  There are three options:
> 
> 1.  do nothing
> 2.  this change is made to RDF and a textual edit in OWL insulates OWL from it
> 3. This change is made to RDF and OWL is damaged as a result
> 
> I am anxious to avoid 3.  I know you prefer 1., but what I was really 
> asking you is whether 2 is possible, or whether the choice is between 
> 1 and 3. If indeed that is the only possible choice, I will have to 
> choose 1.  I myself would vastly prefer 2 to 1, and I think that the 
> overall design will be better and will be more generally useful with 
> 2 rather than 1.  So I know you would vote for 1 given your druthers: 
> but can you please tell me if, given the choice between 2 and 3, 
> whether 2 is even an option?

Well, if I was voting I would vote against 2.

> >[...]
> >
> >>  The substantive change is that, as noted above, the class extension
> >>  of a datatype name is required to contain only the denotations of
> >>  typed literals in the vocabulary, rather than required to *be* the
> >>  value space of the datatype. This brings D-interpretations in line
> >>  with the way that XML typed literals are treated in the RDF
> >>  semantics, which imposes no explicit requirements on
> >>  ICEXT(I(rdf:XMLLiteral))
> >>
> >>  The only example I am aware of which makes the change visible is this:
> >>
> >>  ex:a ex:p "true"^^xsd:boolean .
> >>  ex:a ex:p "false"^^xsd:boolean .
> >>  ex:c rdf:type xsd:boolean .
> >>
> >>    |=?=
> >>
> >>  ex:a ex:p ex:c .
> >
> >What about
> >
> >ex:a ex:p "2"^^xsd:short .
> >
> >|=?=
> >
> >ex:a ex:p _:a .
> >_:a rdf:type xsd:int .
> >
> >I think that this is a valid {xsd:short,xsd:int}-entailment in the PR
> >version of RDF but would not be valid under the proposed change.
> 
> Hmm. It is hard to tell since the XSD specs seem to be ambiguous 
> about whether or not value spaces of distinct datatypes are disjoint 
> or not.  

In this particular case it is even more clear than in the case of xsd:float
vs xsd:int that the values spaces are not disjoint, as xsd:short is a
restriction of xsd:int, and thus its value space is, by definition, a
subset of the value space of xsd:int.

> My best understanding of the intent of the XSD group (and 
> what they were planning to make official in version 2, the last time 
> I checked) is that those value spaces should be considered to be 
> disjoint in the sense that their members are not identical, but that 
> there is a relation called 'equality', distinct from the relation of 
> identity, which holds between them.  (An alternative account of the 
> situation was provided by Henry Thompson, who indicated that the best 
> way to think of the elements of these value spaces is as pairs 
> consisting of the value plus the datatype: rather like typed 
> literals, in fact.)  Whether or not this 'equality' is the same as 
> owl:sameAs, I leave others to decide.  I tend to assume that the 
> safest thing for us to do in the semantics is to go with the most 
> conservative possibility.  As far as the RDFS semantics is concerned, 
> therefore, the value spaces of any two different XSD datatypes MAY be 
> distinct disjoint sets, so your example is not a valid entailment on 
> either the PR or with the proposed change.

I totally disagree.  The normative documents for XML Schema are totally
clear, particularly in this case, that the value spaces are not disjoint.
(Note that this is a separate issue from whether a value typed as a short
would validate as an int.)

> It would be valid if you also asserted
> 
> xsd:short rdfs:subClassOf xsd:int .
> 
> under either semantics.

Well, yes, but so what?  For example what about switching xsd:short and
xsd:int above?

> Pat
> -- 
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Peter F. Patel-Schneider
Bell Labs Research
Received on Tuesday, 13 January 2004 19:09:12 UTC

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