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Re: RDF Semantics: corrections

From: pat hayes <phayes@ihmc.us>
Date: Tue, 13 Jan 2004 17:37:33 -0600
Message-Id: <p06001f10bc2a27ef3daf@[]>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
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 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?

>>  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.  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.

It would be valid if you also asserted

xsd:short rdfs:subClassOf xsd:int .

under either semantics.

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Received on Tuesday, 13 January 2004 18:37:37 UTC

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