W3C home > Mailing lists > Public > w3c-rdfcore-wg@w3.org > December 2002

Should rdf:value have a semantics?? (was: Re: Quick review of RDF primer)

From: pat hayes <phayes@ai.uwf.edu>
Date: Tue, 3 Dec 2002 13:46:23 -0600
Message-Id: <p05111b13ba12b7e86ae6@[10.0.100.247]>
To: fmanola@mitre.org
Cc: w3c-rdfcore-wg@w3.org

>pat hayes wrote:
>>
>>  >
>>  >General comment, not specifically Primer:  the description of
>>  >rdf:value is fine, but how does it relate to a normative
>>  >specification?  What can we say formally about rdf:value?
>>
>>  Right now, we say explicitly that it has no particular meaning.
>>
>>  >What formal semantics (interpretation) allows us to make inferences like:
>>  >
>>  >    my:cat rdf:type ex:DomesticCat .
>>  >    my:cat ex:weight _:x .
>>  >    _:x rdf:value "15" .
>>  >    _:x ex:unit ex:Kilogram
>>  >
>>  >=>
>>  >
>>  >    my:cat rdf:type ex:Obese .
>>
>>  Nothing in the MT provides any connection in meaning between the
>>  third and fourth triple of the first graph.
>
>I assume you mean beside the fact that they both have the same subject?

Well, yes. I mean, there isn't any formal support for that (or any 
other) entailment involving rdf:value

>Anyway, this isn't specific to rdf:value is it?

Well, it seems to me that rdf:value is being used in a way that kind 
of expects a semantics that it currently does not have. SO maybe we 
should give it one.

>I mean, you couldn't
>make the inference if you'd used a datatype "weightInKilograms" either.
>You'd need much more machinery than RDF has.

The point wasnt to do with the particular datatype, but the use of 
rdf:value. Right now rdf:value might as well not be there as far as 
the MT is concerned, it has no meaning at all. You could get the same 
effect by writing ex:foo instead.

>
>>
>>  I was under the impression that we had formally decided NOT to
>>  support this kind of usage. Wasn't that part of the local-datatyping
>>  no-fancy-idioms decision?
>>
>>  >but NOT:
>>  >
>>  >    my:cat rdf:type ex:DomesticCat .
>>  >    my:cat ex:weight _:x .
>>  >    _:x rdf:value "15" .
>>  >    _:x ex:unit ex:Pound
>>  >
>>  >=>
>>  >
>>  >    my:cat rdf:type ex:Obese .
>>  >
>>  >?
>>  >
>>  >My point here is if we are to encourage such usage of rdf:value,
>>  >then there ought to be some normative description to back up such
>>  >usage.
>>
>>  I agree.  Either we should not mention this stuff, or else we should
>>  back it up with some semantics.
>
>Well, that doesn't seem to be the principle on which we've been
>operating!  If we're going to deal with this, let's do rdfs:isDefinedBy
>too!

I would if I thought it had any meaning. The trouble with that one is 
that 'is defined by' really, really is meaningless when applied to 
pieces of RDF. RDF *never* defines anything, it can't possibly define 
anything since it doesn't have negation. So forget about 
rdfs:isDefinedBy, OK?

>
>>  It wouldn't be hard to do and it
>>  would also enable us to do some neat datatyping  entailments that
>>  people seem to think are obvious, such as (with appropriate
>>  wellformedness caveats)
>>
>>  aaa ppp "sss"^^ddd .
>>
>>  -->
>>
>>  aaa ppp _:x .
>>  _:x rdf:value "sss" .
>>  _:x rdf:type ddd .
>>
>
>Yes, keeping in mind that rdf:value can be used with more complicated
>relationships too (i.e., _:x could have multiple properties qualifying
>the value, not just a single one).

Right, that's the limitation, but we can get that from the MT as well.

So more generally, to the WG: should I give rdf:value a model theory? 
Speak soon, guys. Unless I hear otherwise I will do this:

aaa rdf:value "bbb" .

means that there is some conventional mapping M from lexical forms 
under which I(aaa) = M(bbb). Doesn't say much, but it might be 
useful, particularly if we say that any datatype L2V mapping counts 
as a 'conventional mapping'.

Pat


>
>--Frank
>
>--
>Frank Manola                   The MITRE Corporation
>202 Burlington Road, MS A345   Bedford, MA 01730-1420
>mailto:fmanola@mitre.org       voice: 781-271-8147   FAX: 781-271-8752


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Received on Tuesday, 3 December 2002 14:46:05 EST

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