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Re: Input sought on datatyping tradeoff

From: pat hayes <phayes@ai.uwf.edu>
Date: Wed, 17 Jul 2002 12:48:51 -0500
Message-Id: <p05111b05b95b5cb86c02@[65.217.30.45]>
To: Brian McBride <bwm@hplb.hpl.hp.com>
Cc: Piotr Kaminski <piotr@ideanest.com>, www-rdf-interest@w3.org

>At 13:43 11/07/2002 -0700, Piotr Kaminski wrote:
>>A side comment on the datatype question:
>>
>>From: "Brian McBride" <bwm@hplb.hpl.hp.com>:
>>>  Test D:
>>>
>>>     <Jenny>      <ageInYears> "10" .
>>>     <ageInYears> <rdfs:range> <xsdr:decimal> .
>>>
>>>     <John>  <ageInYears>      _:a .
>>>     _:a     <xsdr:decimal>   "10" .
>>
>>According to RDF Model Theory rules, this means that <xsdr:decimal> is both
>>a class and a property.  As far as I can tell this is legal (?),
>
>Yes
>
>>  but is it
>>the desired effect?
>
>Yes.  We did at one point in our discussions have different terms 
>for the value space (Class) and lexical to value mapping (property) 
>of the datatype.  We decided however that it was simpler, and easier 
>for the non-expert user, to have just one term to represent the 
>datatype.  Fortunately, the way the semantics are defined this works.
>
>>   What does it mean?
>
>Remember, that in the model theory, a class is not a set of things, 
>it is a resource which has an associated set, the class extension, 
>which is the set of members of the class.  This is how we addressed 
>the issue of Class being a member of itself.  A class can be a 
>member of its own extension, and we can do that and stick with the 
>most commonly understood set theory.
>
>Similarly a Property is not a set of pairs, but is a resource with a 
>property extension which is the set of pairs defining the relation.
>
>A resource can have both a Class extension and a property extension 
>and it all works fine (so Pat tells me - I'm not a mathematician).
>
>The way we have defined datatypes, a datatype is a resource which 
>has both a class and property extension.  The advantage of doing 
>things this way is, as I said above, users need only understand one 
>concept, the datatype and don't have to use different names in 
>different contexts.
>
>Pat: feel free to jump in here if I'm getting this wrong.

No, you have it just right.

It might help if we were to add that one way to think about this is 
that the use of the same name for a class and a property is really 
only a 'pun'; it doesn't imply that the class extension (which is 
often thought of as being the class itself) and the property 
extension (similarly) are the same extension. But this kind of 
'punning' is harmless in the RDF semantics; and since in this case it 
seems natural, we have decided to take advantage of it.

BTW, notice that it would NOT be OK to have 'punning' between two 
different classes or two different properties. But since classes and 
properties are distinct syntactic categories in RDF, it is harmless 
to use the same name for a class and a property; it couldn't ever be 
ambiguous which you meant.

Pat
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Received on Wednesday, 17 July 2002 13:48:19 GMT

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