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Re: Mistaken identity?

From: Thomas B. Passin <tpassin@comcast.net>
Date: Wed, 23 Jun 2004 22:58:56 -0400
Message-ID: <40DA4370.4010404@comcast.net>
To: "www-rdf-interest@w3.org" <www-rdf-interest@w3.org>

Jon Hanna wrote:

>>&rdf;type and &rdfs;label seem like pretty good identifiers to me, and 
>>they don't have to be retrievable - and if they were, they would not 
>>denote the retrieved thing.
> 
> 
> There is no fragment identifier within the document obtained from
> http://www.w3.org/1999/02/22-rdf-syntax-ns, though I seem to remember that
> rdf:ID was once used there (there were still issues in treating that as a
> fragment identifier). Still at least dereferencing
> http://www.w3.org/1999/02/22-rdf-syntax-ns will tell me that:
> 
> <rdf:type> <rdf:type> <rdf:Property> .
> <rdf:type> <rdfs:isDefinedBy> <rdf:> .
> <rdf:type> <rdfs:label> "type" .
> <rdf:type> <rdfs:comment> "The subject is an instance of a class." .
> <rdf:type> <rdfs:range> <rdfs:Class> .
> <rdf:type> <rdfs:domain> <rdfs:Resource> .
> 
> And dereferencing http://www.w3.org/2000/01/rdf-schema will tell me that:
> 
> <rdfs:label> <rdf:type> <rdf:Property> .
> <rdfs:label> <rdfs:isDefinedBy> <rdfs:> .
> <rdfs:label> <rdfs:label> "label" .
> <rdfs:label> <rdfs:comment> "A human-readable name for the subject." .
> <rdfs:label> <rdfs:domain> <rdfs:Resource> .
> <rdfs:label> <rdfs:range> <rdfs:Literal> .
> 
> So not perfect by my view of what should be done here (I would tend against
> using # to end RDF namespaces anyway) but not bad either.
> 

Right, and you have demonstrated that it can be useful to try to 
dereference a namespace URI, and sometimes to try to dereference URIs 
used as identifiers for rdf resources.  I don't deny that at all, but 
it's different from saying that a namespace _denotes_ the dereferenced 
document (namespace URIs denote nothing outside themselves), or that a 
URI used as an rdf resource automatically and always _denotes_ the thing 
pointed to.

Not only that, but there seems to be at least one error in the document -

<rdf:Property rdf:about="http://www.w3.org/1999/02/22-rdf-syntax-ns#type">

That should be "<rdfs:Property ...>" instead.  Interesting.

In addition, recall that in rdf, there is no unique way to split up a 
uri into a namespace part and a specific part.  Rdf only requires that 
the concatenation comes out to the right URI reference.  So our exemplar 
uri reference here,

http://www.w3.org/1999/02/22-rdf-syntax-ns#type

can rdf-legally be partitioned as we normally think of it -

{http://www.w3.org/1999/02/22-rdf-syntax-ns#}type

or

{http://www.w3.org/1999/02/22}-rdf-syntax-ns#type

or {http://www.w3.org/1999/02/22-rd}f-syntax-ns#type

The point is, you can't really determine even a namespace to try to 
dereference in the hopes of getting some useful information except by 
using heuristics that are not specified or sanctioned by the Rec.

Like I said, it's trickier than it looks.

Cheers,

Tom P

-- 
Thomas B. Passin
Explorer's Guide to the Semantic Web (Manning Books)
http://www.manning.com/catalog/view.php?book=passin
Received on Wednesday, 23 June 2004 22:56:31 GMT

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