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Re: Can RDF say anything about anything?

From: Brian McBride <bwm@hplb.hpl.hp.com>
Date: Wed, 05 Feb 2003 17:31:32 +0000
Message-Id: <5.1.0.14.0.20030205153325.03095af0@localhost>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
Cc: www-rdf-comments@w3.org, graham Klyne <GK@NineByNine.org>, Frank Manola <fmanola@mitre.org>

At 09:45 31/01/2003 -0500, Peter F. Patel-Schneider wrote:

[...]


>Well, if I believed the ``say anything about anything'' wording, I should
>really be able to say anything about, for example, rdf:ID, perhaps even
>saying that it is a class with an instance, as in

At first I was puzzled about whether you meant say something about the URI 
for which rdf:ID is an abbreviation or the thing denoted by rdf:ID ...


>         ...
>         <rdf:ID rdf:about="#IDinstance">
>                 <rdfs:comment>An instance of rdf:ID.</rdfs:comment>
>         </rdf:ID>
>         ...

I think I see some test cases here:

TC1:

_:a rdf:type rdf:ID .

Is this a syntactically legal RDF graph?  Can it be written in RDF/XML?  Is 
there any doubt that the answer to these questions is yes?

TC2:

is

   <rdf:ID rdf:about="#IDinstance">
                 <rdfs:comment>An instance of rdf:ID.</rdfs:comment>
   </rdf:ID>

legal RDF/XML.  Is there any doubt about the answer to that question is no, 
but you could write:

   <rdf:Description rdf:about="#IDinstance">
     <rdf:type rdf:resource="&rdf;ID"/>
     <rdfs:comment>...</rdfs:comment>
   </rdf:Description>



>A better example, perhaps, would be using RDF to critique RDF, as in
>
>         ...
>         <rdfs:Class rdf:about="#broken">
>           <rdfs:comment>The class of broken features in RDF.</rdfs:comment>
>         </rdfs:Class>
>         <broken rdf:about="http://www.w3.org/1999/02/22-rdf-syntax-ns#ID">
>           <rdfs:comment>rdf:ID is broken because it can only occur once in
>                                an rdf document.</rdfs:comment>
>         </broken>
>
>         <broken rdf:about="http://www.w3.org/2000/01/rdf-schema#comment">
>           <rdfs:comment>rdfs:comment is broken because it brings in social
>                                      meaning.</rdfs:comment>
>         </broken>
>
>         ...

Ok, so that all looks syntactically legal - you seem to be able to make 
your assertions.



>A third example, would be to use one of the RDF URIs as a property, as in:
>
>         ...
>         <rdf:Property 
> rdf:about="http://www.w3.org/1999/02/22-rdf-syntax-ns#ID">
>           <http://www.w3.org/1999/02/22-rdf-syntax-ns#ID
>             rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#ID"/>
>         </rdf:Property>
>         ...

Right, whilst

   _:a rdf:ID _:b .

is a syntactically legal graph, it cannot be expressed as RDF/XML.


>Are these three examples legal in RDF?  The first two quotes I presented
>above would indicate so.

Well, "legal in RDF" is rather a vague term. They are syntactically correct 
graphs and there are no semantic constraints that make them illegal.  Some 
of them cannot be represented in RDF/XML, but that is not enough to assert 
they cannot be represented as an RDF graph.

>
>
>The first example, however, is illegal according to RDF Syntax.

Right, but there is another way to express the graph in RDF/XML.

>
>
>The second example is, I think, *legal* but this goes counter to the third
>quote I presented above.

That quote being:

``Certain
URIs are reserved for use by RDF, and may not be used for any purpose not
sanctioned the RDF specifications.''

Hmmm, I suggest that the RDF specs do sanction:

<rdfs:Class rdf:about="#broken">
           <rdfs:comment>The class of broken features in RDF.</rdfs:comment>
         </rdfs:Class>

and also

         <broken rdf:about="http://www.w3.org/2000/01/rdf-schema#comment">
           <rdfs:comment>rdfs:comment is broken because it brings in social
                                      meaning.</rdfs:comment>
         </broken>

Do you agree?

Which leaves:


         <broken rdf:about="http://www.w3.org/1999/02/22-rdf-syntax-ns#ID">
           <rdfs:comment>rdf:ID is broken because it can only occur once in
                                an rdf document.</rdfs:comment>
         </broken>

Aside from the fact that since the WG has not defined a denotation for 
rdf:ID, whoever asserted that would in some sense, not know what they were 
asserting about, is there any doubt that:

   <broken rdf:about="http://example.org/#rdfID">
     <rdfs:comment>The rdf:ID feature in RDF</rdfs:comment>
   </broken>

is ok?  So you can express the idea you seem to want to express. So it 
doesn't contradict your first two quotes. Is your example *sanctioned* by 
the RDF specs?  That I'm having more trouble answering.


>The third example is illegal according to RDF Syntax, and I don't think
>that there is any way of generating this triple in RDF/XML.

That is true.  But I don't see a contradiction with your quote from the 
primer or concepts.

So trying to summarize where we have got to:

a) We have established that not all RDF graphs can be expressed in 
RDF/XML.  Do you still believe that contradicts the statement in the primer 
and concepts that you quoted?

b) We have your third quote "sanctioned by" which may need some 
clarification in the light the test case above.

If that's a reasonable summary, then maybe the doc editors could pick this 
thread up now.

Brian
Received on Wednesday, 5 February 2003 12:35:27 GMT

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