- From: Peter F. Patel-Schneider <pfps@research.bell-labs.com>
- Date: Wed, 05 Feb 2003 12:51:47 -0500 (EST)
- To: bwm@hplb.hpl.hp.com
- Cc: www-rdf-comments@w3.org, GK@NineByNine.org, fmanola@mitre.org
From: Brian McBride <bwm@hplb.hpl.hp.com> Subject: Re: Can RDF say anything about anything? Date: Wed, 05 Feb 2003 17:31:32 +0000 > 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? Yes there is doubt. This is using rdf:ID for a purpose not sanctioned by the RDF specifications, as RDF Syntax indicates that rdf:ID is to be used only for a particular purpose. > 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, Yes there is some small doubt, because RDF is supposed to be able to ``say anything about anything''. > but you could write: > > <rdf:Description rdf:about="#IDinstance"> > <rdf:type rdf:resource="&rdf;ID"/> > <rdfs:comment>...</rdfs:comment> > </rdf:Description> This alternative way of writing the same triples means that the doubt is only very small. > >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. But according to RDF Schema rdfs:comment is supposed to be used to ``provide a human-readable description of a resource'' so this a use of RDF vocabulary for a purpose not sanctioned by the RDF specification. > >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. Well, if an RDF graph cannot be transmitted, is it useful? How can I, a user of RDF, use rdf:ID as a property URI? If so, how? If not, then what is the status of the ``say anything about anything'' wording? > >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. See above. > >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? No. Where is the wording that sanctions this use? > 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. Well, the word ``sanctioned'' comes from the RDF specification, so it is up to the RDF Core Working Group to provide an answer. > >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. I maintain that it is a counter example to the ``say anything about anything'' claim. > 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? Yes. > b) We have your third quote "sanctioned by" which may need some > clarification in the light the test case above. Agreed. > If that's a reasonable summary, then maybe the doc editors could pick this > thread up now. > > Brian peter
Received on Wednesday, 5 February 2003 12:52:07 UTC