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Re: [namespaceDocument-8] RDF and RDDL

From: Jonathan Borden <jonathan@openhealth.org>
Date: Thu, 11 Apr 2002 09:27:26 -0400
Message-ID: <000801c1e15c$c6ef58e0$0301a8c0@ne.client2.attbi.com>
To: "TAG" <www-tag@w3.org>
Tim Bray wrote:
>
> I'd like to thank Stuart for all the spadework on this issue, which
> I realize now that I didn't understand before.  I'm not sure that I
> understand it now, but I'm going to suggest another approach based
> on Stuart's work that I think comes out a little cleaner:
>
> <div ID="schematron" class="resource">
>   <h3>7.7 Schematron</h3>
>        <p>A <a href="rddl.sch">Schematron Schema</a> for RDDL. </p>
>   </div>
>
> <rdf:description rdf:about="rddl.sch">
>   <rddl:prose rdf:resource="#schematron" />
>   <rddl:purpose
>       rdf:resource="http://www.rddl.org/purposes#schema-validation" />
>   <rddl:nature
>       rdf:resource="http://www.ascc.net/xml/schematron" />
>   </rdf:description>

This approach has an easy to read syntax yet I am concerned about _this
particular_ RDF semantics and the applied relationship to a "namespace".
RDF is a very
precise and at the same time sometimes unforgiving tool. The primary
distinction
between RDF and any old XML is that the meaning, the formal semantics, of an
RDF document is formally specified, thanks to the excellent work of Pat
Hayes and the rest of the RDFCore WG. What this means is that you _are not_
allowed to decide what the RDF means, much as you might when writing down
any other XML when devising a preferred syntax. Like it or not, RDF does
have an unambigouos formal meaning aka a 'model theory'. The downside of
this is that you can't just simply devise an RDF compliant syntax because it
looks good, or is readable by a person. So while the syntax of the RDF that
you've written looks terrific, the semantics are way off what a namespace
definition ought to be. My comments are specific to _this specific_ RDF +
XHTML, not the concept of RDF + XHMTL in general.

What you get when parsing such a document is a collection of RDF resources
whose names are the URIs of the namespace related documents. Fine, except...

To my understanding a namespace is a collection of terms, not arbitrary
resources. RDDL was carefully designed to link each term to a related
resource, the RDDL purpose of the link is the name of the arc traveling from
term defined by the rddl:resource id, to the related resource. The
rddl:nature is interpreted as the rdf:type of the related resource.

To me, this is the proper RDF representation of an XML namespace,
and the RDF+XHTML syntax needs to be both readable, and to properly
represent this relationship to a machine.

I am not sure what to think of the names in the namespace being defined by
IDs on <di v> elements, while the semantics of the namespace being defined
by RDF. This feels wrong to me, as if you are saying that the terms in the
namespace denote pieces of XHTML. You see when using anything other than
RDF, what the terms in a namespace _denote_ in a formal sense, are either
undefined or defined by the specification, in RDF, having a model theory,
the denotation is unabiguous and predefined. If you think that's just a
bunch of mumbo jumbo then you really ouight not be using RDF.

>
> ...but there's *lots* of room for argument about these details.
>
> I do think however that the way I've proposed structuring
> the RDF description is the way to go.  And (not a surprise
> when you think about it) it's easier to read & understand
> than the XLink formulation.  -Tim

Some formulations make more sense to people, some to machines. What we need
is a formulation that makes sense to both people and machines, but
particularly 1) that names in the namespace are defined using RDF
terminology/vocabularly definition -- and desirable --2) that browsers
understand these terms e.g. as fragments pointing to the prose descriptions.
e.g. the way a browser handles URI '#'  fragid. This formulation handles the
latter (2) part (which is certainly desirable) but does not handle the
former (1) part which I think is mandatory.

The other issue is that when I think of using RDF to define namespaces, I
don't think of predefining a couple of predicates in the RDDL namespace,
rather really using RDF to define namespaces using arbitrary predicates. The
advantage of RDF is that it is incredibly powerful. The disadvantage of RDF
is that it is incredibly powerful. One of the reasons I think RDDL is that
it constrains what you can say, which is great for XML folks that want to
create namespaces without getting into the details of formal semantics. I've
started to convert the RDDL spec itself into a blend of XHTML + RDF
http://www.rddl.org/rddl-rdf.html -- you'll need to "view source" and look
for the RDF statements intermixed with the XHTML. This RDF is intended to
have exactly the same semantics as the RDDL XLink is intended to convey. If
you don;t like this syntax, we'll need to do more work (it is not totally
clean to intermix RDF + XHTML).

Jonathan
Received on Wednesday, 10 April 2002 21:18:35 GMT

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