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Re: rdfs:isDefinedBy (Was Re: Representing DCMI semantics as RDF schemas versus Web pages)

From: Patrick Stickler <patrick.stickler@nokia.com>
Date: Wed, 05 Jun 2002 11:17:43 +0300
To: ext Bill de hÓra <dehora@eircom.net>, DCMI <DC-ARCHITECTURE@jiscmail.ac.uk>, "'ext Dan Brickley'" <danbri@w3.org>
CC: RDF Interest <www-rdf-interest@w3.org>
Message-ID: <B923A3D7.161AC%patrick.stickler@nokia.com>

On 2002-06-05 2:26, "ext Bill de hÓra" <dehora@eircom.net> wrote:

> Here's the question: Are XML namespaces RDF resources? If a
> collection of names identified by a URI reference can be an RDF
> resource, then an XML namespace can be an RDF resource.
> Bill de hÓra

Obviously, anything in the universe that we can give a URI to
can be a resource, so in that regard, a namespace can be
an RDF resource. Sure. But then that URI should not denote anything
*but* the namespace.

If the URI that is adopted as a namespace prefix already
denotes something else, such as a vocabulary, or a schema,
or other kind of resource, then we introduce ambiguity into
the SW.

It is no better than using a mailto: URL to denote a person.
How do we then differentiate between the properties of the
mailbox and the person?

How do we differentiate between the properties of the namespace
and some other resource with which it shares its URI.

And if a namespace URI denotes "a collection of names" then
it is an abstract resource -- not a schema, or namespace
document, or any other retrievable resource. So if we stick
some schema, say, at that URI, we again introduce ambiguity,
since we can't differentiate between the properties of the
schema and those of the collection of names. And let's not
forget that a given collection of names might be defined
by mulitple schemas in various encodings/languages.

While historically many of the more visible vocabularies
have had a 1:1 correspondence with a namespace, i.e all
terms in the vocabulary are grounded in the same namespace
and the namespace URI also denotes the vocabulary, that is
not because of any inherent characteristic of namespaces
themselves and not IMO because that is the "right" way
to do things.

You could just as well have a vocabulary that has terms,
each of which is grounded in a separate namespace, and
none of those namespace URIs denote the vocabulary. And
that is just as valid a use of namespaces as the first.
And in fact, in the case of complex families of ontologies
with multiple versions and regional/locale variations,
the latter is precisely what is required.

Insofar as *web architecture* is concerned, the fact is
that namespace != vocabulary != schema != doctype, etc.

Finally, if it is argued that namespaces are RDF resources
and they have a special relation to the terms which are
grounded within them, then RDF is behaving rather badly
by discarding the structural partition between namespace
prefix and local name or not automatically defining such
a relation in the graph. The concatenation-based transformation
from qname to URI employed by RDF clearly reflects a view
that the namespace is irrelevant, and all that matters is
the URI. 

And I don't see namespaces or their relations to terms
formally defined in the RDF MT either.

The bottom line is that there are now two conflicting
global naming schemes on the Web: qnames and URIs.

XML uses qnames. RDF and most other web technologies use
URIs.  And RDF/XML uses qnames simply as a hack, similar to
ENTITYs, just to get URIs into element and attribute
names, but disregards all structure and semantics defined
by XML for those qnames. Once you get to the graph, the
namespaces are gone. Poof.


I again assert, there are namespaces in the RDF model, and
namespaces are abstract things, not equivalent to schemas,
and therefore it makes no sense to specify a namespace URI
as the value of rdfs:isDefinedBy.

If rdfs:isDefinedBy is supposed to point to a term's
namespace, then (1) it is not needed, since that information
can be retained from the RDF/XML and (2) it should have
a cardinality of 1 since of course, a term only has one
namespace. (so clearly, such usage is nonsense)

IMO rdfs:isDefinedBy should point to any resource that provides
definition of a term, regardless of encoding/language, and
it should explicitly *not* be a namespace (which is simply
an abstract collection of names). As to the nature of the
defining resource, just use RDF to describe it. Simple.



Patrick Stickler              Phone: +358 50 483 9453
Senior Research Scientist     Fax:   +358 7180 35409
Nokia Research Center         Email: patrick.stickler@nokia.com
Received on Wednesday, 5 June 2002 04:14:17 UTC

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