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RE: xmlns, uri+name pairs or just uris..? Clarification needed.

From: Graham Klyne <GK@Dial.pipex.com>
Date: Sun, 30 Jul 2000 19:28:31 +0100
Message-Id: <4.3.2.7.2.20000730192546.00c50100@pop.dial.pipex.com>
To: Dan Brickley <danbri@w3.org>
Cc: www-rdf-interest@w3.org
Another approach, which I am planning a project to prototype, might be to 
use contexts to capture this kind of information.

#g
--

At 06:45 AM 7/28/00 -0400, Dan Brickley wrote:


>There is the 'rdfs:isDefinedIn' property in rdf schema, which connects a
>resource to its namespace (well, properties, classes etc anyway). Not all
>identified resources (eg. uuid:, tel: URIs) fall into two parts in this
>way, but you might imagine a convention for rdf parsers to emit additional
>'isDefinedIn' triples when parsing xml namespaced rdf serialisations,
>preserving the namespace info more accurately. For the about="..." and
>resource="" constructs, where qnames arent currently used, we're a little
>stuck.
>
>My take on this would be to say that the rdf 1.0 syntax doesn't always
>make this information available, and the rdf model doesnt require that it
>is available, but that model and schema provide representations for it
>when it _is_ available.
>
>One strategy for dealing with this kind of partial knowledge would be to
>explore ways of acquiring, exchanging, aggregating information about rdf
>vocabularies, since once we know their uris we can identify names within
>those namespaces...
>
>dan
>
>
>On Fri, 28 Jul 2000, McBride, Brian wrote:
>
> > Hi Perry,
> > >
> > > My opinion, which I know is shared by at least some of the
> > > implementors on
> > > this list, is that this part of the RDF spec is in error.
> >
> > Yup!  I got very confused by this at one point.  I've come to a
> > sort of 'well we can live with it ...' sort of position in my own
> > mind, but its not very pretty.
> >
> >
> > > ... the relevant stuff justifying the assertion that
> >
> >  as far as
> >      XML is concerned, a qname is a pair (ns, locaname) not the
> >      concatenation of ns and localname, i.e.
> >
> >      <x xmlns:fo="fo" xmlns:foo="foo">
> >        <y fo:obar="a" foo:bar="b"/>
> >
> >      is legal - the attribute names are distinct even though
> >      they concatenate to the same string.
> >
> > Whilst the NS spec is not absolutely precise on this, it seems like best
> > interpretation.
> >
> > >
> > > I think you can guess from the above how I chose to implement
> > > expanded names
> > > in our RDF processor and API.
> > >
> >
> > Me too.  In my SQL stuff I separate out the namespace part and the
> > localname part into different tables.
> >
> > There are different opinions here.  You may recall that I asked
> > Sergey to modify his java api to pass the qname structure through
> > the API rather than the URI.  He refused on the grounds all you
> > need in the model is the URI.
> >
> > With my usual caveats of I'm new to this and not the expert, here
> > is my current reasoning:
> >
> > XML does define a qname to be a pair(namespace, localname).  RDF
> > models however, are defined in terms of URI references.  Resources are
> > identified by a URI reference as are properties.
> >
> > The current RDF XML serialization makes use of XML qnames to represent
> > URI references.  RDF defines a mapping from qnames to URI references,
> > which is, unfortunately, to simply concatenate the two.
> >
> > So given a qname from an XML processor, an RDF processor can determine
> > what URI reference the pair representing the qname represents.
> >
> > The mapping is not a one to one mapping, i.e. there are several qnames, 
> that
> > at least syntactically could represent an arbritary URI reference.
> > Unfortunately, this is a potential problem.  Given a URI reference for
> > a property, it is important to be able to determine the correct namespace
> > for that property because the namespace defines the schema that defines
> > its interpretation.
> >
> > So when, for example, a URI reference for a property is encountred say as
> > the
> > value of an rdf:about or rdf:resource reference, it is not in general
> > possible to determine what schema applies to that property just by
> > processing the URI.  In the words of the sage, BUMMER!
> >
> > However, if a convention that all namespace URI references used for RDF
> > will end in a character that cannot appear in the localname part of qname,
> > then it is possible to determine the correct namespace from the URI of
> > a property alone.  The current SiRPAC implementation relies on this.
> > Given this conduct, the flaw is not deadly.  I have seen published examples
> > of RDF schemas that did not adopt this convention, but I'm not sure
> > which ones - possibly the document on how to represent dublin core in RDF,
> > but I'm not sure.
> >
> > Comments?
> >
> > Perry, do you have a java parser that will pass through the qname pair
> > rather
> > than the URI?
> >
> > Brian
> >

------------
Graham Klyne
(GK@ACM.ORG)
Received on Monday, 31 July 2000 14:11:25 GMT

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