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Re: [rdfmsQnameUriMapping-6] Algorithm for creating a URI from a QName in RDF Model?

From: Graham Klyne <GK@ninebynine.org>
Date: Thu, 23 May 2002 18:46:35 +0100
Message-Id: <5.1.0.14.2.20020523181729.04570ca0@joy.songbird.com>
To: Mark Baker <distobj@acm.org>
Cc: "Sean B. Palmer" <sean@mysterylights.com>, www-talk@w3.org
At 12:58 PM 5/23/02 -0400, Mark Baker wrote:
>On Thu, May 23, 2002 at 04:47:00PM +0100, Graham Klyne wrote:
> > At 03:43 PM 5/23/02 +0100, Sean B. Palmer wrote:
> > >The RDF Core WG would certainly want the SW and WWW to be
> > >interoperable, and yet after repeated debates spurred mainly by Aaron,
> > >fragIDs in RDF haven't been deprecated. That speaks volumes.
> >
> > As one who used to think that fragids in RDF were broken...
> >
> > I've been thinking about this point some, and I'm coming round to a view
> > that fragid's are not only OK with RDF, but their use is to be preferred
> > for many RDF resources, and that the SW/WWW integration can work just
> > fine.  I've not yet had time to sit down and straighten my thoughts ... 
> too
> > many other things to do!
>
>Process foul! 8-)  You can't do that.  We need reasons, damnit!

Er, you're right.  This will be very sketchy:

1. The interpretation of a fragment identifier depends on the MIME type of 
the representation it's applied to.

2. URIs without fragment identifiers are generally presumed to map to some 
resource for which a Web representation (or several) can be retrieved.

3. RDF uses URI-references to denote things that aren't necessarily 
web-retrievable.

I think so far is pretty standard stuff.

The difficulty with someurl#frag in RDF arises when you say that this is 
interpreted by:
(a) dereferencing 'someurl'.
(b) interpreting #frag according to what you get back.
This doesn't work well for RDF, because different MIME types can be 
returned, with different interpretations of the fragment identifier, where 
RDF requires that a URI ref have just one denotation under any given 
interpretation.

So my approach for interpreting someurl#frag (and this is largely inspired 
by comments from TimBL and Pat Hayes, though any errors are of course all 
mine) is this:

(A) *assume* that 'someurl' indicates a resource which has an RDF 
representation.  (If it's not dereferencable as such on the web, so be it, 
but I must assume its notional existence)

(B) when used in an rdf document, 'someurl#frag' means the thing that is 
indicated, according to the rules of application/rdf+xml mime type as a 
"fragment" or "view" of the RDF document at 'someurl'.  If the document 
doesn't exist, or can't be retrieved, then exactly what that view may be is 
somewhat undetermined, but that doesn't stop us from using RDF to say 
things about it.

(C) the RDF interpretation of a fragment identifier allows it to indicate a 
thing that is entirely external to the document, or even to the "shared 
information space" known as the Web.  That is, it can be an abstract idea, 
like my cat or DanC's car.

(D) So any RDF document acts as an intermediary between web retrieval 
documents (itself, at least, and also any other web-retrievable URIs that 
it may use, including schema and references to other RDF documents) and 
some set of abstract or non-Web entities that it may describe.

That's it.  I think it's consistent with all the conventional web axioms, 
but it also provides an handling of URIrefs and their denotation that is 
consistent with the RDF model theory and usage.  The "stretch", if there is 
one, is that it somewhat extends the idea of a "fragment" or "view" beyond 
the conventional idea that it's a physical part of a containing document.

If you accept this, then it becomes natural to take a view that URIs 
without fragment identifiers _should_ be reserved for indicating 
web-retrievable resources (when used in RDF), which is something TimBL has 
promoted.  This goes against quite a lot of actual RDF usage (mine 
included) so I don't think we can be too strict about that, but it seems a 
reasonable principle to aim for.

It also suggests a possible answer to the question about the web and 
URIs.  It is sometimes claimed that to be on the web means to have a 
URI.  So are people and cats and dogs and cars "on the web"?  If I clarify 
the definition of "on the web" to not include things that have URI 
references, then the answer to that question can be "no".  But using RDF, 
we are still free to talk about these things without actually having to 
claim that they are "on the web", by using URI-references rather than "1st 
class" URIs.

#g


-------------------
Graham Klyne
<GK@NineByNine.org>
Received on Thursday, 23 May 2002 14:25:07 GMT

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