W3C home > Mailing lists > Public > semantic-web@w3.org > July 2006

RE: Semantic content negotiation (was Re: expectations of vocabulary)

From: Xiaoshu Wang <wangxiao@musc.edu>
Date: Sun, 30 Jul 2006 10:18:59 -0400
To: "'Richard Newman'" <r.newman@reading.ac.uk>
Cc: "'Semantic Web'" <semantic-web@w3.org>
Message-ID: <000c01c6b3e3$197d2da0$0e241780@bioxiao>

--Richard, 

> >> Sorry, that's nonsense. Not only property URI's are not 
> necessarily 
> >> dereferenceable and the possibly available graph 
> representation may 
> >> or may not contain that statement - do you know about any 
> FOAF client 
> >> behaving as you're suggesting it should? I don't, and I know I 
> >> wouldn't want to install it on my mobile phone of limited 
> resources.
> >
> > Are you sure you are talking RDF? The only thing that is not 
> > dereferencable is literal values because they are not URI.  But 
> > literal can only be an object, not subject and property.
> 
> Not true. Only a subset of URIs are dereferenceable, and even 
> fewer are dereferenceable to yield RDF. The following are a 
> few examples:

If the URI is a property and dereference it does not return a URI is not a
good practice.  I remember that the TAG is working on what is supposed to be
put in the namespace.  In FOAF's case, at least the URI is dereferenable.
But the returned type is HTML.  Here, if GRDDL is standardized, it will
still return an RDF document.  No one is required to make any URI
dereferenable.  But the best practice should recommend so.  If an RDF engine
should follow all the links to retrieve all RDFs.

> - tel:, mailto: etc. URIs
> - tag URIs, which are explicitly not dereferenceable
> - probably the majority of used HTTP URIs, because they are 
> essentially 404s by default, or yield non-RDF data.
> 
> Furthermore, not all properties are of the form
> 
> <dereferenceable RDF URL> + #propertyName
> I often use slashes instead of hashes, and what you get from 
> a web server serving up a representation of that URI is 
> probably not a fragment of the ontology.
> 

This hash/slash debate is irrelevant here.
 
> > I don't know too much about FOAF.  But I do know FOAF does 
> not deploy 
> > its ontology at its namespace (is that why you said a 
> property is not 
> > necessarily dereferenable?) and I think this practice is a very bad 
> > practice.  Because it assumes agent to have preexisting 
> knowledge to 
> > work
> > with something, it sort of make the open-world somewhat closed.   
> > IMHO, it is
> > bad and very bad.
> 
> Last I checked, http://xmlns.com/foaf/0.1/ returns HTML or 
> RDF according to your HTTP request, which might be amusing 
> given the topic under discussion.

The returned MIME type and GRDDL in together to give you a dereferable URI
and in RDF.

Xiaoshu
Received on Sunday, 30 July 2006 14:19:33 GMT

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