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Re: Blank Node Identifiers and RDF Dataset Normalization

From: Pat Hayes <phayes@ihmc.us>
Date: Mon, 25 Feb 2013 18:36:15 -0600
Cc: Markus Lanthaler <markus.lanthaler@gmx.net>, <public-rdf-wg@w3.org>
Message-Id: <5137A9E6-5003-4E88-B5C3-34A203FC15DE@ihmc.us>
To: Steve Harris <steve.harris@garlik.com>

On Feb 25, 2013, at 11:17 AM, Steve Harris wrote:

> On 2013-02-25, at 17:12, Markus Lanthaler <markus.lanthaler@gmx.net> wrote:
> 
>> On Monday, February 25, 2013 4:46 PM, Steve Harris wrote:
>> 
>>> I believe it would be more correct to say that graph labels do not HAVE
>>> to demote the graph, they're allowed to if you want them to.
>>> 
>>> Regardless, the example is valid regardless on whatever graph labelling
>>> semantics are being used - within some system with a known relationship
>>> between graph labels and metadata.
>>> 
>>> If the graph label refers to the document which was parsed, and the
>>> metadata refers to the parsing (which is a very common situation), then
>>> the example is equally valid.
>>> 
>>> I think you may be attaching too much important to the idea of
>>> denoting.
>> 
>> 
>> Yes, maybe I am. The future will show.
> 
> I think the past has shown quite conclusively.
> 
> Graph identifier URIs have been in use for easily 10 years, the URI denoting the graph is probably one of the rarer situations. It's not caused any problems that I'm aware of.

If true, this strongly suggests to me that graph identifier URIs are not in fact used in RDF triples very often. In which case, of course, what they denote is of no importance. If the URI acts only as a graph label, so that this is the only meaningful role it plays on the dataset syntax, then indeed it can denote whatever anyone wants it to. But we have to face up to the fact that this decision renders the use of those IRIs *inside RDF triples* effectively separated from their role as graph labels, so they cannot be used to convey, on an open Web, RDF-expressed metadata information about the graphs.

> I'd agree that reusing one URI to mean both a person and a graph (for e.g.) is ugly modelling, and should be discouraged, but that's a separate issue.

Its not a separate issue, and its worse than ugly: it is DIRECTLY in contradiction to the normative RDF specs. When used in RDF triples, an IRI means what it denotes, and it can only denote one thing. Even those who want RDF graphs to be interpreted contextually, so that the IRIs in one graph might have a different meaning from those in another, havn't argued for allowing two instances of the same IRI in a single graph have different meanings. If we allow or endorse this, then we have effectively given up on the basic idea of Web identifiers used in data at all. 

Pat

> 
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> 
> -- 
> Steve Harris
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Received on Tuesday, 26 February 2013 00:36:58 GMT

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