Re: ISSUE-30: How does SPARQL's notion of RDF dataset relate our notion of multiple graphs?

On Apr 18, 2011, at 15:00 , Pierre-Antoine Champin wrote:

<snip/>
>>> Not so easy. When you *use* a URI in RDF, like the <g> here, it is
>>> not referring to itself, but to what it denotes. (Put another way,
>>> the URI is not quoted.) Which means that rdf:tags isn't going to
>>> have the meaning you intend. Now, we could change this, and say
>>> that rdf:tag is (uniquely) referentially opaque in its subject
>>> position. This would however be a major change to the RDF semantics
>>> and data model, and would require us to re-wrote the semantic spec.
>>> And it has other knock-on consequences eg for OWL, since OWL
>>> equality reasoning would have to be blocked from such triples. So I
>>> think we should think very hard before going there.
>>> 
>>> However, XML Schema has a datatype for making literals refer to
>>> URIs. http://www.w3.org/TR/xmlschema-2/#anyURI  and we could use
>>> that. OK, we can't have a literal in subject position (sigh), so we
>>> have to turn it around:
>>> 
>>> <h> rdf:taggedBy xsd:anyURI^^"<the URI written as a string>" .
>>> 
>>> and then it would all work, without breaking the RDF semantics.
>> 
>> I understand, but isn't this problem a reflection of the fact that we
>> try to model here the common term of tagging, ie, attaching a string
>> to a resource as some sort of a characterization of the latter? In
>> fact, as we said at the f2f, SPARQL is blissfully silent on how that
>> URI is used. If we want to avoid misunderstandings through the usage
>> of the word tagging, we can say something like
>> 
>> <g> rdf:loose_association_of_resources <h> .
> 
> nah...
> 
>  <i> owl:sameAs <g> .
> 
> entails
> 
>  <i> rdf:loose_assoctiation_of_resources <h> .
> 
> Is this what you want to say??

Hm. I do not know...

Given that SPARQL is loose in the way they use URI-s from graphs in datasets, I would not be shocked by this. The question is what would the SPARQL 1.1 entailment regime say about this. 

But, again: I wonder whether we have to say anything in formal terms at all about SPARQL's behaviour, except to make it clear that (<g>,G) is _not_ a shorthand for <g> identifying G.

Ivan


> 
> I prefer Pat's proposa: if you want to associate something with a *uri*
> (and not a resource), use a xsd:anyURI literal.
> 
>    pa
> 

----
Ivan Herman, W3C Semantic Web Activity Lead
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