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Re: Question about literals in subject position

From: Steve Harris <steve.harris@garlik.com>
Date: Thu, 24 Sep 2009 11:52:11 +0100
Cc: "public-rdf-dawg@w3.org Group" <public-rdf-dawg@w3.org>
Message-Id: <DF1A4AF1-E7C5-443C-ADDB-68174E5381C6@garlik.com>
To: Bijan Parsia <bparsia@cs.man.ac.uk>
On 24 Sep 2009, at 10:50, Bijan Parsia wrote:

>> SPARQL/Query 1.0 is restricted to legal RDF graphs though, as per  
>> (my reading at least) of section 12.6.
> Oh, right. Because entailment regimes are constrained to be subsets  
> of RDF graphs.
> That seems like pointless restriction frankly. I mean, let's say I  
> violate it (by supporting the natural, but pointless, queries above)  
> in my entailment regime. What happens? Well, nothing really. It  
> doesn't hurt interop. It just means that we have to revisit this  
> spec if we want such an entailment regime to be compliant.
> [snip]

Well, not really, because the entailment regime can just say that such  
triples are legal in it's data (you can't write that in RDF, but  
that's not critical), and then a SPARQL query, using that regime can  
return such bindings. Again, by my reading.

>> Certainly there are RDF-like systems which permit literals in the  
>> subject position, so for some people at least it is significant.
> I guess the question is does it make sense to have the *extension*  
> point forbid those as possible extensions? I guess I don't see the  
> value.

I think the extension point is allowed to change the behaviour there.  
The spec doesn't explicitly mandate RDFs restrictions, it's just  
implied by the fact that SPARQL is defined in terms of RDF. If your  
extension implies some other model, then I would expect it to be able  
to have a different idea of valid data.

A bit out of my depth here though :)

> (I don't see the huge value in allowing subject literals, but  
> perhaps for some datatype cases it could be useful. I'll see if I  
> can come up with an example.)

FWIW, my stores use the fact that literals can't appear in the subject  
position in its data as a source of optimisations, but I don't see why  
other people shouldn't allow it, given a different entailment regime  
that allows such things.

- Steve

Steve Harris
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Received on Thursday, 24 September 2009 10:52:46 UTC

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