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Re: Question about literals in subject position

From: Birte Glimm <birte.glimm@comlab.ox.ac.uk>
Date: Tue, 22 Sep 2009 12:39:55 +0100
Message-ID: <492f2b0b0909220439m7cab7442t9df783aa09289e51@mail.gmail.com>
To: SPARQL Working Group <public-rdf-dawg@w3.org>
<snip>

> Section 12.6 of the current spec says here (there are people in the group
> who can probably shed more light on the meaning, in case I got it wrong):
>
> "For any basic graph pattern BGP and pattern solution mapping P, P(BGP) is
> well-formed for E."

So well-formed means it is a valid statement (makes sense) and even
though the entailment holds, it is an illegal query since stating such
a thing is forbidden. Then, however, I would say that even under
simple entailment the systems should classify such a query as illegal
or return no answers because the results for the query are not
well-formed. They are simply invalid RDF. I just noticed also that
SPARQL 1.0 says under 12.1.4 Triple Patterns:
Because RDF graphs may not contain literal subjects, any SPARQL triple
pattern with a literal as subject will fail to match on any RDF graph.
This would mean that what Paul describes (returning literal values as
subjects that occur in the signature) is not a correct behaviour
although I can understand it and we had that in mind for other
entailment regimes too, but according to the specs it wouldn't be the
right thing to do. I guess I will give an example for the entailment
regimes and take the approach that such queries are ill-formed, which
is in accordance with the spec.

Thanks,
Birte

> So, in your case:
>
>  say you we have solution mapping binging ?X to literal "xyz", then
>
> P(BPG) =  "xyz" rdf:type rdf:XMLLiteral .
>
> which is not an RDF Graph and thus not well-formed wrt. any entailment
> regime that falls under the
> scope of the BGP Matching extension, since the definition of well-formedness
> requires well-formedness and



>  "An entailment regime specifies a subset of *RDF graphs* called well-formed
> for the regime"
>
> In other words, BGP matching extensions can only allow solutions
> which yield valid RDF Graphs when applied to the BGP.
>
> Agreed?
>
> cheers,
> Axel
>
>
>
>
>
>
>
>
>
>> --
>> Dr. Birte Glimm, Room 306
>> Computing Laboratory
>> Parks Road
>> Oxford
>> OX1 3QD
>> United Kingdom
>> +44 (0)1865 283529
>>
>>
>
> --
> Dr. Axel Polleres
> Digital Enterprise Research Institute, National University of Ireland,
> Galway
> email: axel.polleres@deri.org  url: http://www.polleres.net/
>
>
>
>



-- 
Dr. Birte Glimm, Room 306
Computing Laboratory
Parks Road
Oxford
OX1 3QD
United Kingdom
+44 (0)1865 283529
Received on Tuesday, 22 September 2009 11:40:37 GMT

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