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Re: Question about literals in subject position

From: Axel Polleres <axel.polleres@deri.org>
Date: Tue, 22 Sep 2009 12:02:35 +0100
Cc: "SPARQL Working Group" <public-rdf-dawg@w3.org>
Message-Id: <A887C38D-8555-4EBA-B1A4-06D7435D62C7@deri.org>
To: "Birte Glimm" <birte.glimm@comlab.ox.ac.uk>

On 22 Sep 2009, at 11:44, Birte Glimm wrote:

> Hi all,
> I just wanted to confirm that I understand the specs correctly...
> As I read the SPARQL query grammar, I understand it that literals are
> allowed in subject position. Thus, a query such as
> SELECT ?x WHERE { ?x rdf:type rdf:XMLLiteral }
> under say RDF or RDFS entailment would have to return all valid RDF
> XML literals (if not sufficently restricted) of which there are
> infinitely many. Although RDF itself does not allow literals in
> subject position, SPARQL queries do allow it and in fact according to
> the RDF(S) semantics (under RDF(S) entailment) the statement XXX
> rdf:type rdf:XMLLiteral is entailed by any graph for XXX a valid XML
> Literal lexical form. All correct?
> Birte
>

Hi Birte,

interesting catch! However, I think that we have to look at it from the
perspective of how extending BGP matching rather than purely from the  
definitions of RDF(S) entailment only:

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, 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/
Received on Tuesday, 22 September 2009 11:03:16 GMT

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