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Re: Question about literals in subject position

From: Bijan Parsia <bparsia@cs.man.ac.uk>
Date: Thu, 24 Sep 2009 10:16:34 +0100
Cc: Birte Glimm <birte.glimm@comlab.ox.ac.uk>, SPARQL Working Group <public-rdf-dawg@w3.org>
Message-Id: <5936F94A-EA36-48F7-849C-D81A51C15090@cs.man.ac.uk>
To: Steve Harris <steve.harris@garlik.com>
On 24 Sep 2009, at 10:01, Steve Harris wrote:
> My understanding is that it is a legal query, it's prohibited  
> neither by the syntax, nor the query algebra.
> However, it can't ever match any RDF triples in the query processors  
> data, as there can be no legal triples with literal subjects.

Without inference. In the presence of RDFS interpretations, there are  
situations where one might reasonably think that
	"<foo/>"^^rdf:XMLLiteral rdf:type rdfs:Literal.

is entailed by:
	:s :p "<foo/>"^^rdf:XMLLiteral.

The only thing preventing that entailment would be a syntactic  
restriction on the form of things entailed (e.g., if we restrict  
ourselves to legal RDF graphs.)

> I don't remember the exact motivation, though I seem to remember  
> that it came up in discussion in the DAWG group, but it does give us  
> a level of future proofing, and allows systems which allow literal  
> subjects to easily extend SPARQL to query their full data.

I guess the relevant question is do we want RDFS entailment in SPARQL  
queries to work strictly on RDFS graphs, in which case:
	ASK WHERE {"<foo/>"^^rdf:XMLLiteral rdf:type rdfs:Literal.}
is always false, or work with ground BGPs, in which case that query  
would sometimes return true. (E.g., against the above triple).

In other words, are we in the future we proofed against :)

Received on Thursday, 24 September 2009 09:17:10 UTC

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