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Re: Question about literals in subject position

From: Steve Harris <steve.harris@garlik.com>
Date: Thu, 24 Sep 2009 10:31:36 +0100
Message-Id: <123CC927-04F3-4957-866D-8DCFDE3658F5@garlik.com>
To: "public-rdf-dawg@w3.org Group" <public-rdf-dawg@w3.org>
On 24 Sep 2009, at 10:16, Bijan Parsia wrote:

> On 24 Sep 2009, at 10:01, Steve Harris wrote:
> [snip]
>> 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.)

SPARQL/Query 1.0 is restricted to legal RDF graphs though, as per (my  
reading at least) of section 12.6.

>> 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 :)

Heh, perhaps :)

I think currently it's always false, due to 12.6.

Certainly there are RDF-like systems which permit literals in the  
subject position, so for some people at least it is significant.

- Steve

-- 
Steve Harris
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Received on Thursday, 24 September 2009 09:38:58 GMT

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