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Re: Question about literals in subject position

From: Bijan Parsia <bparsia@cs.man.ac.uk>
Date: Thu, 24 Sep 2009 10:50:53 +0100
Cc: "public-rdf-dawg@w3.org Group" <public-rdf-dawg@w3.org>
Message-Id: <1FBFBD30-9B01-414C-A627-0314B26C99E1@cs.man.ac.uk>
To: Steve Harris <steve.harris@garlik.com>
On 24 Sep 2009, at 10:31, Steve Harris wrote:

> 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.

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]
>>
>> In other words, are we in the future we proofed against :)
>
> Heh, perhaps :)
>
> I think currently it's always false, due to 12.6.

I concur.

> 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 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.)

Cheers,
Bijan.
Received on Thursday, 24 September 2009 10:03:49 GMT

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