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Re: Skolemization and RDF Semantics

From: Pat Hayes <phayes@ihmc.us>
Date: Sun, 17 Apr 2011 08:41:27 -0500
Cc: Dan Brickley <danbri@danbri.org>, David Wood <dpw@talis.com>, "public-rdf-wg@w3.org" <public-rdf-wg@w3.org>
Message-Id: <A418B3E7-81E0-4F6A-89BA-B3FDE6B8CA09@ihmc.us>
To: Steve Harris <steve.harris@garlik.com>

On Apr 17, 2011, at 5:58 AM, Steve Harris wrote:

> On 2011-04-17, at 05:38, Pat Hayes wrote:
> 
>> 
>> On Apr 16, 2011, at 9:46 AM, Steve Harris wrote:
>> 
>>> On 2011-04-15, at 20:35, Dan Brickley wrote:
>>> 
>>>> On 15 April 2011 21:29, David Wood <dpw@talis.com> wrote:
>>>>> Hi all,
>>>>> 
>>>>> Ivan and IIRC Steve or Richard tried to convince me that Skolemization is already mentioned and allowed in the 2004 version of the RDF Semantics.  I can't find it.  Can someone please enlighten me?  Thanks.
>>>> 
>>>> http://www.w3.org/TR/rdf-mt/#prf
>>> 
>>> This section also contains the text which means it's not currently OK (by my reading) to expose skolem constants to the outside world:
>> 
>> No, the cited passage does not imply that. I am at a loss to see how you think it can be interpreted that way. 
>> 
>>> “Intuitively, this lemma shows that asserting a Skolemization expresses a similar content to asserting the original graph, in many respects. However, a graph should not be thought of as being equivalent to its Skolemization, since these 'arbitrary' names would have the same status as any other URI references once published. Also, Skolemization would not be an appropriate operation when applied to anything other than the antecendent of an entailment.”
> 
> “a graph should not be thought of as being equivalent to its Skolemization, since these 'arbitrary' names would have the same status as any other URI references once published”
> 
> I don't know how else to interpret that - what is it supposed to convey?

That skolem URIs are URIs. 

> 
>>> There's some more context too.
>>> 
>>> Interestingly it also touches on the issue of being able to identify URIs that are skolem constants, here's a quick summary of the issues in that area:
>>> 
>>> SPARQL stores which skolemise on export need to be able to identify their own skolemised bNodes so they can correctly handle SPARQL Query/Update operations which use them.
>> 
>> Can you (or anyone) explain this point more carefully? What exactly is a "SPARQL store" ? Is this simply RDF, or does it have more structure? What exactly does it mean for a SPARQL store to "export"? If this simply publishing RDF, or does it have a SPARQL-specific meaning? And if so, what is it exactly? 
> 
> Like Ivan said, I was referring to SPARQL SELECT and CONSTRUCT results. I couldn't think of a more neutral term for exposing nodes in the graph to he outside world.

OK, got that now. I was worried there might be some huge pile of SPARQL machinery which I wasn't aware of :-)

Pat

> 
> - Steve
> 
> -- 
> Steve Harris, CTO, Garlik Limited
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Received on Sunday, 17 April 2011 13:42:03 GMT

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