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Re: Why empty named graphs make sense

From: Andy Seaborne <andy.seaborne@epimorphics.com>
Date: Wed, 30 May 2012 09:03:50 +0100
Message-ID: <4FC5D466.9020902@epimorphics.com>
To: RDF-WG <public-rdf-wg@w3.org>


On 30/05/12 01:41, Sandro Hawke wrote:
> On Wed, 2012-05-16 at 17:33 +0100, Andy Seaborne wrote:
>> 1/ Scenario:
>>
>> A site publishes a page, in RDFa, with triples in it.
>>
>> The site replaces that page, with one saying, "changes in progress,
>> please come back later" with no triples.
>>
>> Recording this seems like a reason to have empty named graphs.
>
> I agree it's useful, I just don't know how to make it work with
> partial-graph semantics.   See "subset semantics" in recent message,
> http://lists.w3.org/Archives/Public/public-rdf-wg/2012May/0650.html
>
> If
>       <g1>  { }
> and
>       <g1>  { :a :b :c }
> entails
>       <g1>  { :a :b :c }
>
> then information was the first graph conveying?

Existence.

Change - had triples, now it does not.

Not a 404.


> I think it was
> conveying that<g1>  had at least no triples, which isn't conveying much
> at all.

> I think this use case could be addressed with a vocabulary instead of
> syntax, as you suggest I think for dealing with quadstores.  That is,
> instead of allowing in a dataset:
>
>     <g>  { }
>
> we could put in the default graph something like:
>
>     <g>  a rdf:EmptyResource.
>
> It's certainly not as elegant, though.   (and not compatible with
> existing SPARQL systems.)

Nor RDF/XML, N-Triples or Turtle.

> Oh.   I guess with partial-graph semantics (aka subset semantics) one
> could also make it complete with a count of the triples.  So
>
>     <g>  { :a :b 1,2,3 }.
>     <g>  rdf:tripleCount 3.
>
> would tell us<g>  has those three triples and ONLY those three triples.
> With that, the complete-graph-semantics dataset:
>
>     <g>  { }
>
> could be conveyed by the partial-graph-semantics dataset:
>
>     <g>  rdf:tripleCount 0.
>
> Still not elegant, but it might be about as simple as possible.

Seems like jumping through weird hoops for something that just works out 
anyway.

	Andy


>
>      -- Sandro
>
>
>> 2/ The default graph can be empty.
>>
>> 3/ A point made has been that N-quads can't represent an empty named graph.
>>
>> A syntax fix could be:
>>
>> _<http://space>  .
>>
>> or other style indicate a space name.
>>
>>
>> To my reading, the spaces document works if "quads" are removed, not
>> that I'm suggesting that.
>>
>> How about defining as a convenience of useful vocabulary rather than
>> core definition?
>>
>> 	Andy
>>
>>
>
>
>
Received on Wednesday, 30 May 2012 08:04:28 GMT

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