Union or not union for the default graph...

I have sent this to the wrong mailing list yesterday evening; Andy pinged me that I did that:-( so here is the mail again.

Since then I have also added the text to the Wiki page. 

Ivan

Begin forwarded message:

> From: Ivan Herman <ivan@w3.org>
> Subject: union or not union for the default graph...
> Date: April 11, 2012 21:21:41 GMT+02:00
> To: Sandro Hawke <sandro@w3.org>, Andy Seaborne <andy.seaborne@epimorphics.com>, Pat Hayes <phayes@ihmc.us>
> Cc: W3C Public RDF Mailing list <public-rdf@w3.org>
> 
> Just some thoughts late at night for me... I just realized something.
> 
> I think we may have to acknowledge that what I would call the "quoting" semantics (which is the one on the wiki page) and a "union" semantics may both be necessary, depending on the application. Tom's frbr example needed the union semantics, ie, where the default graph includes the union of all constituent graphs in the datase for the interpretationt, whereas the "quote" semantics is clearly used in other settings. We know that SPARQL engines exist that operate with one, others with the other.
> 
> Defining the two semantics formally is obvious. The "quote" semantics is the one on the wiki page; the "union" semantics differs only in term of the definion of the vocabulary which would have to include the union of all graphs, too. Otherwise it is the same.
> 
> Should that be reflected in the TriG file? Actually, maybe not. It depends on the application and the underlying system which semantics is used. The same dataset can be interpreted this way or that way. We already have such a situation: the same OWL RL data can be interpreted through the Direct Semantics and the RDF Compatible Semantics. There are subtle differences between the two and it is up to the application to choose. The difference between the "quoting" and the "union" semantics is much less subtle, but the situation is similar. We may also have engines that do RDF semantics, others do D-entailment, or RDFS entailment...
> 
> Bottom line: we may define two different dataset semantics (ie, two different interpretations) and... leave it that.
> 
> Thoughts?
> 
> Ivan
> 
> ---
> Ivan Herman
> Tel:+31 641044153
> http://www.ivan-herman.net
> 
> (Written on mobile, sorry for brevity and misspellings...)
> 
> 


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Ivan Herman, W3C Semantic Web Activity Lead
Home: http://www.w3.org/People/Ivan/
mobile: +31-641044153
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