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Re: Union or not union for the default graph...

From: Andy Seaborne <andy.seaborne@epimorphics.com>
Date: Thu, 12 Apr 2012 14:36:01 +0100
Message-ID: <4F86DA41.6020908@epimorphics.com>
To: Ivan Herman <ivan@w3.org>
CC: public-rdf-wg@w3.org


On 12/04/12 14:19, Ivan Herman wrote:
>
> On Apr 12, 2012, at 15:12 , Andy Seaborne wrote:
>
>>>> Should that be reflected in the TriG file? Actually, maybe
>>>> not.
>>
>> I confess I don't see the need for this in the semantics.
>>
>> Whether the default graph is treated as the union of the named
>> graphs is a decision made by the application (see William's point),
>> in which case the default graph is just some graph it has (locally)
>> created.  It will do that based on whether it thinks the incoming
>> data is trustworthy and appropriate to the task.
>>
>> It might be useful to (non-normatively) discussion default union
>> graphs but it is just a union graph like any other.
>>
>> It might be useful to be able to have syntax to say "for this TriG
>> file make the default graph the union of named graph" but that is a
>> syntactic shortcut to avoid having to repeat the data, no semantics
>> implications.
>>
>
> What seems to be the case is that (a) there are use cases where the
> union semantics is needed

Hmm - I see use cases which can be addressed by the application deciding 
to use the default union graph.  I'm not sure that I see any where it is 
needed as part of the semweb web-wide architecture, i.e. the publishing 
entity making the declaration and expecting it to be the only way it's used.

> and (b) this may not be the only semantics,

:-) so many to choose from

> so there is a need for a switch. Now we can put that switch in the
> TriG file via some syntax (which is what you seem to propose)or an
> out-of-band switch that is used to control the implementation; that
> what I was proposing. I would not _really_ care as long as the choice
> is clear (and defining the semantics is a way of making it really
> clear) and there is a way to make the choice, although Steve's recent
> mail seems to indicate that for his system at least the 'out-of-band'
> approach may be more appropriate.
>
> Cheers
>
> I.

	Andy

>
>> Andy
>>
>> On 12/04/12 13:35, Ivan Herman wrote:
>>> I have sent this to the wrong mailing list yesterday evening;
>>> Andy pinged me that I did that:-( so here is the mail again.
>>
>> and only because it didn't end up in the right folder.
>>
>>>
>>> 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...)
>>>>
>>>>
>>>
>>>
>>> ---- Ivan Herman, W3C Semantic Web Activity Lead Home:
>>> http://www.w3.org/People/Ivan/ mobile: +31-641044153 FOAF:
>>> http://www.ivan-herman.net/foaf.rdf
>>>
>>>
>>>
>>>
>>>
>>
>
>
> ---- Ivan Herman, W3C Semantic Web Activity Lead Home:
> http://www.w3.org/People/Ivan/ mobile: +31-641044153 FOAF:
> http://www.ivan-herman.net/foaf.rdf
>
>
>
>
>
Received on Thursday, 12 April 2012 13:36:37 UTC

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