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Re: Minimal dataset semantics

From: Sandro Hawke <sandro@w3.org>
Date: Thu, 23 Aug 2012 15:09:09 -0400
Message-ID: <50367FD5.2070102@w3.org>
To: Pat Hayes <phayes@ihmc.us>, Richard Cyganiak <richard@cyganiak.de>
CC: RDF Working Group WG <public-rdf-wg@w3.org>
[combining responses to Pat and Richard]

On 08/23/2012 02:02 PM, Pat Hayes wrote:
>> >Partial-graph quoting semantics might also be called "quad" semantics.   You can decompose the dataset into quads that each stands on its own; merging datasets is just the set-union of the quads.  Each quad tells you that a particular triple is in a particular named graph.    There is no way to form a contradiction among such quads.
> True, but this also makes the "naming" useless for most purposes, as with this semantics there is no way to ever discover what graph the "name" actually names. It would be consistent to assume that all graph "names" denote the full RDF graph containing all possible RDF triples. This negates the whole point of having such a semantics, IMO. It is certianly not what the original term 'named graph' was supposed to mean, and I strongly suspect is not a sense of 'named graph' that is in use anywhere outside the WG.
This was a surprise to me, too, but Lee reported that they often wrote 
TriG like this:

=== file 1
... stuff

<g1> { ... some triples that go in g1 ... }

... more stuff

<g1> { ... more triples that go in g1 ... }

=== file 2

<g1> { ... even more triples that go in g1 ...}

============

When I suggested that file1 and file2 contradicted each other, I recall 
several people agreeing with Lee.  They want to be able to have the 
triples in g1 spread out throughout multiple locations in multiple files.

When we talked about this, it seemed to many people to be perfectly 
natural, to be "Open World."   Saying that some triples are in some 
named graphs isn't as useful as enumerating all the triples in that 
named graph (and declaring you are doing so), but it's not useless.

On 08/23/2012 11:46 AM, Richard Cyganiak wrote:
> On 23 Aug 2012, at 16:17, Sandro Hawke wrote:
>>> The quoting semantics makes it a contradiction if dataset A and dataset B contain the same graph IRI with different associated graphs. We cannot do semantic extensions that produce useful additional entailments from a contradiction.
>> Not true.   There are two versions of the quoting semantics -- partial-graph semantics and complete-graph semantics.
> The "partial-graph quoting semantics" is really just a broken form of "truth semantics". It is broken because it arbitrarily emulates one effect of entailment (the fact that dropping any triple from a true graph yields a true graph), while not emulating all the other effects of entailment (e.g., the fact that substituting any subject or object with a blank node in a true graph yields another true graph, or that adding redundant inferred triples doesn't affect truth, or that substituting one literal with an equal-valued literal doesn't affect truth, or that replacing one blank node with another fresh blank node doesn't affect truth).

I agree the machinery of partial-graph quoting semantics is the same as 
that of a very limited truth semantics, as you describe it.    I think 
the intuitions and motivations are quite different, however.

>>    The discussion here in recent days has focused on the complete-graph quoting semantics, but in previous telecons we had near consensus (everyone but Eric) on using the partial-graph semantics.
> No, we had near consensus that "complete-graph quoting semantics" is not desirable. I think what you interpret as support for "partial-graph quoting semantics" was mostly support for "some form of truth semantics".
>

That's not how I recall it, and looking over the record doesn't change 
my mind.   It doesn't really matter whether we had consensus or not, 
except as far as it helps shed light on where we are now, so I only 
bring it up for that purpose.   But please do look over point 7 on 
http://lists.w3.org/Archives/Public/public-rdf-wg/2012Apr/0165.html .

Also, to be clear, I'm not trying to advocate for any particular 
semantics -- I just would like something that gets folks closer to being 
able to address the use cases.   I think complete-graph quoting 
semantics is probably that, but I took guidance from that 25 April 
meeting that partial-graph quoting semantics was a more promising 
route.   In my implementation, as I recall, partial worked fine, with 
the usual RDF best-practice that systems semantically *can* drop triples 
but in general they do/should not.

      -- Sandro
Received on Thursday, 23 August 2012 19:09:20 UTC

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