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Re: Minimal dataset semantics

From: Pat Hayes <phayes@ihmc.us>
Date: Thu, 23 Aug 2012 13:02:31 -0500
Cc: Richard Cyganiak <richard@cyganiak.de>, RDF Working Group WG <public-rdf-wg@w3.org>
Message-Id: <BE82DE05-2F74-48A5-A2B4-81EA27701618@ihmc.us>
To: Sandro Hawke <sandro@w3.org>

On Aug 23, 2012, at 10:17 AM, Sandro Hawke wrote:

> On 08/23/2012 11:06 AM, Richard Cyganiak wrote:
>> If we define a semantics for datasets at all, it should be such that:
>> 
>> 1. It formalizes what is denoted by graph IRIs
>> 2. Interesting semantic extensions can be made by third parties or future WG
>> 3. It doesn't preclude any reasonable semantic extensions
>> 
>> The problem with the quoting semantics is that it fails #3. 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 discussion here in recent days has focused on the complete-graph quoting semantics, but in previous telecons we had near consensus (everyone but Eric

And but Pat, if you are going to say that the graph "name" actually refers to a graph. 

> ) on using the partial-graph semantics.    That's what my draft had.
> 
> 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. 

>> 
>> No other proposed semantics does have that problem. All of the other proposed semantics can be easily extended with an additional clause that requires equal graph names to be associated with equal graphs.
>> 
>> Therefore, the quoting semantics is *not minimal*. Quite the opposite. It is not a "weak" semantics at all, because it makes it very easy to derive contradictions, and contradictions are *very strong* semantic effects.
>> 
>> I also agree with Antoine that formalizing the notion of "no semantics" is pointless.
>> 
>> My conclusion is that our viable options are:
>> 
>> a) to say nothing regarding the semantics of datasets, or
> 
> Can we do that and still do your point (1) and (2) above?    If so, I like that idea.    I'm just not sure if that's possible (or how to do it).
> 
>> b) to define a minimal version of a "truth-based"/"entailment-based" semantics (where [[ :i1 { G } ]] entails [[ :i1 { G' } ]] if graph G entails graph G').

Which means that the "name" refers to the entailment closure of the graph, rather than the graph itself. Right? 

Pat

>> 
> 
> (I'm going to stay out of this option for now; need to think about it more.)
> 
>    -- Sandro
> 
> 

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Received on Thursday, 23 August 2012 18:03:04 UTC

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