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Re: Minimal dataset semantics

From: Sandro Hawke <sandro@w3.org>
Date: Thu, 23 Aug 2012 11:17:36 -0400
Message-ID: <50364990.1010503@w3.org>
To: Richard Cyganiak <richard@cyganiak.de>
CC: RDF Working Group WG <public-rdf-wg@w3.org>
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) 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.

>
> 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').
>

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

     -- Sandro
Received on Thursday, 23 August 2012 15:17:42 UTC

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