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Re: Minimal dataset semantics

From: Richard Cyganiak <richard@cyganiak.de>
Date: Thu, 23 Aug 2012 17:02:07 +0100
Cc: RDF Working Group WG <public-rdf-wg@w3.org>
Message-Id: <E5B0D2D6-020C-4FBD-B0D0-C1EC7FF437EF@cyganiak.de>
To: Ivan Herman <ivan@w3.org>
On 23 Aug 2012, at 16:23, Ivan Herman wrote:
> the text says:
> 
> [[[
> The interpretation of an RDF dataset is the interpretation of its default graph. The presence or absence of named graphs does not affect the truth of a dataset.
> ]]]

So the second sentence rules out any semantic extensions that make a dataset's truth dependent on the content of some named graph, right?

> whether the mathematical formalism reflects that or not is secondary; if it does not, than it is wrong.
> 
> That being said, I presume you refer to the entry in the formalism that says 
> 
> ∀i,j, i,j=1,…,n: if ui = uj then Gi and Gj are equal.

No, I refer to the entry that says I(ui) = Gi.

Let's say I have datasets A and B, where the graph associated with u1 in A is a, and the graph associated with u1 in B is b. Let's say that a≠b.

Now from I(ui)=Gi it follows that in any interpretation that satisfies A, I(u1)=a. It also follows that in any interpretation that satisfied B, I(u1)=b. Now, can there be any interpretation that satisfies both A and B? No, because that would require a=b.

In other words, there is no interpretation -- no possible arrangement of the universe -- that can make both A and B true. In other words, A contradicts B.

> but it also says: 
> 
> [[[
> In this section the “equality” of graphs in a dataset means that they are mutually inferable through simple entailment.
> ]]]
> 
> So yes, the current quoting semantics does not do your entry (b) below, but I am not sure I understand your statement about the contradiction.

Does the explanation above clear this up?

> But all that may be moot with the recent proposals of Antoine, so this issue may be overran by events.

I'm afraid I didn't manage to follow everything that went on in the thread. What was the outcome? Is there a new minimalist proposal on the table?

Best,
Richard



> 
> Ivan
> 
> 
> On Aug 23, 2012, at 17:06 , 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.
>> 
>> 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
>> 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').
>> 
>> Best,
>> Richard
> 
> 
> ----
> 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, 23 August 2012 16:02:41 UTC

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