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Re: Draft for a "minimal dataset semantics"

From: Peter F. Patel-Schneider <pfpschneider@gmail.com>
Date: Thu, 13 Sep 2012 14:13:27 -0400
Message-ID: <50522247.7010809@gmail.com>
To: Antoine Zimmermann <antoine.zimmermann@emse.fr>
CC: RDF WG <public-rdf-wg@w3.org>

On 09/13/2012 12:50 PM, Antoine Zimmermann wrote:
> Le 13/09/2012 03:08, Peter F. Patel-Schneider a écrit :
>
[...]

>
> Here is another test case:
>
> { :n  owl:sameAs  :m }
> :n { :s  :p  :o }
> :m { :x  owl:differentFrom  :x }
>
> OWL-dataset-entails:
>
> :n { :t  owl:sameAs  rdf:type }
>
>

Well, depending on which version of OWL.  OWLfull, yes.

>> I don't think that this means that 2.16 is not tricky.  There is some
>> interaction.  However, this means that named graphs are not independent
>> from each other.  In fact, there is a much easier situation showing that
>> named graphs are not independent, namely 2.13 T11.2  Similarly, it means
>> that named graphs are not independent from the default graph.
>>
>> It is also the case that an inconsistent default graph makes the named
>> graphs irrelevant.
>
> It makes the dataset inconsistent, which is fortunate.

I don't consider this to be particularly fortunate.  Why should an 
inconsistent default graph make the named graphs irrelevant?  Why should the 
default graph situation bleed into the named graphs at all?
>
>
>> This last is, I think, a particularly strong point against providing
>> this sort of semantics at all.
>
> By "this last", what do you mean? This last test case (T14.1) or this last 
> sentence that you wrote above?
>
> I think you mean the former (if it's the latter, I don't see why). Do you 
> think that, if the graphs--named and default--were independent, it would be 
> acceptable?  That's the alternative proposal where IGEXT maps IRIs to 
> graphs, instead of resources to graphs.

I prefer having the named graphs independent of each other, and independent of 
the default graph.  I might be able to live with the situation where an 
inconsistent default graph makes the named graphs irrelevant, but why should I 
have to?  We could just go back to the proposals from much earlier where these 
sorts of issues do not arise.

>
> But even with IGEXT mapping resources, I find the test case to be even more 
> satisfying if it's consistent. It makes it easy to implement a reasoner in 
> that case. Take a reasoner for entailment regime E.
> Check if any two of the graph IRIs denote the same thing in the default 
> graph. For each group of owl:same graph IRIs, reason on the merge of the 
> graphs in the group, using the E-reasoner.

This is not sufficient.  You can do the forcing n into m trick to require lots 
of case splitting.

If named graphs are just named by IRIs then this is not needed.

peter

PS:  Here is a reiteration of the old proposals.

There is no independent notion of interpretations of RDF datasets.    If you 
want to do something like entailment between RDF datasets you can either look 
at one component of the dataset, so you ask whether the default graphs sit in 
an entailment relationship or ask whether the graphs with a particular name 
sit in an entailment relationship, or you can look at the entire dataset, so 
you ask whether the default graphs sit in an entailment relationship and all 
the similarly-named graphs sit in an entailment relationship.  In each case 
you probably want to consider a missing named graph to be the empty graph.  
This ends up being more flexible and considerably simpler than the minimal 
semantics currently being proposed, as well as requiring no new reasoning 
machinery.
Received on Thursday, 13 September 2012 18:13:59 GMT

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