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Re: Minimal dataset semantics

From: Richard Cyganiak <richard@cyganiak.de>
Date: Mon, 27 Aug 2012 10:34:53 +0100
Cc: public-rdf-wg@w3.org
Message-Id: <052F6761-CA9E-4704-B69A-CF3C08D5B86D@cyganiak.de>
To: Antoine Zimmermann <antoine.zimmermann@emse.fr>
Antoine,

On 24 Aug 2012, at 17:15, Antoine Zimmermann wrote:
> However, we could as well make the graph extension be a function from IRIs to RDF Graphs (let us call it IRI-GEXT) instead of a function from resources to RDF Graphs (let us call it RES-GEXT).
> 
> With RES-GEXT, the following:
> 
> <ex:bob>  owl:sameAs  <eg:bob> .
> <ex:bob> { <ex:bob> <name> "Robert Doe" .
>           <x>  owl:sameAs  <y> }
> <eg:bob> { <eg:bob> <name> "Robert Doe" .
>           <x>  owl:differentFrom  <y> }
> 
> is inconsistent. The problem is, perhaps ex:bob and eg:bob really refer to the same thing, and the dataset is using the scheme "graph IRI denotes primary topic" which we agreed is an acceptable scheme. Yet, here, the semantics with RES-GEXT does not allow to separate the inferences of the two graphs.
> This problem does not occur with IRI-GEXT.

I see this as a fairly minor distinction.

Yes, if you use the “graph IRI denotes primary topic” convention and put “owl:sameAs” triples into the default graph, then the two graphs will be semantically merged and you cannot keep their inferences apart. I can live with that.

I prefer RES-GEXT because it is clearer about what the graph IRIs denote. IRI-GEXT doesn't tell us anything about what the graph IRIs denote. RES-GEXT says that the graph IRIs denote some entity with a graph extension. This would make it more natural to say (in semantic extensions, not in core RDF Semantics) things like: “The graph extension of any RDF document published on the web is the RDF graph encoded in the document.” Or: “An RDF graph is itself a resource, and it has itself as its graph extension.”

These things *can* be said using IRI-GEXT too, but require one more step of indirection (at least with the mental model I currently have in my head.)

(This discussion is a nice example for the subtle differences that arise from the choice of formalisation, and why saying nothing about dataset semantics might end up being a bad idea: Different users of datasets will make their own and often incompatible assumptions.)

> here is a possible formalisation of this.
> Let us assume an entailment regime E.
> A dataset-interpretation is an E-interpretation I plus a function GEXT from the set of IRIs to the set of RDF Graphs.
> For a dataset D = (DG,(n1,G1),...,(nk,Gk)),
> - for an IRI n and a graph G, I(n,G) is true iff there exists a graph G' such that (n,G') is in GEXT and G' E-entails G;

I guess for RES-GEXT, this line would be changed to: ...such that (I(n),G') is in GEXT... ?

> - if I(G) is false, then I(D) is false;

I think you mean: if I(DG) is false...?

> - if I(ni,Gi) is false for some i, then I(D) is false;
> - I(D) is true otherwise.
> 
> Nice, concise, and achieve essentially the same as my initial dataset semantics.

+1

Best,
Richard



> 
> Le 24/08/2012 13:23, Richard Cyganiak a écrit :
>> On 24 Aug 2012, at 11:48, Antoine Zimmermann wrote:
>>> Is it the resource denoted by the IRI, or is it the IRI that is
>>> associated with the graph?
>> 
>> Please be precise what you mean by "associated".
>> 
>> In the abstract syntax, we have <i,G> pairs. So i and G are paired in
>> a named graph (as per existing SPARQL terminology). Resources do not
>> exist in the abstract syntax.
>> 
>> In the semantics, the pair <i,G> says that the IRI denotes some
>> resource, and that resource has some graph as its graph extension,
>> and that graph entails G.
>> 
>> Or if you want less formal, informative language: In the named graph
>> <i,G>, G is a claim about the state of the resource denoted by i.
>> 
>>> Consider the following, IMHO realistic, example:
>>> 
>>> ex:url1 and ex:url2 are the URLs RDF Documents found on the Web.
>>> The dafult graph is used to put mappings between different graphs.
>>> 
>>> # EXAMPLE BEGINS # ex:url1  owl:sameAs  ex:url2 . ex:url1 { ex:url1
>>> foaf:mbox  <mailto:frank@sinatra.net>} ex:url2 { ex:url2  foaf:mbox
>>> <mailto:frank@sinatra.net>} # EXAMPLE ENDS #
>>> 
>>> (I don't want to hear that this incorrect according to httpRange14.
>>> httpRange14 is not part of RDF and, in any case, this example shows
>>> something that happens on the Web for real.)
>>> 
>> 
>>> Now, if it's the resource that is associated with the graph, then
>>> we have a resource associated with two graphs.
>> 
>> Please be precise what you mean by "associated".
>> 
>> In the abstract syntax, we have two different IRIs paired with two
>> different named graphs. No ambiguity.
>> 
>> In the semantics, assuming OWL entailment, the dataset states that
>> ex:url1 and ex:url2 both denote some resource, and that graph has
>> some graph as its graph extension, and that graph entails the two
>> graphs you showed. No ambiguity.
>> 
>> Less formal wording of the semantic situation: We have two named
>> graphs that both make claims about the state of the same resource.
>> The claims don't contradict, so all is fine. No ambiguity.
>> 
>>> Why not, but then it becomes ambiguous when you talk about the
>>> graph associated with a resource. Which graph? Do you talk about
>>> both graphs all the time.
>> 
>> I never talked about associating graphs with resources. I talked
>> about associating graphs with IRIs, in the sense of "pairing them in
>> a named graph as per existing SPARQL terminology".
>> 
>>> I think that, in conversation, when we say "the graph associated
>>> with ex:url1", we tend to mean whatever is in the curly brackets
>>> after the IRI rather than an RDF graph that is supposed to be
>>> attached to a thing in the world.
>> 
>> I agree.
>> 
>>> We can put IRIs in the interpretation, because IRIs are not only
>>> part of the syntax, they are also made part of the semantics by
>>> being the literal values of literals with datatype IRI xsd:anyURI.
>> 
>> Yes, but I'm not sure what this has to do with anything.
>> 
>>> Anyway, this idea of having <i,G> pairs in the interpretation
>>> should be formalised as it may be a solution that avoids putting
>>> several RDF interpretations into one Dataset interpretation.
>> 
>> +1
>> 
>> Richard
>> 
>> 
>> 
>>> 
>>> 
>>> --AZ.
>>> 
>>> 
>>> Le 23/08/2012 21:43, Richard Cyganiak a écrit :
>>>> On 23 Aug 2012, at 19:02, Pat Hayes wrote:
>>>>>>> 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?
>>>> 
>>>> I'm not sure what you mean by "refers to".
>>>> 
>>>> Regarding the formal mechanism of associating IRIs with their
>>>> respective graphs, I've started to like the idea that Alan
>>>> mentioned the other day: Dataset interpretations contain a
>>>> mapping from resources to graphs, called the graph extension.
>>>> This mapping associates graphs with (some) resources. A
>>>> name-graph-pair (a.k.a. abstract named graph)<i,G>  satisfies a
>>>> dataset interpretation I if the graph extension in I of I(i)
>>>> entails G.
>>>> 
>>>> If I'm not messing up, then this mechanism works the same as the
>>>> property extension and class extension mechanisms that already
>>>> exist in RDF Semantics.
>>>> 
>>>> So, the graph IRI then *denotes* a resource (one that is in the
>>>> domain of the class extension function). And in the abstract
>>>> syntax, the graph IRI is "associated" or "paired" with a certain
>>>> RDF graph. I don't know if one can say that the IRI "refers to"
>>>> the entailment closure of the graph.
>>>> 
>>>> Does this make any sense?
>>>> 
>>>> Best, Richard
>>>> 
>>> 
>>> -- Antoine Zimmermann ISCOD / LSTI - Institut Henri Fayol École
>>> Nationale Supérieure des Mines de Saint-Étienne 158 cours Fauriel
>>> 42023 Saint-Étienne Cedex 2 France Tél:+33(0)4 77 42 66 03
>>> Fax:+33(0)4 77 42 66 66 http://zimmer.aprilfoolsreview.com/
>>> 
>> 
>> 
>> 
> 
> 
> -- 
> Antoine Zimmermann
> ISCOD / LSTI - Institut Henri Fayol
> École Nationale Supérieure des Mines de Saint-Étienne
> 158 cours Fauriel
> 42023 Saint-Étienne Cedex 2
> France
> Tél:+33(0)4 77 42 83 36
> Fax:+33(0)4 77 42 66 66
> http://zimmer.aprilfoolsreview.com/
> 
Received on Monday, 27 August 2012 09:35:22 UTC

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