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Re: Semantics for stateful resources

From: Antoine Zimmermann <antoine.zimmermann@emse.fr>
Date: Thu, 24 May 2012 01:36:47 +0200
Message-ID: <4FBD748F.8050108@emse.fr>
To: Richard Cyganiak <richard@cyganiak.de>
CC: Pat Hayes <phayes@ihmc.us>, RDF Working Group WG <public-rdf-wg@w3.org>
Quick reply:

if I have the following TriG file:

{ #default graph
   :Employee  rdfs:subClassOf  :Person .
   :Unemployed  rdfs:subClassOf  :Person .
:year2008 {
  :Joe  :worksFor  :AcmeCorp .
  :worksFor  rdfs:domain  :Employee .
:year2009 {
  :Joe  a  :Unemployed .
  :Unemployed  owl:disjointWith  :Employee .

1. Would this be entailed:

:year2008 { :Joe  a  :Employee }


2. and this:

:year2008 { :Joe  a  :Person }

3. Would it be inconsistent?


Considering that "graph changes over time" is a *PRIORITY A* use case, 
and this in fact applies to all sorts of dimensions of context 
(including provenance, also in the high priorities---where are 
inferences coming from?), if these inferences are non-entailments, there 
will be extremely important use cases not addressed by the design. 
Therefore, either there must be a semantics where you can make 
inferences inside a "named" graph and put back the inferred statement 
inside the graph (for instance, the semantics I proposed, but it's not 
the only way) or we'd better not normalise any semantics as pfps 
suggests to do. It is also possible to have 2 semantics, it's been done 
in OWL.


Le 24/05/2012 01:09, Richard Cyganiak a écrit :
> Pat,
> On 23 May 2012, at 19:41, Pat Hayes wrote:
>> Richard, I am confused.
> No, in this case, I am. I used the phrase “identifying subgraphs”
> sloppily in my mail to Yves because I echoed the wording of the
> original issue. I should have said “managing subgraphs” or something
> like that.
> But let's talk, this is interesting.
>> Sometimes I get the sense that you want the graph names to refer
>> not to graphs as such, but rather to 'stateful resources' (or
>> whatever) which have a robust identity and emit graphs when poked,
>> a REST-inspired kind of a thing.. (Cf. your responses on other
>> threads.)
> Yes, this. Well, I'd weaken that a bit: I'd like graph names to
> denote things with robust identity that somehow have a graph
> associated. “Emit graph when poked” is one particular and probably
> the most useful kind of association, but I'd like to be less specific
> about the nature of the association. The less specific we are, the
> closer we remain to SPARQL semantics, and the less we get in the way
> of current practice. Tighter definitions can be done as semantic
> extensions where required.
>> At other times, however (as here) you seem to want the graph names
>> to refer to an actual set of triples, a true Platonic RDF graph.
> No, not in general.
> Although “is same as” is just another kind of association, and that
> can be sometimes useful too. If it works as a tortured edge case,
> then that's a plus.
>> It really does matter which we choose, and I don't see how we can
>> choose both (or not without a lot of new machinery to make the
>> distinction, that we have not even discussed yet) and I don't think
>> it is viable to just be muddled or ambiguous about it, as that is
>> the muddle we are in already and are trying to get straight.
> I believe we can do both. I'll try to show how. I'm an amateur at
> this stuff, so forgive me if it's a horrible mess, but it might be
> enough to give you an idea where I'm trying to go with this “stateful
> resource” and “state relationship” business.
> A DS-interpretation is a simple interpretation plus a “state
> relationship”, let's call it ISREL, that contains pairs of resources
> and graphs.
> We could say that if<x,G>  is in ISREL, then x is an
> rdfs:StatefulResource.
> And a state pair<i,G>  is true iff<I(i),G>  is in ISREL.
> Borrowing an idea from Antoine's proposed semantics, I think that
> every rdfs:StatefulResource should have an associated set of
> interpretations, let's call that the “state extension” of the
> stateful resource, that contains exactly the interpretations that
> satisfy the graph associated by the state relationship.
> Something along these lines would probably be the maximum amount of
> normative semantics I'd go along with for datasets.
> But if this works as I hope, then this would give us a base from
> which one could go further. If we want to rigidly denote graphs, for
> example, then we can define a semantic extension that imposes an
> additional semantic condition: For every<x,G>  in ISREL, x = G.
> Done!
> Or, if we want to capture “web semantics”, so that a state pair is
> true iff prodding x yields the graph G, then that's a different
> semantic extension with a different additional condition:<I(i),G>  is
> in ISREL if and only if dereferencing i and parsing as RDF yields
> graph G.
> This keeps the semantics of different graphs in the dataset entirely
> separate. As you know, I think this is a feature. However, I suppose
> that again, additional semantic conditions could change this. I can
> definitely see how it could be useful in the case of “web semantics”
> to require that the names of stateful resources actually denote the
> resource in all the interpretations in the state extension. I suppose
> this could be imposed by requiring that all these interpretations in
> the state extensions share at least the state relationship with the
> “main” interpretation.
>> For example, if the graph names refer to stateful resources, then
>> there are two rather different ways to identify a subgraph or a
>> larger graph. ONe is to speak of a subset (defined somehow) of the
>> graph that is the current state of the stateful resource, the other
>> is to have a relation between two resources such that one returns a
>> subset of what the other returns, at any time. These behave
>> differently and would need to be implemented differently.
> The second approach sounds better to me because the relationship
> between names and graphs is the same for both the larger graph and
> the subgraph. As I said, I think that being noncommittal about the
> actual nature of the relationship between resource and graph (is it
> identity, dereference, or something else?) is a feature.
>> I have no axe to grind here. I would be quite happy if we were to
>> declare that graph names in datasets always refer to stateful
>> resources.
> Then let's go with that.
>> I would also be happy if we decide they always refer to graphs.
> I think *always* doing that is unacceptable. That's because of the
> case where I want to fetch RDF from the web and stick it into a
> dataset using the source URL as a graph name. The source URL denotes
> something out there on the web (an RDF document probably); it
> certainly doesn't denote a graph. So I'm contradicting the web.
>> But I am not happy about it being ambiguous or undecided. I do feel
>> that it is very important that we choose one story and stick to it.
>> Which one do you want to pitch for?
> I feel that the semantic model *needs* an indirection between the
> denoted resource and the graph.
> (What we call the class of denoted resources, and what we call the
> relationship to the graph, then becomes a somewhat secondary
> question. I'm currently trying to see whether “stateful resource” and
> “state” will stick, but that's not actually so terribly important.)
> Best, Richard
>> Pat
>> On May 23, 2012, at 1:12 PM, Richard Cyganiak wrote:
>>> Hi Yves,
>>> I took an action to propose some informative wording regarding
>>> the possibility of identifying subgraphs of a larger graph. See
>>> below for a first attempt. I suppose this would go somewhere near
>>> the definition of “RDF dataset” or whatever we end up calling
>>> these things. The terminology (named graphs etc.) still may have
>>> to change of course. Is this wording ok for you?
>>> Best, Richard
>>> [[ Note: Graphs in an RDF dataset may overlap. The same
>>> underlying set of triples may be divided up into named graphs
>>> along multiple dimensions (such as data owner or subject area) by
>>> repeating each triple in multiple graphs. Whether such a setup
>>> would be realized by storing each triple multiple times, or
>>> through views of some sort, is up to the implementation. ]]
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Antoine Zimmermann
ISCOD / LSTI - Institut Henri Fayol
École Nationale Supérieure des Mines de Saint-Étienne
158 cours Fauriel
42023 Saint-Étienne Cedex 2
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Received on Wednesday, 23 May 2012 23:36:32 GMT

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