Re: Really minimal dataset semantics

Le 21/09/2012 20:06, Pat Hayes a écrit :
>
> On Sep 21, 2012, at 10:44 AM, Peter F. Patel-Schneider wrote:
>
>> I don't think that this is converging, even on technical grounds.
>>
>> My view is that many or most of the use cases in
>> http://www.w3.org/2011/rdf-wg/wiki/TF-Graphs-UC have to do with
>> talking about actual graphs.
>
> I agree this is most obvious way to interpret the idea of a graph
> having a name. And it sems to be required in order to have metadata
> about graphs being expressed in RDF.
>
> My favorite semantics for datasets would simply say that the meaning
> of a dataset containing a pair <u, G> is that u denotes G. Fomally,
> I(<u,G>) is true when I(u)=G.  This gives the entailment rule that
> {G, N} entails {G' N'} just when N' is a subset of N. We could add in
> asserting the default graph if that makes people happy.

We've debated this over and over again, and we know very well that it is 
incompatible with certain use cases. In any case, often, the graph name 
would be used to talk about a container, and sometimes something else.

>
> We might want to tweak this to have u denoting a resource with G as
> its initial state, etc., if we ever get this kind of stuff into a
> semantics, but I won't hold my breath on that.
>
>> My view is that having semantic interpretations of RDF datasets
>> being insensitive to the actual named graph doesn't fully support
>> these use cases.  I'm even a bit worried about defining entailment
>> between RDF datasets that doesn't distinguish between equivalent
>> named graphs.
>
> I agree. Semantic equivalence is not identity of graphs, and
> (contrary to what Antoine calims), the RDF semantics does not say
> that you have blanket permission to replace a graph with an
> equivalent graph. All the semantics does is to define semantic
> equivalence.

The dataset semantics does not say you have blanket permission to do 
anything. It does not tell you what to do and not do, it simply defines 
what's a valid conclusion, just like any semantics.

AZ

>
> Pat
>
>> peter
>>
>> PS:  From http://www.w3.org/2011/rdf-wg/wiki/TF-Graphs-UC I see at
>> least 1.1,
>>
>>
>>
>> On 09/21/2012 11:33 AM, Antoine Zimmermann wrote:
>>> Le 21/09/2012 17:25, Peter F. Patel-Schneider a écrit :
>>>>
>>>> On 09/20/2012 12:00 PM, Antoine Zimmermann wrote:
>>>>> Le 20/09/2012 16:54, Peter F. Patel-Schneider a écrit :
>>>>>>
>>>
>>> [skip]
>>>
>>>>>>
>>>>>> In the semantics there is no notion of a relationship
>>>>>> between a name and an actual graph.
>>>>>
>>>>> In the semantics to which I refer (viz., first version of the
>>>>> Minimal dataset semantics) there is a function IGEXT that
>>>>> maps graph IRIs to RDF graphs. Isn't this a notion of a
>>>>> relationship between a name and an actual graph?
>>>> No, as it does not distinguish between equivalent graphs.
>>>> Suppose you have two equivalent graphs, then you can use them
>>>> interchangeably in your semantics.
>>>
>>> If I have 2 equivalent graphs, I can use them interchangeably
>>> according to the *RDF semantics*. It's not my fault.
>>>
>>>
>>>>> Or maybe, by "actual graph", you mean a graph that is
>>>>> actually "written" in a given dataset? Normally, the
>>>>> semantics defines the notion of interpretation independently
>>>>> of a given formula, and an interpretation makes true all
>>>>> sorts of formulas.
>>>>>
>>>>> In what I propose, for an interpretation to make a named
>>>>> graph true, the name has to be related (via IGEXT) to
>>>>> whatever graph makes the graph inside the pair true.
>>>>>
>>>> Yes, but this doesn't pick out the actual graph, just one of
>>>> many possible graphs.
>>>
>>> If you want the graph inside a <name,graph> pair, just read the
>>> dataset. There are APIs for this.
>>>
>>> Dataset d = loadDatasetFromFile(new File("foo.trig")); Graph g =
>>> d.getGraph("http://ex.com/g");
>>>
>>> That's it.
>>>
>>>>>
>>>>>> If named graphs and RDF datasets are supposed to carry a
>>>>>> relationship between a name and an actual graph, then
>>>>>> shouldn't the semantics reflect this?
>>>>>
>>>>> By IGEXT, it does, but a dataset interpretation is not
>>>>> defined in function of a given dataset, so there is no reason
>>>>> that the name be associated with the "actual graph" in a
>>>>> given dataset.
>>>>
>>>> Why not? Isn't a major use case for RDF graphs to record where
>>>> graphs (actual graphs, not equivalence classes of graphs) come
>>>> from?
>>>
>>> And what's this has to do with semantics? If I want to record
>>> someone's speach, I don't need to know the semantics of what
>>> he/she says.
>>>
>>>
>>>>>>
>>>>>> This is totally different from properties. No one should be
>>>>>> arguing that RDF graphs are supposed to carry a
>>>>>> relationship between a name and a set of pairs. Instead
>>>>>> this is what the semantics does.
>>>>>>
>>>>>>
>>>>>>
>>>>>>> (Of course, you
>>>>>>>> could always just ignore the semantics and directly use
>>>>>>>> the graph from the dataset, but then what is the point
>>>>>>>> of having the named graph there?)
>>>>>>>
>>>>>>> The data structure is also very important, just as in RDF
>>>>>>> graphs, the data structure is already a nice way of
>>>>>>> organising the data, linking data together, etc.
>>>>>>> Semantics does not have to come into play where it has no
>>>>>>> role.
>>>>>>>
>>>>>>>
>>>>>>> --AZ
>>>>>>
>>>>>>
>>>>>> Huh? If the meaning of a named graph is tied up with
>>>>>> relating names to graphs, then the semantics certainly has
>>>>>> a role there.
>>>>>
>>>>> Sorry, maybe I misunderstood what you were saying, but then I
>>>>> don't understand your point.
>>>>>
>>>>> What I'm saying is that, if you find a dataset somewhere in
>>>>> the wild, or if you have a dataset in memory, you can get the
>>>>> graph associated with a graph IRI by simply parsing the
>>>>> dataset representation. Semantics does not come into play in
>>>>> that case.
>>>>>
>>>>>
>>>>> --AZ
>>>>
>>>> Sure, you can look right in the dataset to find the graph, no
>>>> semantics involved. However, if RDF datasets is supposed to be
>>>> able to carry some meaning about graphs and their sources then
>>>> shouldn't its semantics actually use graphs?
>>>
>>> No, they are not supposed to carry information about the graph.
>>> That's only one use case, and we know there are people against
>>> this idea (e.g., the default as merge case). I want the common
>>> denominator that would not lead to erroneous entailments
>>> according to anybody's understanding of datasets.
>>>
>>> For metadata about graphs, provenance, dates, whatever, define a
>>> vocabulary and make sure that all applications that support the
>>> vocabulary interpret it in the same way. It has been done before
>>> and it works.
>>>
>>>
>>> AZ
>>>
>>>>
>>>> peter
>>>>
>>>>
>>>
>>
>>
>>
>
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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
France
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Received on Saturday, 22 September 2012 08:46:57 UTC