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Re: Really minimal dataset semantics

From: Peter F. Patel-Schneider <pfpschneider@gmail.com>
Date: Fri, 21 Sep 2012 11:25:02 -0400
Message-ID: <505C86CE.9070200@gmail.com>
To: public-rdf-wg <public-rdf-wg@w3.org>
CC: Antoine Zimmermann <antoine.zimmermann@emse.fr>

On 09/20/2012 12:00 PM, Antoine Zimmermann wrote:
> Le 20/09/2012 16:54, Peter F. Patel-Schneider a écrit :
>>
>> On 09/20/2012 05:49 AM, Antoine Zimmermann wrote:
>>> Le 19/09/2012 22:12, Peter F. Patel-Schneider a écrit :
>>>
>> [...]
>>>
>>>> I also have problems with any dataset semantics that isn't based on the
>>>> actual form of the named graphs.
>>>
>>> I don't understand this.
>>>
>>>> Isn't a major use of datasets
>>>> supposed to be for associating a graph with its source?
>>>
>>> Sure. This type of use case is compatible with the semantics.
>>>
>>>
>>>> if so, neither
>>>> of these two semantics seems to be correct, as the meaning of a named
>>>> graph in a dataset is not a graph but is instead something like an
>>>> equivalence class of graphs.
>>>
>>> I don't understand. Where do you see equivalence classes?
>>
>> Interpretations for named graphs talk directly about entailment. If you
>> change an interpretation by replacing a named graph with an equivalent
>> one, then the truth values don't change.
>
> Sure, but that's generally the case in any semantics, there are usually many
> interpretations that provide the same truth value.
> In RDF, interpretations for properties talk about membership. If you change
> an interpretation by replace IEXT(p) by a superset of it, then the truth
> values don't change either. Where is the problem here?
>

The problem is that RDF graphs don't talk about names and sets of pairs per 
se, whereas RDF datasets do talk about names and graphs. This makes the 
situation very different for RDF datasets.

>
>>>> The semantics then destroys the
>>>> relationship between the name and the actual graph.
>>>
>>> It does not destroy anything because a semantics does not *do*
>>> anything. The actual relationship between the name and the actual
>>> graph is written in the dataset.
>>> You could argue along these lines saying that the RDF semantics
>>> destroys the relationship between the property names and the actual
>>> pairs (subject,object) that are actually in the graph.
>>> If you want to know what subjects or objects occur with a predicate
>>> inside a graph, just look at the graph. There are APIs for this.
>>> Same for datasets.
>>
>> 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.
> 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 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?
>
>>
>> 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?

peter
Received on Friday, 21 September 2012 15:25:36 GMT

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