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Re: New Terminology Section

From: Tom Johnson <johnson.tom@gmail.com>
Date: Mon, 9 May 2016 21:37:24 -0700
Message-ID: <CAJeHiNHUUp7gSHzDVHFqsgG06wgS70Q4f47Aq_URKwN49-x0Rw@mail.gmail.com>
To: Holger Knublauch <holger@topquadrant.com>
Cc: RDF Data Shapes Working Group <public-data-shapes-wg@w3.org>
"the data graph counts" should read "the shapes graph counts", in the
previous message.

On Mon, May 9, 2016 at 9:31 PM, Tom Johnson <johnson.tom@gmail.com> wrote:

>
>
> On Mon, May 9, 2016 at 8:18 PM, Holger Knublauch <holger@topquadrant.com>
> wrote:
>
>> On 10/05/2016 12:30, Tom Johnson wrote:
>>
>>
>>
>> On Mon, May 9, 2016 at 5:29 PM, Holger Knublauch <
>> <holger@topquadrant.com>holger@topquadrant.com> wrote:
>>
>>>
>>>
>>> On 10/05/2016 10:11, Tom Johnson wrote:
>>>
>>> Irene, you say:
>>>
>>> > "Doing more" doesn't create a problem, but, on the other hand, it is
>>> not required.
>>>
>>> I'm really uncertain about this. Couldn't inferring further class
>>> relations (e.g., by using the entailment mechanism included in the spec)
>>> cause different results for basically every operation in SHACL?
>>>
>>>
>>> Can you think of a specific example? sh:entailment would potentially
>>> produce additional triples. But this is the user's choice, and then the
>>> user may expect to see additional validation results...
>>>
>>
>> We seem to be in agreement that inferring additional triples will change
>> results. Examples seem obvious; adding a `subClassOf` statement whose
>> subject is any class referenced in a shape will do the trick, but that's
>> far from the only example.
>>
>> This seems like a problem to me because I don't see that it's clear where
>> triples like `subClassOf` must appear (data graph? shapes graph? any
>> graph?) for a resource to count as a shape, or to match various constraint
>> components.
>>
>>
>> To have an effect on sh:scopeClass and sh:class, the subClassOf triples
>> must be in the data graph.
>>
>
> Is this stated somewhere in the current spec? I haven't been able to find
> it, if so.
>
> Also, the question applies equally to cases where the intent is presumably
> that (only?) the data graph counts. For instance: which resources count as
> sh:Shapes?
>
>> Note that adding a `subClassOf` triple to a shapes graph to effect
>> validation could be considered a feature; I'm unsure whether that feature
>> is supported.
>>
>>
>> Currently the spec only looks at the data graph.
>>
>>
>> Additionally, `sh:entailment` seems generally under/un-defined. Can
>> inference effect data graphs only? or also shapes graphs? Which triples can
>> be considered by a reasoner and how are inferred triples used by the SHACL
>> semantics?
>>
>>
>> I have just clarified this to the sh:entailment section:
>>
>>
>> https://github.com/w3c/data-shapes/commit/71a9eeaff0317de0cdca6b36500412dabc922f78
>>
>> I am unsure how many people will actually use sh:entailment, so any
>> feedback/requirement may help us add missing details. It is very brief
>> right now, indeed.
>>
>
> I think some clear definition is called for; otherwise, I would simply
> remove the feature; is there a functional difference between entailment (in
> this case) and providing a mechanism for the user/engine to add arbitrary
> triples to the data or shapes graph during pre-processing? This could be a
> simpler way to think of the problem.
>
> - Tom
>
>
> Holger
>>
>>
>>
>> Some of my other concerns about the specifics of `class` and `instance`
>> definitions seem to be in the process of being fixed up; from a quick
>> reading of the latest editor's draft, this is looking promising.
>>
>> - Tom
>>
>>
>>> Thanks, i
>>> Holger
>>>
>>>
>>>
>>>
>>> In lieu of a repeat of previous conversations, I'll just say: For me, as
>>> an implementer in waiting, this is a huge problem. On last reading, very
>>> little seemed unambiguously defined.
>>>
>>> - Tom
>>>
>>> On Mon, May 9, 2016 at 12:14 PM, Irene Polikoff <
>>> <irene@topquadrant.com>irene@topquadrant.com> wrote:
>>>
>>>> Karen,
>>>>
>>>> As I understand it, RDFS inferencing is one way to address this.
>>>> However,
>>>> RDFS inferencing would do more than what is specified here. "Doing more²
>>>> doesn¹t create a problem, but, on the other hand, it is not required.
>>>>
>>>> Another way to address this is to run a query as follows:
>>>>
>>>> SELECT ?resource
>>>> WHERE {
>>>>
>>>> ?class rdfs:subClassOf* example:Class1 .
>>>> ?resource a ?class .
>>>>
>>>> }
>>>>
>>>> Running this query would not change any graphs. As an aside, RDFS
>>>> inferencing is also often done without modifying any graphs. Inferences
>>>> are calculated on the fly when users/systems query data without any
>>>> materialization of inferred triples. At least, this is how triple stores
>>>> that support RDFS inferencing typically work.
>>>>
>>>> Does your concern have to do with where the rdfs:subClassOf triples come
>>>> from - would they exist in the data graph, would they exist in the
>>>> shapes
>>>> graph? They could be in either. If no subclass triples are there, then
>>>> the
>>>> first triple match simply binds ?class to example:Class1 and the query
>>>> result is the same as if we were only looking for nodes that are
>>>> connected
>>>> to example:Class1 via rdf:type link.
>>>>
>>>> It doesn¹t seem to be a role of SHACL to mandate where these triples
>>>> should be located. If they are available in either of the graphs, a
>>>> SHACL
>>>> engine should take them into account. If they are not available, than it
>>>> doesn¹t take them into account.
>>>>
>>>> In our experience, users typically put the subclass triples into the
>>>> shapes graph. At the same time, they need flexibility to do whatever
>>>> fits
>>>> their architecture and processes.
>>>>
>>>>
>>>> Irene Polikoff
>>>>
>>>>
>>>> On 5/9/16, 1:47 PM, "Karen Coyle" < <kcoyle@kcoyle.net>
>>>> kcoyle@kcoyle.net> wrote:
>>>>
>>>> >Type
>>>> >The types of a node are its values of rdf:type as well as the
>>>> >superclasses of these values.
>>>> >
>>>> >This conflates two different relationships: the relationship of a
>>>> >subject to a class (as defined in RDF/RDFS), defining the subject as an
>>>> >instance of the class; and the sub-/super-class relationships between
>>>> >classes. I dont' see how this can be achieved without inferencing.
>>>> >
>>>> >If we assume some pre-processing of the data graph to include the
>>>> >superclasses, then type is precisely as it is defined in RDF - there
>>>> are
>>>> >just more type statements in the graph.
>>>> >
>>>> >As stated, this is quite an expansion of the meaning of type. In
>>>> >addition, it appears to require modifications to the data graph to
>>>> >include the super classes of each class (presumably up to and including
>>>> >rdfs:Resource).
>>>> >
>>>> >I think it would be best if SHACL defined the shape and data graphs as
>>>> >immutable, thus expecting that all operations read but do not modify
>>>> the
>>>> >graphs. I thought we had come to that conclusion.
>>>> >
>>>> >kc
>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>> --
>>> -Tom Johnson
>>>
>>>
>>>
>>
>>
>> --
>> -Tom Johnson
>>
>>
>>
>
>
> --
> -Tom Johnson
>



-- 
-Tom Johnson
Received on Tuesday, 10 May 2016 04:38:33 UTC

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