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Re: New Terminology Section

From: Tom Johnson <johnson.tom@gmail.com>
Date: Mon, 9 May 2016 19:30:17 -0700
Message-ID: <CAJeHiNG8MRJXdXxSN9-yq-1yaCRjc6MxKb9vZfoi-o9mNxQ3YQ@mail.gmail.com>
To: Holger Knublauch <holger@topquadrant.com>
Cc: RDF Data Shapes Working Group <public-data-shapes-wg@w3.org>
On Mon, May 9, 2016 at 5:29 PM, Holger Knublauch <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. 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.

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?

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>
> 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
Received on Tuesday, 10 May 2016 02:31:25 UTC

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