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Re: type and instance and subclass in SHACL documents

From: Irene Polikoff <irene@topquadrant.com>
Date: Sat, 12 Mar 2016 21:13:23 -0500
To: Tom Johnson <johnson.tom@gmail.com>
CC: "Peter F. Patel-Schneider" <pfpschneider@gmail.com>, Karen Coyle <kcoyle@kcoyle.net>, RDF Data Shapes Working Group <public-data-shapes-wg@w3.org>
Message-ID: <D30A304F.95C94%irene@topquadrant.com>
Tom,

I am trying to understand what makes SHACL notion of the word instance to be
"very different" from RDFS notion of the word instance.

RDFS specification says:

"Resources may be divided into groups called classes. The members of a class
are known as instances of the class. Classes are themselves resources. They
are often identified by IRIs
<http://www.w3.org/TR/rdf11-concepts/#section-IRIs>  and may be described
using RDF properties. The rdf:type
<https://www.w3.org/TR/rdf-schema/#ch_type>  property may be used to state
that a resource is an instance of a class.¡±

SHACL relies on rdf:type property to determine if something is an instance
of a class. If there is a difference here, it is not a very obvious
difference, but rather a quite nuanced one. And there is certainly no
contradiction of any kind. Any time SHACL considers X to be an instance of
Y, so would RDFS. As SHACL relies on rdf:type triples, a SHACL engine may
sometimes not recognize that a resource is an instance of a particular class
when a fully RDFS aware program would. I believe when explained in such
terms, the difference sounds clearer and much less alarming.

RDFS specification prescribes class membership for the resources it defines.
Such as "rdfs:range is an instance of rdf:Property
<https://www.w3.org/TR/rdf-schema/#ch_property> ¡±. RDFS vocabulary encodes
these definitions with triples like ¡°rdfs:range rdf:type rdf:Property¡±.

If this whole controversy is about whether SHACL engines should act as if
RDFS vocabulary triples  were always a part of the data graph when
validation is performed, then such statement could have been made very
plainly. I think it is worth recording this as an issue. Whatever the cost
of doing such inclusion, it may be well worth it to avoid the confusion of
pondering the subtleties of the word ¡°instance¡±.

Irene 

From:  Tom Johnson <johnson.tom@gmail.com>
Date:  Saturday, March 12, 2016 at 5:17 PM
To:  Irene Polikoff <irene@topquadrant.com>
Cc:  "Peter F. Patel-Schneider" <pfpschneider@gmail.com>, Karen Coyle
<kcoyle@kcoyle.net>, RDF Data Shapes Working Group
<public-data-shapes-wg@w3.org>
Subject:  Re: type and instance and subclass in SHACL documents

I suspect my email crossed streams with Irene's, but I want to answer the
call for someone who sees the problem here directly: I do, quite clearly; at
least with respect to the `sh:scopeClass` example.

`sh:scopeClass`'s definition relies on RDFS terminology throughout. It would
be reasonable for a reader of the spec to assume that the spec (and
validation process) imports RDFS directly. The total impact of this is
ambiguous to me, but it clearly creates a problem for terms that are defined
directly the in RDF/RDFS. Text disavowing this *is* included, but is rather
hidden--and, I think more to Peter's point, flies in the face of any claim
that SHACL "conforms to RDFS".

It seems to me that SHACL is on safer ground defining itself in terms of
basic graph patterns evaluated on the data graph, and should avoid using
RDFS terminology in its formal definitions. Such terminology should continue
to be used in naming SHACL abstractions where it is deemed to be an aid to
intuitive understanding (`sh:scopeClass` seems like a good example, here).

- Tom

On Sat, Mar 12, 2016 at 2:08 PM, Irene Polikoff <irene@topquadrant.com>
wrote:
> 
> Peter,
> 
> Repeating that "SHACL instance is indeed very different from RDFS
> instance¡± doesn¡¯t move the conversation forward. The question was ¡°why and
> how¡± and you have not answered this question in a way that I could
> understand.
> 
> Since no one else in the working group jumped in to answer this question
> and, on contrary, several people joined me in asking it, I have to
> conclude that no one else understand this either. If I am wrong, and there
> is someone other than Peter who does, please, answer it. If something is
> indeed very different from another thing, such difference should be
> apparent to most group members.
> 
> As I recall, this topic has been brought up by Peter on and off during (at
> least) the last 12 months. Realistically and practically speaking, if no
> one else in the working group that is made of experts and experienced
> practitioners understands this difference, even after a year of
> discussions, I see this topic as having absolutely no practical relevance.
> The chance that the broader community would understand it or care to
> understand it or be impacted by it in any way whatsoever is infinitely
> close to zero.
> 
> I would venture even further and say that such unwavering focus on obscure
> points that make no practical difference is they key obstacle to adoption
> of RDF technology. As a community, we must overcome this tendency in order
> to move forward.
> 
> Irene
> 
> 
> 
> 
> On 3/12/16, 4:30 PM, "Peter F. Patel-Schneider" <pfpschneider@gmail.com>
> wrote:
> 
>> >The SHACL documents talk about instance.  If this is RDFS instance, then,
>> >yes,
>> >SHACL engines would always have to treat rdfs:label as an instance of
>> >rdf:Property.
>> >
>> >This is why I say that the SHACL documents should be very clear every time
>> >that they talk about instance that it is not the common RDFS instance that
>> >they are talking about but some new notion particular to SHACL,
>> >particularly
>> >as SHACL uses RDFS vocabulary.
>> >
>> >SHACL instance is indeed very different from RDFS instance.
>> >
>> >peter
>> >
>> >
>> >On 03/12/2016 10:05 AM, Irene Polikoff wrote:
>>> >> We need rdf:type to know if something is an instance of a class (note
>>> >>that I am saying simply 'instance' because I do not see the difference).
>>> >>
>>> >> If {rdfs:label rdf:type rdf:Property} triple was provided to a SHACL
>>> >>engine, then the violation would be raised.
>>> >>
>>> >> How else could it be known from the data graph that rdfs:label is a
>>> >>property? Or are you saying that SHACL engines should always include
>>> >>triples in RDFS vocabulary when they do their processing?
>>> >>
>>> >> Sent from my iPhone
>>> >>
>>>> >>> On Mar 11, 2016, at 10:36 PM, Peter F. Patel-Schneider
>>>> >>><pfpschneider@gmail.com> wrote:
>>>> >>>
>>>> >>> Using the RDFS definition of instance, rdfs:label is an instance of
>>>> >>> rdf:Property so it is in the scope of the shape and there is a
>>>> >>>violation.
>>>> >>> Using the SHACL definition of instance, rdfs:label is *not* an
>>>> >>>instance of
>>>> >>> rdf:Property so it is *not* in scope and there is *no* violation.
>>>> >>>
>>>> >>> peter
>>>> >>>
>>>> >>>
>>>>> >>>> On 03/11/2016 04:50 PM, Karen Coyle wrote:
>>>>> >>>> Peter, I admit that I, too, am having trouble understanding this.
>>>>> >>>>(And so it
>>>>> >>>> isn't all on Peter, if anyone else "gets it" maybe they could weight
>>>>> >>>>in.) The
>>>>> >>>> SHACL document uses the term "instance" 78 times. I admit I only
>>>>> >>>>looked at the
>>>>> >>>> first couple of dozen of those uses. For the most part they appear to
>>>>> >>>>me to
>>>>> >>>> conform to the RDFS definition of "instance" - meaning an instance of
>>>>> >>>>class.
>>>>> >>>> In some cases the term is used more colloquially, but those places in
>>>>> >>>>the
>>>>> >>>> document don't seem to be definitional.
>>>>> >>>>
>>>>> >>>> You say that it doesn't validate, but can you say what the difference
>>>>> >>>>is in
>>>>> >>>> the two definitions? I still see it as having to do with the
>>>>> >>>>vocabulary
>>>>> >>>> definition as opposed to the SHACL validation, but you didn't buy
>>>>> >>>>that when I
>>>>> >>>> suggested it. If I were to use a typical OWL-based validation,
>>>>> >>>>rdfs:range
>>>>> >>>> ex:label "range" would be flagged as inconsistent. The same would be
>>>>> >>>>true if I
>>>>> >>>> would have
>>>>> >>>>  ex:someSubject dct:type "text" .
>>>>> >>>> (dct:type has a range of rdf-schema#Class)
>>>>> >>>>
>>>>> >>>> If this isn't the issue, I would sure like to know what is.
>>>>> >>>>
>>>>> >>>> Thanks,
>>>>> >>>> kc
>>>>> >>>>
>>>>>> >>>>> On 3/11/16 2:22 PM, Peter F. Patel-Schneider wrote:
>>>>>> >>>>> The definition of SHACL depends on "instance".  This can be read to
>>>>>> >>>>>mean
>>>>>> >>>>> "RDFS instance" or "SHACL instance".  Under the former meaning the
>>>>>> >>>>>data graph
>>>>>> >>>>> does not validate against the shape.   Under the latter meaning the
>>>>>> >>>>>data graph
>>>>>> >>>>> does validate against the shape.
>>>>>> >>>>>
>>>>>> >>>>> peter
>>>>>> >>>>>
>>>>>> >>>>>
>>>>>>> >>>>>> On 03/11/2016 02:15 PM, Irene Polikoff wrote:
>>>>>>> >>>>>> I don©öt understand what you mean by
>>>>>>> >>>>>>
>>>>>>> >>>>>> "validates against this shape under SHACL instance but not under
>>>>>>> >>>>>>RDFS
>>>>>>> >>>>>> instance.©÷
>>>>>>> >>>>>>
>>>>>>> >>>>>> I am not able to parse the sentence.
>>>>>>> >>>>>>
>>>>>>> >>>>>> What are you doing? Taking a shape described and the graph
>>>>>>> >>>>>>described and
>>>>>>> >>>>>> running it against SHACL engine? What execution validates and
what
>>>>>>> >>>>>> execution doesn©öt validate?
>>>>>>> >>>>>>
>>>>>>> >>>>>>
>>>>>>> >>>>>>
>>>>>>> >>>>>> Irene
>>>>>>> >>>>>>
>>>>>>> >>>>>>
>>>>>>> >>>>>>
>>>>>>> >>>>>> On 3/11/16, 5:03 PM, "Peter F. Patel-Schneider"
>>>>>>> >>>>>><pfpschneider@gmail.com>
>>>>>>> >>>>>> wrote:
>>>>>>> >>>>>>
>>>>>>>>> >>>>>>>> On 03/11/2016 01:01 PM, Karen Coyle wrote:
>>>>>>>>> >>>>>>>>
>>>>>>>>> >>>>>>>>
>>>>>>>>>> >>>>>>>>> On 3/11/16 11:43 AM, Peter F. Patel-Schneider wrote:
>>>>>>>>>> >>>>>>>>> Consider the following shape (using obvious prefix
>>>>>>>>>> declarations)
>>>>>>>>>> >>>>>>>>>
>>>>>>>>>> >>>>>>>>> sh:propertyShape a sh:Shape ;
>>>>>>>>>> >>>>>>>>>   sh:scopeClass rdf:Property ;
>>>>>>>>>> >>>>>>>>>   sh:property [ sh:predicate rdfs:label ;
>>>>>>>>>> >>>>>>>>>                 sh:minCount 1 ] .
>>>>>>>>>> >>>>>>>>>
>>>>>>>>>> >>>>>>>>> The data graph (using obvious prefix declarations)
>>>>>>>>>> >>>>>>>>>
>>>>>>>>>> >>>>>>>>> rdfs:range ex:label "range" .
>>>>>>>>>> >>>>>>>>>
>>>>>>>>>> >>>>>>>>> validates against this shape under SHACL instance but not
under
>>>>>>>>>> >>>>>>>>>RDFS
>>>>>>>>>> >>>>>>>>> instance.
>>>>>>>>> >>>>>>>>
>>>>>>>>> >>>>>>>> Isn't this a problem with every vocabulary and not just RDFS?
If
>>>>>>>>> >>>>>>>>the
>>>>>>>>> >>>>>>>> rules of
>>>>>>>>> >>>>>>>> the vocabulary (such as domain and range) are not encoded as
such
>>>>>>>>> >>>>>>>>in
>>>>>>>>> >>>>>>>> SHACL
>>>>>>>>> >>>>>>>> then the SHACL result can be "in violation" of the vocabulary
>>>>>>>>> >>>>>>>> definition.
>>>>>>>>> >>>>>>>>
>>>>>>>>> >>>>>>>> Now, if that is the case then I understand that violating the
>>>>>>>>> >>>>>>>>foundation
>>>>>>>>> >>>>>>>> vocabulary of RDF/RDFS may be more grave than violating a
>>>>>>>>> >>>>>>>>user-developed
>>>>>>>>> >>>>>>>> vocabulary, and in some cases doing the latter may indeed be
the
>>>>>>>>> >>>>>>>> intention of
>>>>>>>>> >>>>>>>> the SHACL definition. So do we want to build into SHACL that
it
>>>>>>>>> >>>>>>>>must
>>>>>>>>> >>>>>>>> follow
>>>>>>>>> >>>>>>>> RDF/RDFS property and class definitions? And how feasible is
that?
>>>>>>>>> >>>>>>>>
>>>>>>>>> >>>>>>>> kc
>>>>>>>> >>>>>>>
>>>>>>>> >>>>>>> This is only a real problem because SHACL uses "instance" in
its
>>>>>>>> >>>>>>> specification, this term is also used centrally in RDFS, and
SHACL
>>>>>>>> >>>>>>>uses
>>>>>>>> >>>>>>> RDFS
>>>>>>>> >>>>>>> vocabulary.
>>>>>>>> >>>>>>>
>>>>>>>> >>>>>>> The question then is how to read "instance" in SHACL
>>>>>>>> >>>>>>>documentation, i.e.,
>>>>>>>> >>>>>>> how
>>>>>>>> >>>>>>> to prevent readers of the SHACL documentation from seeing "RDFS
>>>>>>>> >>>>>>>instance"
>>>>>>>> >>>>>>> where "SHACL instance" is meant.
>>>>>>>> >>>>>>>
>>>>>>>> >>>>>>>
>>>>>>>> >>>>>>> peter
>>>> >>>
> 
> 
> 



-- 
-Tom Johnson
Received on Sunday, 13 March 2016 02:14:12 UTC

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