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Re: Shapes vs Classes (in LDOM)

From: Jose Emilio Labra Gayo <jelabra@gmail.com>
Date: Sat, 24 Jan 2015 05:47:41 +0100
Message-ID: <CAJadXXLKA3f-_5XTQQw7mH5gThraxE-_4ptoYBh2rS_=-T0ZvA@mail.gmail.com>
To: Holger Knublauch <holger@topquadrant.com>
Cc: RDF Data Shapes Working Group <public-data-shapes-wg@w3.org>
On Fri, Jan 23, 2015 at 11:42 AM, Holger Knublauch <holger@topquadrant.com>
wrote:

>  I think that separation of classes and types (and of course global
> constraints) is fine - our differences are largely syntactical. I will
> experiment with adding the class ldom:Shape and a property ldom:shape that
> links a class with its (additional) ldom:Shapes and publish an update,
> hopefully early next week. I think this will provide the freedom of
> separating things (that is advocated by Resource Shapes/ShEx), while at the
> same time supporting the pattern of attaching constraints to classes (that
> is working well for SPIN users). Users will be able to mix those types of
> declarations.
>

I think that is a good step forward and I encourage LDOM to go more in that
direction. After taking a look a LDOM, I think one of the main differences
between it and ShEx is precisely the impossibility to separate shapes (or
sets of constraints) from classes.

In my opinion, it is not practical when one is trying to describe the
contents of linked data portals and one is reusing concepts/properties from
different vocabularies.

As a practical example, I would recommend the following paper [1] where we
used the concept qb:Observation in two different linked data portals. The
observations had different shapes in both portals with different
properties, but all the observations had the same type: qb:Observation. I
think that situation happens will happen a lot in real life linked data
portals.

Yesterday, I proposed to add a user story inspired by that example.

Best regards, Jose Labra

[1] Validating and Describing Linked Data Portals using RDF Shape
Expressions, Jose Emilio Labra Gayo, Eric Prud'hommeaux, Harold Solbrig,
1st Workshop on Linked Data Quality, Sept. 2014, Leipzig, Germany
PDF: http://labra.github.io/ShExcala/papers/ldq2014.pdf
Slides: http://www.slideshare.net/jelabra/linked-dataquality-2014



>
>
> Holger
>
>
>
> On 1/23/15, 8:05 PM, Dimitris Kontokostas wrote:
>
> I am in no way saying that your proposal is wrong, I am just suggesting my
> idea for separating distinct validation types (class, global, shape).
> (only one comment inline)
>
> On Fri, Jan 23, 2015 at 11:35 AM, Holger Knublauch <holger@topquadrant.com
> > wrote:
>
>>
>> On 1/23/15, 7:03 PM, Dimitris Kontokostas wrote:
>>
>>  First of all, great work initiating this Holger!!!
>>
>>  Maybe I miss something in the semantics of the class declarations but I
>> would suggest a simplification of the constraint definitions. Examples:
>>
>>  # class example
>>
>>  ex:constraintA
>>    a ldom:ClassConstraint ;
>>   ldom:class ex:ClassA, ex:ClassB, ex:ClassC ; #  (oslc:describes)
>>   ldom:sparql """ ..?this ... """ ;
>>   ldom:property [
>>             ldom:predicate ex:propA ;
>>             ldom:minCount 1 ;
>>         ] ;
>>
>>  in this case, all classes (A,B & C) have a min cardinality 1
>> restriction on ex:propA which is not possible if we subclass the constraint
>> to a single class.
>>
>>
>>  Hi Dimitris,
>>
>> to me this looks like the wrong direction. It is much more natural to
>> write
>>
>> ex:ClassA
>>     ldom:property [
>>         ...
>>     ]
>>
>> Sharing the same property across multiple classes is also not a scenario
>> that I have come across yet.
>>
>
>  I saw that in an OSLC example document and liked the idea.
>
>
>> And why the extra burden of creating a URI for the constraint - I guess
>> most people will be perfectly happy with blank nodes. Likewise, why should
>> they have to explicitly declare the type ldom:ClassConstraint, if it is
>> implicit from the context.
>>
>>  We also decouple the schema declaration with the constraint declaration
>> (*)
>>
>>
>>  I don't think this decoupling is often desirable. When someone defines a
>> class, then of course the properties should be defined together with it
>> (just like owl:Restrictions did). What else would a class definition good
>> for?
>>
>> In case someone really has to define shapes independently from classes,
>> then we can easily add a property such as the inverse of the ldom:class
>> that you have above, e.g. ldom:shape as in
>>
>> ex:ClassA
>>     ldom:shape ex:ShapeB ;
>>
>> This would offer the same flexibility but have it in a more natural
>> direction to cover the most common use cases.
>>
>>
>>  # global constraint example, the rdfs:Resource / owl:Thing declaration
>> is redundant
>>
>>  ex:constraintB
>>   a ldom:GlobalConstraint ;
>>   ldom:sparql """ ... """ ;
>>
>>  # ShExC / RS shapes in a similar way these are currently defined
>>  ex:constraintC
>>   a ldom:ShapeConstraint ;
>>   ldom:sparql """ ... """ ;
>>    ldom:property [
>>             ldom:predicate ex:propA ;
>>             ldom:minCount 1 ;
>>         ] ;
>>
>>  For the ShapeConstraints we can define how validation can performed
>> e.g. starting from a node or inferring the types of the nodes based on the
>> shape definition and then validating in a similar way to the
>> ClassConstraint.
>> Would something like this solve the class/shape problem?
>>
>>
>>  Why would the solution that I proposed not work?
>>
>> Thanks,
>> Holger
>>
>>
>>
>>
>>
>>  (*) Another reason for not defining constraints as classes is that
>> automated Agents try to profile datasets for classes / properties used
>> which, might confuse them and give false statistics.
>>
>>  Best,
>> Dimtiris
>>
>>
>> On Fri, Jan 23, 2015 at 5:57 AM, Holger Knublauch <holger@topquadrant.com
>> > wrote:
>>
>>> May I suggest we try to resolve the long-standing issue of Shapes versus
>>> Classes in the specific context of LDOM. Maybe we can make progress if we
>>> have a specific metamodel in front of us.
>>>
>>> In the current draft, class definitions are containers of constraints,
>>> i.e.
>>>
>>>     rdfs:Class
>>>         a rdfs:Class ;
>>>         rdfs:subClassOf rdfs:Resource ;
>>>         ldom:property [
>>>             ldom:predicate ldom:constraint ;
>>>             ldom:valueType ldom:Constraint ;
>>>         ] ;
>>>         ldom:property [
>>>             ldom:predicate ldom:property ;
>>>             ldom:valueType ldom:PropertyConstraint ;
>>>         ] ;
>>>
>>> which means that you can define a class such as
>>>
>>>     ex:Rectangle
>>>         ldom:property [
>>>             ldom:predicate ex:height ;
>>>             ...
>>>         ] ...
>>>
>>> This could (easily) be generalized by moving the properties into a new a
>>> class
>>>
>>>     ldom:Shape
>>>         a rdfs:Class ;
>>>         rdfs:subClassOf rdfs:Resource ;
>>>         ldom:property [
>>>             ldom:predicate ldom:constraint ;
>>>             ldom:valueType ldom:Constraint ;
>>>         ] ;
>>>         ldom:property [
>>>             ldom:predicate ldom:property ;
>>>             ldom:valueType ldom:PropertyConstraint ;
>>>         ] ;
>>>
>>>  which serves as superclass of rdfs:Class
>>>
>>>     rdfs:Class
>>>         a rdfs:Class ;
>>>         rdfs:subClassOf ldom:Shape ;
>>>
>>> This would mean that users could define stand-alone shapes
>>>
>>>     ex:MyShape
>>>         a ldom:Shape ;
>>>         ldom:property [
>>>             ...
>>>         ] ...
>>>
>>> And this shape could be reused such as in
>>>
>>>     ex:MyClass
>>>         a rdfs:Class ;
>>>         ldom:constraint [
>>>             a ldom:ShapeConstraint ;
>>>             ldom:all ex:MyShape ;
>>>         ] ...
>>>
>>> or as an entry point to the validation:
>>>
>>>     FILTER ldom:violatesConstraints(?resource, ex:MyShape)
>>>
>>> (maybe renaming the function above to ldom:hasShape).
>>>
>>> Since rdfs:Class is a subclass of ldom:Shape, class definitions become
>>> special kinds of shape definitions. The main differences between classes
>>> and shapes would be:
>>>
>>> - Classes can be instantiated, i.e. you can have ex:MyRectangle a
>>> ex:Rectangle
>>> - Class-based constraints get inherited (Shapes cannot have
>>> rdfs:subClassOf)
>>>
>>> I don't see practical problems with such a design, and in fact it may be
>>> a cleaner separation of concerns. The reason why these two concepts are
>>> currently merged into one is that the differences are fairly small, and
>>> people could simply define an anonymous (even typeless) class as a
>>> collection of constraints, as in Example 9
>>>
>>>     http://spinrdf.org/ldomprimer.html#template-constraints
>>>
>>> Thoughts?
>>>
>>> Cheers,
>>> Holger
>>>
>>>
>>>
>>
>>
>>  --
>>  Dimitris Kontokostas
>> Department of Computer Science, University of Leipzig
>> Research Group: http://aksw.org
>> Homepage:http://aksw.org/DimitrisKontokostas
>>
>>
>>
>
>
>  --
>  Dimitris Kontokostas
> Department of Computer Science, University of Leipzig
> Research Group: http://aksw.org
> Homepage:http://aksw.org/DimitrisKontokostas
>
>
>


-- 
Saludos, Labra
Received on Saturday, 24 January 2015 04:48:29 UTC

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