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Re: Shapes vs Classes (in LDOM)

From: Eric Prud'hommeaux <eric@w3.org>
Date: Fri, 23 Jan 2015 04:27:45 -0500
To: Dimitris Kontokostas <kontokostas@informatik.uni-leipzig.de>
Cc: Holger Knublauch <holger@topquadrant.com>, RDF Data Shapes Working Group <public-data-shapes-wg@w3.org>
Message-ID: <20150123092744.GB23984@w3.org>
* Dimitris Kontokostas <kontokostas@informatik.uni-leipzig.de> [2015-01-23 11:03+0200]
> 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.
> We also decouple the schema declaration with the constraint declaration (*)
> 
> # 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?

If I understand you, you propose that § 2. Class Declarations be
instead Shape Declarations. I believe this addresses the issues raised
in the November Class vs. Shape wars:

  https://lists.w3.org/Archives/Public/public-data-shapes-wg/2014Nov/thread#msg37
  https://lists.w3.org/Archives/Public/public-data-shapes-wg/2014Nov/thread#msg199

I believe the answer to the Can Shapes always be Classes thread
  https://lists.w3.org/Archives/Public/public-data-shapes-wg/2014Nov/thread#msg22
was "no", at least not for classes used elsewhere in the enterprise or
public.

— 


> (*) 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.

This seems sort of like the difference between classes and interfaces
in Java. (They're the same thing in C++, which is why C++ is so much
more popular than Java.)


> 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

-- 
-ericP

office: +1.617.599.3509
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