Re: ISSUE-139: uniform descriptions and implementations of constraint components

On 6/9/16 10:30 PM, Irene Polikoff wrote:

> What is missing?
>
>
> As an aside, I would probably slightly change the definition of SHACL type
> to:
>
> The SHACL types of a node in an RDF graph are its property values for
> rdf:type as well as the SHACL superclasses of these values.


This is complicated by the requirement to explicitly declare subclass 
relationships, so it would need to be tied closely to an explanation 
that includes that.

>
>
> Property values are defined in the spec, but values are not, so this is a
> bit more precise.
>
>
> And I would also probably change SHACL Class definition to:
>
> Nodes in an RDF graph that are subjects or objects in graph’s triples that
> have rdfs:subClassOf predicate or objects of triples that have rdf:type
> predicate. SHACL makes no assumption whether any particular value for
> rdf:type of a SHACL class can be found in the graph.

This is what I think is missing in the definition and the examples - the 
"subjects or objects". An example using a class-instance in the object 
position would make this more obvious. Otherwise, the examples that 
exist do not show this affordance of "class" which is what distinguishes 
it from a nodeScope.

kc

>
> Assuming that the above definition is correct. I am not 100% sure.
>
>
>
> Irene
>
>
>
>
>
>
> On 6/10/16, 12:08 AM, "Karen Coyle" <kcoyle@kcoyle.net> wrote:
>
>> It isn't a question of switching them. I find that the examples do not
>> show the difference between class-defined nodes and predicate-defined
>> node that uses rdf:type as the predicate. Since all rdf:type/s must be
>> explicitly defined, these are either the same, or they are different,
>> and if they are different, that needs to be made clear.
>>
>> kc
>>
>> On 6/9/16 9:26 AM, Irene Polikoff wrote:
>>> Examples in the spec would not have the same result if scopeNode and
>>> scopeClass were switched. They look pretty clear to me as they all
>>> identify what focus nodes would be selected.
>>>
>>> May be the following will help:
>>>
>>> Let¹s say there is a graph like so:
>>>
>>> ex:Person rdfs:label ŒPerson¹.
>>> ex:Person rdfs:label ŒHuman Being¹.
>>> ex:Alice rdf:type ex:Person.
>>> ex:Alice rdfs:label ŒAlice¹.
>>> ex:Alice rdfs:label ŒAlice Jones¹.
>>> ex:Bob rdf:type ex:Person.
>>> ex:Joe rdf:type ex:Person.
>>> ex:Joe rdfs;label ŒJoe¹.
>>>
>>> And a shape
>>>
>>> ex:Shape1
>>>      a sh:Shape ;
>>>      sh:scopeNode ex:Person ;
>>>      sh:property [
>>>          sh:predicate rdfs:label ;
>>>          sh:maxCount 1 ;
>>>      ] .
>>>
>>>
>>> The node in focus is ex:Person and there will be a violation because it
>>> has two labels. No other nodes are in focus, no other violations.
>>>
>>> If there was a different shape
>>>
>>> ex:Shape2
>>> a sh:Shape ;
>>> sh:scopeClass ex:Person ;
>>> sh:property [
>>> sh:predicate rdfs:label ;
>>> sh:maxCount 1 ;
>>> ] .
>>>
>>>
>>> Then, three nodes are in scope - ex:Alice, ex:Bob and ex:Joe. There will
>>> be one violation for ex:Alice.
>>>
>>> Lets look at the shape Holger has below. This shape demonstrates, among
>>> other things, that the validation can look at triples with the focus
>>> nodes
>>> as objects by using sh:inverseProperty
>>>
>>> ex:MyShape
>>>      a sh:Shape ;
>>>      sh:scopeNode ex:Person ;
>>>      sh:inverseProperty [
>>>          sh:predicate rdf:type ;
>>>          sh:maxCount 2 ;
>>>      ] .
>>>
>>> The focus node is ex:Person and there will be a violation since there
>>> are
>>> three triples that follow the {?x rdf:type ex:Person} pattern.
>>>
>>>
>>> Irene
>>>
>>>
>>>
>>> On 6/9/16, 1:33 AM, "Karen Coyle" <kcoyle@kcoyle.net> wrote:
>>>
>>>> Holger, that still doesn't explain what the difference is. What is the
>>>> quality of a SHACL class that is different to a triple with a predicate
>>>> of rdf:type? Are you saying that scopeClass implies/allows subclass
>>>> relationships to be included? If so, that must be said in the
>>>> specification, and it should be illustrated in the examples. As it is,
>>>> the examples given would have the same result using either predicate.
>>>>
>>>> Also, the section that introduces scopeNode does not say that it
>>>> applies
>>>> only to the subject of a triple. If that is the case, then it needs to
>>>> specify that.
>>>>
>>>> kc
>>>>
>>>> On 6/8/16 9:44 PM, Holger Knublauch wrote:
>>>>>
>>>>> On 9/06/2016 14:40, Karen Coyle wrote:
>>>>>> sh:scopeClass <foaf:Person> and sh:scopeNode <foaf:Person> appear to
>>>>>> identify the same focus node(s) in the data graph.
>>>>>
>>>>> sh:scopeNode means "the (class) node itself".
>>>>> sh:scopeClass means "all SHACL instances of the class".
>>>>>
>>>>> So they do not identify the same focus nodes.
>>>>>
>>>>> Holger
>>>>>
>>>>>
>>>>>
>>>>>>
>>>>>> ***shape1***
>>>>>> ex:MyShape
>>>>>>     a sh:Shape ;
>>>>>>     sh:scopeNode foaf:Person ;
>>>>>>     sh:inverseProperty [
>>>>>>         sh:predicate rdf:type ;
>>>>>>         sh:maxCount 2 ;
>>>>>>     ] .
>>>>>>
>>>>>> ***shape2***
>>>>>> ex:PersonShape
>>>>>>     a sh:Shape ;
>>>>>>     sh:scopeClass ex:Person .
>>>>>>
>>>>>> ***data graph***
>>>>>>
>>>>>> ex:Alice a ex:Person .
>>>>>> ex:Bob a ex:Person .
>>>>>> ex:NewYork a ex:Place .
>>>>>>
>>>>>> Where does the spec address the reason for this?
>>>>>>
>>>>>> kc
>>>>>>
>>>>>> On 6/7/16 10:09 PM, Holger Knublauch wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 8/06/2016 14:57, Karen Coyle wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> On 6/7/16 7:38 PM, Holger Knublauch wrote:
>>>>>>>>> Yes and SHACL should implement the same policy, because
>>>>>>>>> sh:maxCount
>>>>>>>>> also
>>>>>>>>> only makes sense for predicate-based constraints and not node
>>>>>>>>> constraints.
>>>>>>>>
>>>>>>>> Does this then rule out a constraint like "n things of type x"? For
>>>>>>>> example, if you want to limit the number of foaf:Person nodes?
>>>>>>>
>>>>>>> No. To express "A graph must have at most 2 instances of
>>>>>>> foaf:Person"
>>>>>>> you would write
>>>>>>>
>>>>>>> ex:MyShape
>>>>>>>     a sh:Shape ;
>>>>>>>     sh:scopeNode foaf:Person ;
>>>>>>>     sh:inverseProperty [
>>>>>>>         sh:predicate rdf:type ;
>>>>>>>         sh:maxCount 2 ;
>>>>>>>     ] .
>>>>>>>
>>>>>>> In other words "there must be at most 2 triples that have
>>>>>>> foaf:Person
>>>>>>> as
>>>>>>> object and rdf:type as predicate".
>>>>>>>
>>>>>>> Peter's suggested use of sh:maxCount at node constraints would mean
>>>>>>>
>>>>>>> "Verify that the set of value nodes is not larger than two. Oh, and
>>>>>>> regardless of the actual data, I already know that this set of value
>>>>>>> nodes has size 1, because it always consists of the focus node only.
>>>>>>> So
>>>>>>> actually I only need to test whether the value of sh:maxCount > 0."
>>>>>>>
>>>>>>> which is a rather useless construct. You have just confirmed that
>>>>>>> misusing sh:maxCount as node constraints will likely confuse users.
>>>>>>>
>>>>>>> Is this difference clearer now, or what else could I clarify?
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Holger
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>> --
>>>> Karen Coyle
>>>> kcoyle@kcoyle.net http://kcoyle.net
>>>> m: 1-510-435-8234
>>>> skype: kcoylenet/+1-510-984-3600
>>>>
>>>
>>>
>>>
>>
>> --
>> Karen Coyle
>> kcoyle@kcoyle.net http://kcoyle.net
>> m: 1-510-435-8234
>> skype: kcoylenet/+1-510-984-3600
>
>
>
>

-- 
Karen Coyle
kcoyle@kcoyle.net http://kcoyle.net
m: 1-510-435-8234
skype: kcoylenet/+1-510-984-3600

Received on Friday, 10 June 2016 08:19:30 UTC