Re: shapes-ISSUE-129 (existential constraints): Existential constraints should be consistent [SHACL - Core]

My take on existential constraints is if the sparql definitions contains
"WHERE { $this $predicate ?object . ... }" or not
if it does it means that the constraint is applied on existing values and
does not require the value to exist.

Using this definition the list is different
non-existential constraints:
- sh:class, sh:classIn, sh:datatype, sh:datatypeIn, sh:directType
- sh:max/min/In/Exclusive, sh:maxLength, sh:minLength
- sh:nodeKind
- sh:in
- sh:valueShape
- sh:closed
- sh:uniqueLang
- sh:notEquals, sh:lessThan, sh:lessThanOrEquals


existential ones:
- sh:minCount, sh:maxCount
- sh:qualified*
- sh:hasValue
- sh:equals (sorry, I didn't catch equals before)

I didn't mean to include sh:and/or/not as they are different than the other
core constraints

I suggested to limit them to - sh:minCount, sh:maxCount (and sh:qualified*
of course) but if we don't we should keep them consistent
sh:hasValue should be consistent with sh:in
sh:equals should be consistent with sh:notEquals, sh:lessThan,
sh:lessThanOrEquals


On Tue, Mar 8, 2016 at 12:48 AM, Holger Knublauch <holger@topquadrant.com>
wrote:

> Many constraint types indeed apply to "all values" only:
> - sh:class, sh:classIn, sh:datatype, sh:datatypeIn, sh:directType
> - sh:max/min/In/Exclusive, sh:maxLength, sh:minLength
> - sh:nodeKind
> - sh:in
> - sh:valueShape
>
> These are handled by NodeValidationFunctions in the current draft.
>
> Other constraint types are more heterogeneous, and calling them
> "existential" is IMHO an over-simplification:
> - sh:and, sh:or, sh:not
> - sh:closed
> - sh:equals, sh:notEquals, sh:lessThan, sh:lessThanOrEquals
> - sh:minCount, sh:maxCount
> - sh:qualifiedXY
> - sh:uniqueLang
> - sh:hasValue
>
> Just looking at sh:equals for example, the query is something like
>
>
>         SELECT $this ($this AS ?subject) $predicate ?object
>         WHERE {
>             {
>                 $this $predicate ?object .
>                 FILTER NOT EXISTS {
>                     $this $equals ?object .
>                 }
>             }
>             UNION
>             {
>                 $this $equals ?object .
>                 FILTER NOT EXISTS {
>                     $this $predicate ?object .
>                 }
>             }
>         }
>
> i.e. the system will produce violations if the "other" property has a
> value that isn't present but also vice-versa. It may even fire a violation
> if there is no value at all, but the other property has values.
>
> Coming up with an artificial limitation is likely going to hurt extensions
> that we cannot anticipate yet, so I believe we must treat them as
> "arbitrary" constraint types that may do anything they like.
>
> I also have seen many use cases of sh:hasValue, esp in filterShapes, so i
> would be against dropping that.
>
> Holger
>
>
>
>
> On 7/03/2016 20:49, RDF Data Shapes Working Group Issue Tracker wrote:
>
>> shapes-ISSUE-129 (existential constraints): Existential constraints
>> should be consistent [SHACL - Core]
>>
>> http://www.w3.org/2014/data-shapes/track/issues/129
>>
>> Raised by: Dimitris Kontokostas
>> On product: SHACL - Core
>>
>> The current core spec defines three existential constraints: sh:minCount,
>> sh:maxCount and sh:hasValue.
>> sh:hasValue requires for a value to exist and match the one supplied in
>> the shape definition.
>>
>> This is not consistent with sh:in which is a variation of sh:hasValue and
>> probably not easy for users to understand the different of sh:hasValue and
>> other constraints e.g. sh:minLength, sh:class, etc
>>
>> I suggest we restrict the core existential constraints to sh:minCount and
>> sh:maxCount only. The rest of the constraints will apply only when there is
>> a value.
>>
>>
>>
>>
>
>


-- 
Dimitris Kontokostas
Department of Computer Science, University of Leipzig & DBpedia Association
Projects: http://dbpedia.org, http://rdfunit.aksw.org, http://
http://aligned-project.eu
Homepage:http://aksw.org/DimitrisKontokostas
Research Group: AKSW/KILT http://aksw.org/Groups/KILT