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Re: ISSUE-135: Proposed changes to implement syntax simplification

From: Holger Knublauch <holger@topquadrant.com>
Date: Thu, 5 May 2016 18:37:31 +1000
To: "public-data-shapes-wg@w3.org" <public-data-shapes-wg@w3.org>
Message-ID: <dfe3fe64-fa3b-a443-df59-2e486f359e17@topquadrant.com>
On 5/05/2016 17:58, Dimitris Kontokostas wrote:
>
> Thanks Holger
>
> see inline for a couple of questions
>
> On Thu, May 5, 2016 at 5:49 AM, Holger Knublauch 
> <holger@topquadrant.com <mailto:holger@topquadrant.com>> wrote:
>
>     Several WG members supported the idea of allowing constraints to
>     be used as values in places such as sh:or. I was asked to make
>     some specific suggestions on what would need to be changed in the
>     spec, so that the following syntax options would behave
>     identically. (Both scenarios state that the values of
>     schema:address must be string literals or instances of
>     schema:Address):
>
>     a) Currently supported: sh:or can only be used with
>     sh:NodeConstraints and operands of sh:or must be shapes
>
>     ex:MyShape
>         a sh:Shape ;
>         sh:constraint [
>             sh:or (
>                 [ a sh:Shape ;
>                     sh:property [
>                         sh:predicate schema:address ;
>                         sh:datatype xsd:string ;
>                     ] ;
>                 ]
>                 [ a sh:Shape ;
>                     sh:property [
>                         sh:predicate schema:address ;
>                         sh:class schema:Address ;
>                     ] ;
>                 ]
>             )
>         ]
>
>     which lacks on multiple fronts - it is too verbose and also forces
>     repetition of the predicate.
>
>     b) Proposed: generalize sh:or and values of sh:or may be
>     constraints of the same kind as the surrounding constraint.
>
>     ex:MyShape
>         a sh:Shape ;
>         sh:property [
>             sh:predicate schema:address ;
>             sh:or (
>                 [ sh:datatype xsd:string ]
>                 [ sh:class schema:Address ]
>             )
>         ]
>
>     In this proposal, the members of the sh:or List may be
>     sh:PropertyConstraints if sh:or is used within a
>     sh:PropertyConstraint.
>
>
> Questions:
>  - can we have nested sh:or / sh:and / sh: not in these positions?

Yes I think so. Example sh:or [ sh:and [ ... the sh:or is a 
sh:PropertyConstraint, and so is the sh:and, thus the same default 
constraint type would apply there too.

>  - are we allowed to provide shapes on only 
> (Inverse)PropertyConstraints parameters i.e. we must always follow the 
> current context?

I believe it would make most sense to follow the current context. To 
"escape" that, you can always explicitly make the nested constraint an 
instance of something else via rdf:type, including rdf:type sh:Shape as 
a complete generalization.

>  - Is this simplification allowed in sh:constraint as well?

Yes, the default type then would be sh:NodeConstraint.

HTH
Holger


>
> Thanks,
> Dimitris
>
>     Required changes (all incremental to current spec):
>
>     1) Generalize sh:hasShape from sh:hasShape(?node, ?shape,
>     ?shapesGraph) to
>
>         sh:validateNode(?node, ?shapeOrConstraint, ?shapesGraph,
>     ?defaultConstraintType, ?defaultPredicate)
>
>     The two arguments at the end are optional, and are used to
>     complement the provided ?shapeOrConstraint unless it is a
>     sh:Shape. Legal values for ?defaultConstraintType would be
>     sh:PropertyConstraint, sh:InversePropertyConstraint and
>     sh:NodeConstraint. ?defaultPredicate is only supported if
>     ?defaultConstraintType is given and != sh:NodeConstraint.
>
>     The algorithm would be
>
>     a) if ?shapeOrConstraint rdf:type sh:Shape, then behave as currently
>     b) otherwise, assume ?defaultConstraintType (unless the node has
>     an rdf:type)
>         and assume ?defaultPredicate for sh:predicate.
>     c) report failure if the node has rdf:type that is neither
>     sh:Shape nor ?defaultConstraintType.
>
>     While this function isn't pretty it's mostly used internally
>     anyway and may therefore be regarded as an implementation detail.
>     The name sh:validateNode is better than sh:hasShape because it may
>     also return unbound.
>
>     2) Generalize sh:or to also have contexts: sh:PropertyConstraint
>     and sh:InversePropertyConstraint
>
>     3) Add a sh:propertyValidator to sh:OrConstraint similar to what
>     we have as sh:nodeValidator, but with the sh:validateNode function:
>
>     SELECT $this ?failure ...
>     WHERE {
>       	{
>     		$this $predicate ?value .
>     	}
>     	 {
>     		SELECT (SUM(?s) AS ?count)
>     		WHERE {
>     			GRAPH $shapesGraph {
>     				$or rdf:rest*/rdf:first ?shape .
>     			}
>     			BIND (sh:validateNode(?value, ?shape, $shapesGraph, sh:PropertyConstraint, $predicate) AS ?valid) .
>     			BIND (IF(bound(?valid), IF(?valid, 1, 0), 'error') AS ?s) .
>     		}
>     	}
>     	BIND (!bound(?count) AS ?failure) .
>     	FILTER IF(?failure, true, ?count = 0) .
>     }
>
>
>     and similar for sh:inversePropertyValidator. The same approach
>     would work for sh:and and sh:not. I guess also for sh:valueShape
>     if that's desirable.
>
>     Regards,
>     Holger
>
>
>
>
> -- 
> Dimitris Kontokostas
> Department of Computer Science, University of Leipzig & DBpedia 
> Association
> Projects: http://dbpedia.org, http://rdfunit.aksw.org, 
> http://http://aligned-project.eu <http://aligned-project.eu/>
> Homepage:http://aksw.org/DimitrisKontokostas
> Research Group: AKSW/KILT http://aksw.org/Groups/KILT
>
Received on Thursday, 5 May 2016 08:38:05 UTC

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