Re: Question: SHACL for describing parameterized operations?

Dear Holger,

Thank you for your answer.
I'll try to clarify my use case a bit more.

The operation I mentioned before represents an API call, which is described using the Hydra vocabulary [1].
At the moment, we describe triple pattern queries as follows:

_:search hydra:template                  "http://example.org{?subject,predicate,object}";
         hydra:variableRepresentation hydra:ExplicitRepresentation;
         hydra:mapping                        _:subject, _:predicate, _:object;
_:subject   hydra:variable "subject";
   hydra:property rdf:subject.
_:predicate hydra:variable "predicate";
                    hydra:property rdf:predicate.
_:object       hydra:variable "object";
                    hydra:property rdf:object.

Next to that, we also want to describe the RDF results of this API call,
where the _:subject, _:predicate and _:object variables are used as parameters.
Which we could indeed do using SHACL-SPARQL as you suggested:

_:search hydra:resultShape ex:findMatchingTriples.
ex:findMatchingTriples
    sh:parameter [
        sh:path _:subject ;
        sh:order 0 ;
        sh:nodeKind sh:BlankNodeOrIRI ;
    ] ;
    sh:parameter [
        sh:path _:predicate ;
        sh:order 1 ;
        sh:nodeKind sh:IRI ;
    ] ;
    sh:parameter [
        sh:path _:object ;
        sh:order 2 ;
    ] ;
    sh:propertyValidator [
    a sh:SPARQLSelectValidator ;
    sh:select """
     SELECT ?subject ?predicate ?object
     WHERE {
      ?subject ?predicate ?object .
     }
     """
   ] .

This seems like a solution that would definitely work,
but I'm just wondering what the advantage of this SHACL-based definition
would be compared to a SPIN-based definition, such as the following:

_:search hydra:resultShape ex:findMatchingTriplesAlt.
ex:findMatchingTriplesAlt
        my:hasParameters ( _:subject, _:predicate, _:object );
        a sp:Select;
        sp:resultVariables ( _:subject, _:predicate, _:object );
        sp:where ([
            sp:subject _:subject;
            sp:predicate _:predicate;
            sp:object _:object
        ]).

One obvious advantage of SHACL is that it's a W3C recommendation.
But to me, it seems like SPIN might be more fitting to our use case,
as there is an explicit RDF-based link between the parameters of the API call
and the parameters within the query.

Furthermore, this might enable other RDF-based techniques (such as reasoners)
to take advantage of this query structure, which would not be possible with the SHACL definition as the query is text-based.

What is your opinion on using SPIN or SHACL in this case?

Kind regards,
Ruben Taelman

[1] http://www.hydra-cg.com/spec/latest/core/

On 29 August 2017 at 01:28:10, Holger Knublauch (holger@topquadrant.com) wrote:

Hi Ruben,

it is unclear to me what exactly you are attempting to represent. Below you present an outline for a solution but the problem statement isn't clear. Could you elaborate a bit?

Note that the SHACL vocabulary (see TTL file) includes the class sh:Parameterizable that can be used as superclass of all kinds of "templates". For example to state that something takes three arguments, you could do

my:Operation rdfs:subClassOf sh:Parameterizable .

ex:findMatchingTriples
    a my:Operation ;
    sh:parameter [
        sh:path ex:subject ;
        sh:order 0 ;
        sh:nodeKind sh:BlankNodeOrIRI ;
    ] ;
    sh:parameter [
        sh:path ex:predicate ;
        sh:order 1 ;
        sh:nodeKind sh:IRI ;
    ] ;
    sh:parameter [
        sh:path ex:object ;
        sh:order 2 ;
    ] ;
    ...

(and then attach whatever you like as other triples to the ex:findMatchingTriples - in the case of SHACL-SPARQL there are SPARQL queries, in the case of SHACL-JS there are JS snippets).

The above may be on the wrong meta-level, but as I said I don't really understand your use case yet.

Regards
Holger



On 28/08/2017 21:41, Ruben Taelman wrote:
Dear all,

I have a question regarding the potential use of SHACL for a certain use-case,
and I'm wondering if someone here can help me identify whether or not SHACL is a good solution for this.

I am looking for a vocabulary that can declaratively describe RDF results
of a certain operation, based on certain parameters.

For example, assume the following operation can evaluate triple pattern queries:
findMatchingTriples(subject, predicate, object)

As far as I can see, SHACL doesn't provide a way to bind 'variables' to nodes in a shape.
The closest I have come to describing such a triple pattern operation
is using the following (invalid) SHACL shape:

  _:shape a sh:NodeShape;
          sh:targetNode _:subject;
          sh:property [
              sh:path _:predicate;
              sh:hasValue _:object.
          ].

_:subject, _:predicate and _:object refer to the variable parameters of the operation.

I assume I need some kind of abstraction layer above SHACL,
that basically 'instantiates' SHACL shapes based on certain parameters.

So my question is:
is such an abstraction already possible in some form, or are there future plans for something like this?
Or would you suggest using a SPARQL-based vocabulary for this, such as SPIN,
as I don't really need SHACL's validation and constraint checking-features.

Kind regards,
Ruben Taelman