Unify reusable and parametrised functions

[tpluscode]

I'd like to combine some of the ideas brought up in #279, #314, #315 + the prior art of sh:Function, sh:SPARQLFunction and https://datashapes.org/sparql#

I think there is significant overlap to combine these in a single solution which could solve multiple function (pun intended). A generalised function definition would then be backed by one or more implementations

1. Reusable Node Expressions (like Add a node expression type for calling a node expression #279 + parameters)
2. SPARQL (like Add sh:SPARQLFunction #314)
3. Native built-ins (like dash-sparql:)
4. JS or other native code for ad-hoc solutions

[HolgerKnublauch]

Yes absolutely. Such a generalization was implemented for TopBraid, e.g. with JS-based functions. And they can all be invoked consistently using https://w3c.github.io/shacl/shacl-af/#node-expressions-function

[tpluscode]

Here's my idea for Reusable Node Expressions, implementing #168

Used as

ex:ShapeB
    sh:targetNode [
        ex:directlyTypedAs ( <https://data.exemple.fr/B> )
   ] ;

The expression ex:directlyTypedAs would need to wrap a more verbose expression, borrowing the way sh:SPARQLFunction are parametrised in SHACL-AF.

ex:directlyTypedAs 
  sh:parameter [
    sh:path ex:arg1 ;
    sh:nodeKind sh:IRI ;
    sh:order "0"^^xsd:decimal ;
  ] ;
  sh:expression [
      sh:path [
        sh:inversePath rdf:type ;
        sh:nodes [ sh:var "arg1" ] ;
      ] ;
  ] ;
.

[tpluscode]

For SPARQL, it already provides sh:select Select Expression in the FPWD, so it's pretty much te same as #316 (comment)

ex:directlyTypedAs 
  sh:parameter [
    sh:path ex:arg1 ;
    sh:nodeKind sh:IRI ;
    sh:order "0"^^xsd:decimal ;
  ] ;
  sh:expression [
    sh:select """
      SELECT ?instance {
        ?instance a $arg1
      }
    """
  ] ;
.