Validate output mappings with RML SHACL shapes

[niegrzybkowski]

Hi, based on the discussion in #69 I tried creating a constant valued blank node, but such a setup emitted invalid RML.

Steps to reproduce:

Run YATTER with the following yarrrml:

prefixes:
 ex: "http://example.com/"

mappings:
  person:
    sources:
      - ['data.json~jsonpath', '$.persons[*]']
    s:
        value: constant
        type: blank
    po:
      - [a, foaf:Person]
      - [ex:name, $(firstname) $(lastname)]

Which outputs the following RML subject map:

rr:subjectMap [
        a rr:SubjectMap;
        rr:constant "constant";
        rr:termType rr:BlankNode
];

Which is invalid according to the RML spec 6.1:

If the constant-valued term map is a subject map, predicate map or graph map, then its constant value must be an [IRI].

And is (understandably) rejected by some RML engines.

Expected behavior

YATTER works around this or fails when mapping, instead of emitting invalid RML.

Additional context

Templated blank nodes work correctly

[dachafra]

Validation of the output mappings is not currently part of Yatter. Please use the official RML spec, CG-W3C, not IO: https://w3id.org/rml/core/spec.

There is a set of SHACL shapes to validate the mappings, so one possibility would be to include the validation process with shacl shapes in Yatter: https://github.com/kg-construct/rml-core/blob/main/shapes/core.ttl