How to specify return type from function-mappings?

[webhart]

I try to map the following data:

[
    {
        "k":"v1"
    },
    {   
        "k":"v2"
    }
]

using the following mapping:

@prefix rml: <http://semweb.mmlab.be/ns/rml#> .
@prefix rr: <http://www.w3.org/ns/r2rml#> .
@prefix ex: <https://example.org/> .
@prefix ql: <http://semweb.mmlab.be/ns/ql#>.
@prefix fnml: <http://semweb.mmlab.be/ns/fnml#>.
@prefix fno: <https://w3id.org/function/ontology#>.
@prefix grel: <http://users.ugent.be/~bjdmeest/function/grel.ttl#>.

ex:my_Map a rr:TriplesMap ;
    rml:logicalSource [
        rml:source "test_data.json";
        rml:referenceFormulation ql:JSONPath;
        rml:iterator "$[*]"
    ] ; 
  
    rr:subjectMap [ rr:termType rr:BlankNode ] ;

    rr:predicateObjectMap [
        rr:predicate ex:my_prop;
        rr:objectMap [ 
            fnml:functionValue [
                rr:predicateObjectMap [
                    rr:predicate fno:executes;
                    rr:objectMap [ 
                        rr:constant grel:controls_if
                    ]   
                ];
                rr:predicateObjectMap [
                    rr:predicate grel:bool_b;
                    rr:objectMap [ 
                        fnml:functionValue [
                            rr:predicateObjectMap [
                                rr:predicate fno:executes;
                                rr:objectMap [
                                    rr:constant grel:string_contains
                                ]   
                            ];
                            rr:predicateObjectMap [
                                rr:predicate grel:valueParameter;
                                rr:objectMap [ rml:reference "k" ]
                            ];
                            rr:predicateObjectMap [
                                rr:predicate grel:string_sub;
                                rr:objectMap [ rr:constant "v1" ]
                            ]
                        ]
                    ]
                ];
                rr:predicateObjectMap [
                    rr:predicate grel:any_true;
                    rr:objectMap [rr:constant ex:my_v1_resource]
                ];
                rr:predicateObjectMap [
                    rr:predicate grel:any_false;
                    rr:objectMap [rr:constant ex:my_default_resource]
                ]
            ]
        ]
    ].

If I use a resource in the line rr:objectMap [rr:constant ex:my_default_resource], I get a literal like "https://example.org/my_default_resource" instead of the expected <https://example.org/my_default_resource> If I use a typed literal as object for the rr:constant, the type gets lost. Is there a way to force a conversion to a specific type? How to create resources from functions?