How to create custom functions?

[ajjmai]

Could someone please help me understand how to create custom functions for rml-mapper?

I'm using yarrrml and rmlmapper-java for xml -> rdf conversion. I have been able to use the pre-defined functions from grel:http://users.ugent.be/~bjdmeest/function/grel.ttl# and idlab-fn: http://example.com/idlab/function/ namespaces but I have not managed to add any custom functions. I have tried getting started with the example for dynamic loading provided here: https://github.com/RMLio/rmlmapper-java, but I'm not able to make it work. I have probably misunderstood something?

I have all the relevant files in same folder:

.
├── CustomFunctions.class
├── CustomFunctions.jar
├── CustomFunctions.java
├── functions.ttl
├── main.xml
├── rmlmapper-6.2.2-r371-all.jar
├── rules.rml.ttl
└── rules.yml

CustomFunctions.java (copied from the example)

public class CustomFunctions {
  public static String parsePointLat(String s) {
    return s.replace("POINT ", "").replace('(', ' ').replace(')', ' ').trim().split("\\s+")[0];
  }
}

functions.ttl (copied from the example)

@prefix dcterms: <http://purl.org/dc/terms/> .
@prefix doap:    <http://usefulinc.com/ns/doap#> .
@prefix fno:     <https://w3id.org/function/ontology#> .
@prefix fnoi:    <https://w3id.org/function/vocabulary/implementation#> .
@prefix fnom:    <https://w3id.org/function/vocabulary/mapping#> .
@prefix grel:    <http://users.ugent.be/~bjdmeest/function/grel.ttl#> .
@prefix grelm:   <http://fno.io/grel/rmlmapping#> .
@prefix rdfs:    <http://www.w3.org/2000/01/rdf-schema#> .

grel:parsePointLat a fno:Function ;
  fno:name "parsePointLat" ;
  rdfs:label "parsePointLat" ;
  dcterms:description "Parse the latitude from a point." ;
  fno:expects ( grel:valueParam ) ;
  fno:returns ( grel:stringOut ) .

grelm:javaString
    a                  fnoi:JavaClass ;
    doap:download-page "CustomFunctions.jar" ;
    fnoi:class-name    "CustomFunctions" .

grelm:parsePointLat
    a                    fno:Mapping ;
    fno:function         grel:parsePointLat ;
    fno:implementation   grelm:javaString ;
    fno:methodMapping    [ a                fnom:Function ;
                           fnom:method-name "parsePointLat" ] .

rules.yml

prefixes:
  rdf: http://www.w3.org/1999/02/22-rdf-syntax-ns#
  rdfs: http://www.w3.org/2000/01/rdf-schema#
  grel: http://users.ugent.be/~bjdmeest/function/grel.ttl#
  idlab-fn: http://example.com/idlab/function/
  temp: http://example.com/temp/

mappings:
  act:
    sources:
      - [main.xml~xpath, //act]
    subjects: "temp:$(//preface/p/docNumber)"
    predicateobjects:
      - predicates: temp:id
        objects:
          - function: grel:parsePointLat
             parameters:
              - [grel:valueParameter, "50.2"]
  #  - predicates: temp:id
      #   objects:
      #     - function: grel:string_replace
      #        parameters:
      #         - [grel:valueParameter, "50.2"]
      #         - [grel:p_string_find, "."]
      #         - [grel:p_string_replace, "--"]

rules.rml.ttl (generated with yarrrml-parser)

@prefix rr: <http://www.w3.org/ns/r2rml#>.
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>.
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>.
@prefix fnml: <http://semweb.mmlab.be/ns/fnml#>.
@prefix fno: <https://w3id.org/function/ontology#>.
@prefix d2rq: <http://www.wiwiss.fu-berlin.de/suhl/bizer/D2RQ/0.1#>.
@prefix void: <http://rdfs.org/ns/void#>.
@prefix dc: <http://purl.org/dc/terms/>.
@prefix foaf: <http://xmlns.com/foaf/0.1/>.
@prefix rml: <http://semweb.mmlab.be/ns/rml#>.
@prefix ql: <http://semweb.mmlab.be/ns/ql#>.
@prefix : <http://mapping.example.com/>.
@prefix grel: <http://users.ugent.be/~bjdmeest/function/grel.ttl#>.
@prefix idlab-fn: <http://example.com/idlab/function/>.
@prefix temp: <http://example.com/temp/>.

:rules_000 a void:Dataset;
    void:exampleResource :map_act_000.
:map_act_000 rml:logicalSource :source_000.
:source_000 a rml:LogicalSource;
    rml:source "main.xml";
    rml:iterator "//act";
    rml:referenceFormulation ql:XPath.
:map_act_000 a rr:TriplesMap;
    rdfs:label "act".
:s_000 a rr:SubjectMap.
:map_act_000 rr:subjectMap :s_000.
:s_000 rr:template "http://example.com/temp/{//preface/p/docNumber}".
:pom_000 a rr:PredicateObjectMap.
:map_act_000 rr:predicateObjectMap :pom_000.
:pm_000 a rr:PredicateMap.
:pom_000 rr:predicateMap :pm_000.
:pm_000 rr:constant temp:id.
:pom_000 rr:objectMap :om_000.
:om_000 a fnml:FunctionTermMap;
    rr:termType rr:Literal;
    fnml:functionValue :fn_000.
:fn_000 rml:logicalSource :source_000;
    rr:predicateObjectMap :pomexec_000.
:pomexec_000 rr:predicateMap :pmexec_000.
:pmexec_000 rr:constant fno:executes.
:pomexec_000 rr:objectMap :omexec_000.
:omexec_000 rr:constant "http://users.ugent.be/~bjdmeest/function/grel.ttl#parsePointLat";
    rr:termType rr:IRI.
:fn_000 rr:predicateObjectMap :pom_001.
:pom_001 a rr:PredicateObjectMap;
    rr:predicateMap :pm_001.
:pm_001 a rr:PredicateMap;
    rr:constant grel:valueParameter.
:pom_001 rr:objectMap :om_001.
:om_001 a rr:ObjectMap;
    rr:constant "50.2";
    rr:termType rr:Literal.

I have installed yarrrm-parser and rmlmapper-java locally. I use these commands:

yarrrml-parser -i rules.yml -o rules.rml.ttl
javac CustomFunctions.java && jar cvf CustomFunctions.jar CustomFunctions.class
java -jar rmlmapper-6.2.2-r371-all.jar -f functions.ttl -m rules.rml.ttl -o dump.ttl

As per the example, the expected output is "POINT (50.2 5.3)", but I get "50.2", so it seems that the function is not called at all.
output in dump.ttl:
<http://example.com/temp/30.12.1993%2F1501> <http://example.com/temp/id> "50.2".

Furthermore, if I try to use both a custom function and a pre-defined function from grel-namespace (adding the commented lines in rules.yml), the rmlmapper throws an error.

 18:00:41.170 [main] ERROR be.ugent.rml.cli.Main               .run(453) - Function with id http://users.ugent.be/~bjdmeest/function/grel.ttl#string_replace not found
be.ugent.idlab.knows.functions.agent.functionModelProvider.fno.exception.FunctionNotFoundException: Function with id http://users.ugent.be/~bjdmeest/function/grel.ttl#string_replace not found

[ajjmai]

Okay, so I apparently had misunderstood how the example custom function works. The expected input was "POINT (50.2 5.3)" and output "50.2", not the other way around. Realising this solved the issue of the function not working.

However, the problem remains that the example custom function cannot be used together with functions in the grel namespace without making these adjustments:

In custom functions.ttl change namespace of the grelm:javaString to something else.

Include both the custom functions and grel-functions:
java -jar mapper.jar -f functions.ttl -f functions_grel.ttl -f grel_java_mapping.ttl <other options>