Conditional steps

[tpluscode]

Consider a CSVW pipeline which often looks like below, where we remove the csvw: triples.

@prefix code: <https://code.described.at/> .
@prefix p: <https://pipeline.described.at/> .

<transform> a p:Pipeline, p:ReadableObjectMode ;
    p:steps
        [
            p:stepList
                (
                    _:readCsv
                    _:parseCsv
                    _:toDataset
                    _:toObservation
                    _:buildCubeShape
                    _:flatten
                    _:removeCsvwTriples
                    _:generateShapeTarget
                ) ;
        ] ;
.

_:removeCsvwTriples a p:Step ;
    code:implementedBy
        [
            a code:EcmaScriptModule ;
            code:link <node:barnard59-base/filter.js#default> ;
        ] ;
    code:arguments
        (
            [
                a code:EcmaScriptModule ;
                code:link <node:barnard59-csvw/filter.js#excludeCsvwTriples> ;
            ]
        ) ;
.

The simple way to disable a step (skip it) would be to resolve a variable

<transform>
  p:variables [
    p:variable [ p:name "KEEP_CSVW_META" ; p:value false ] ;
  ] ;
.

_:removeCsvwTriples a p:Step ;
+   p:if "KEEP_CSVW_META"^^p:VariableName ;
.

So if KEEP_CSVW_META is truthy, the step will run.

In to make more complex decision, the a function could be used

_:removeCsvwTriples a p:Step ;
+   p:if """
+       async () => {
+         // make a decision in code
+       }
+     """^^code:EcmaScript ;
.

This function would be evaluated once, at the beginning.

[tpluscode]

If not too complex, we should implement this in a way that the skipped step if effectively removed from the pipeline. Otherwise a PassThrough would be enough