Breaking referentially transparent

Question

I have following function:

_processSelectedPlantWcOb: function (oSelectOb, oSetNewDates, oWpServiceOb, fnQueryDatesPlantWcOb) {
  let self = this;

  return oSelectOb
    .map(function (oEvent) {
      return oEvent.getSource();
    })
    .switchMap(function (oSelect) {
      return oSetNewDates.mapTo(oSelect);
    })
    .map(function (oSelect) {
      let oItem = oSelect.getSelectedItem();
      let aKeys = oItem.getKey().split("/");
      return {sPlant: aKeys[0], sWc: aKeys[1]};
    })
    .switchMap(function (oSelected) {
      return fnQueryDatesPlantWcOb(oWpServiceOb, oSelected.sPlant, oSelected.sWc);
    });
},

as you can see the last parameter is expect a function and the implementation of the function fnQueryDatesPlantWcOb looks as follow:

_processQueryDatesPlantWcOb: function (oWpServiceOb, sPlant, sWc) {

  return oWpServiceOb
    .switchMap(function (oModel) {
      let oPlantFilter = new sap.ui.model.Filter("Plant", sap.ui.model.FilterOperator.EQ, sPlant);
      let oWcFilter = new sap.ui.model.Filter("WorkCenter", sap.ui.model.FilterOperator.EQ, sWc);

      return Rx.Observable.create(function (subscriber) {
        oModel.read("/CostCenterCalendarSet", {
          success: function (oData, oResponse) {
            subscriber.next(oResponse);
          },
          error: function (oError) {
            subscriber.error(oError);
          },
          filters: [oPlantFilter, oWcFilter]
        });
      });
    })
    .filter(function (oData) {
      return oData.data.results.length > 0
    })
    .mergeMap(function (oData) {
      return Rx.Observable.from(oData.data.results);
    })
    .map(function (oData) {
      let oDate = oData.InspectionDate;
      return new Date(oDate.getFullYear(), oDate.getMonth(), oDate.getDate());
    })
    .filter(function (oDate) {
      let oToday = new Date();
      return oDate.getTime() > oToday.getTime();
    })
    .map(function (oDate) {
      return oDate.getTime();
    });
},

As you can see, the parameters sPlant and sWc will be use in the switchMap function.

What I want to know is, do I break the functional paradigm? In my opinion, I do not break it, because every time when I pass the same sPlant and sWc values, I will get the same result, but I am not sure.

score 0 · Answer 1 · edited May 23 '17 at 12:17

I don't program in JS, but the "functional paradigm" actually defines two properties regarding (pure) functions:

It always returns the same result if given the same arguments. This is called referential transparency, and you can add it to your list of $5 programming terms.

It can’t cause any side effects. That is, the function can’t make any changes that are observable outside the function itself—for example, by changing an externally accessible mutable object or writing to a file.

Since oModel.read("/CostCenterCalendarSet", ...) seems to do I/O, it's not a pure function (check out this question to see why reading is a side effect, too).

Furthermore, it's a controversial topic if functions using date/time can be referentially transparent at all—especially if it's not part of the parameter list.

Breaking referentially transparent

1 Answers1