higher order reducers

better-map-filter-hors.ts

// types
type T_XF<XF_Before, XF_After> = (a: XF_Before) => XF_After;

type T_Pred<Pred_Fn> = (fn: Pred_Fn) => boolean;

type T_Reducer<Before, After> = (acc: After, cur: Before) => After;

type T_Hor<Inner_Before, Before, After> = (
  reducer: T_Reducer<Before, After>
) => T_Reducer<Inner_Before, After>;

type T_Map = <Before, After, Inner_Before = Before>(
  xf: T_XF<Inner_Before, Before>
) => T_Hor<Inner_Before, Before, After>;

type T_Filter = <Before, After>(
  pred: T_Pred<Before>
) => T_Hor<Before, Before, After>;


const map: T_Map = (xf) => (reducer) => (acc, cur) => reducer(acc, xf(cur));

const filter: T_Filter = (pred) => (reducer) => (acc, cur) => pred(cur) ? reducer(acc, cur) : acc;