monoid.ts

class Monoid<T>{
  mzero: T;
  mplus: (a: T, b: T) => T;
  constructor(mzero: T, mplus: (a: T, b: T) => T) {
    this.mzero = mzero;
    this.mplus = mplus;
  }
}

const add = new Monoid<number>(0, (a, b) => a + b);
const mul = new Monoid<number>(1, (a, b) => a * b);
const arr = new Monoid<any[]>([], (a, b) => a.concat(b));

const foldM = <T>(monoid: Monoid<T>, data: T[]) => data.reduce(monoid.mplus, monoid.mzero);

let a = [[1], [2, 3], [4], [5]];
let b = [1, 2, 3, 4, 5];

alert(foldM(arr, a));
alert(foldM(add, b));