monoid1.ts

interface Monoid<A, B> {
  mempty: () => Monoid<A, B>;
  mconcat: (r: Monoid<A, B>) => Monoid<A, B>;
  runMonoid: () => B;
}

const mconcat = <A, B>(a : Monoid<A, B>) => (b: Monoid<A, B>): Monoid<A, B> => a.mconcat(b);

class Sum implements Monoid<Sum, number> {
  constructor(private value: number) {}
  
  mempty() {
    return new Sum(0);
  }

  mconcat(sum: Sum) {
    return new Sum(this.value + sum.value);
  }

  runMonoid() {
    return this.value;
  }
}

class Prod implements Monoid<Prod, number> {
  constructor(private value: number) {}
  
  mempty() {
    return new Prod(1);
  }

  mconcat(prod: Prod) {
    return new Prod(this.value * prod.value);
  }

  runMonoid() {
    return this.value;
  }
}

let s = new Sum(1);
let t = new Sum(2);

let u = new Prod(2);
let v = new Prod(3);

let a = mconcat (s) (t.mempty());
let b = mconcat<Prod, number> (u) (s);

console.log(a.runMonoid());
console.log(b.runMonoid());
// interface Functor<F> {
//   map<A, B>(f: (a: A) => B): F<A> => T<B>;
// }