Reader Monad in TypeScript

reader-monad.ts

class ReaderMonad<A, B> {
  constructor(public runReader: (a: A) => B) {
  }

  bind<C>(func: (b: B) => ReaderMonad<A, C>): ReaderMonad<A, C> {
    return new ReaderMonad<A, C>((a: A) => {
      const b = this.runReader(a);
      return func(b).runReader(a);
    });
  }

  static return<A, B>(b: B): ReaderMonad<A, B> {
    return new ReaderMonad((a: A) => b);
  }
}

const uppercase = new ReaderMonad((s: string) =>
  s.toUpperCase())
const reverse = new ReaderMonad((s: string) =>
  s.split('').reverse().join(''));


const reader = uppercase
  .bind(s1 => reverse
    .bind(s2 => ReaderMonad.return([s1, s2])));


const result = reader.runReader("hello"); //["HELLO", "olleh"]