Task.js

const compose = (f, g) => (...args) => f(g(...args));

class Task {
  constructor(fork) { this.fork = fork; }

  static of (fork) { return new Task(fork); }

  map(f)   { return Task.of((rej, res) => this.fork(rej, compose(res, f))); }
  chain(f) { return Task.of((rej, res) => this.fork(rej, x => f(x).fork(rej, res))); }
  ap(task) { return Task.of((rej, res) => this.fork(rej, f => task.fork(rej, compose(res, f)))); }

  static fromPromise(f, ...args) {
    return Task.of((reject, resolve) => f(...args).then(resolve, reject));
  }
}

const getTimer = wait => Task.of((rej, res) => setTimeout(res, wait, `Wait for ${wait}`));

Task.of((rej, res) => res(x => y => ({x, y})))
    .ap(getTimer(500).map(x => `First ${x}`))
    .ap(getTimer(100).map(x => `Second ${x}`))
    .fork(console.error, console.log);