state-safe-mdo.js

const after = (n, x) => Future((l, r) => setTimeout(r, n, x));

const Do = (f, of) => {
  return of(undefined).chain(() => {
    const g = f()
    const step = value => {
    const result = g.next(value)
    return result.done ?
           of(result.value) :
           result.value.chain(step)
    }
    return step()
  })
}

const fut = Do(function*(){
  const a = yield after(300, 1);
  const b = yield after(300, 2);
  return a + b;
}, Future.of);

fut.fork(console.log, console.log)
fut.fork(console.log, console.log)