const doM = <A>(gen: Lazy<IterableIterator<any>>): A => {
  let g = gen();
  const step = (value: any) => {
    const result = g.next(value);
    if (result.done) {
      g = gen();
      return result.value;
    } else {
      return result.value.chain(step);
    }
  };
  return step(undefined);
};

const id = doM<number>(function *() {
  const begin = yield identity.of(identity.of(41));
  const add = yield begin.ap(identity.of((x: number) => x + 1));
  return add;
});

expect(id).toEqual(42);