An es6 js maybe monad, using generators

maybe.traceur

log = console.log.bind(console);

maybe = v => {

  function * f () {
    if (v) yield v;
  }

  f.bind = f =>
    v ? f(v) : maybe();

  f.map = f =>
    v ? maybe(f(v)) : maybe();

  f.toString = () => `[Maybe ${v}]`;

  return f;
};

maybe.lift = f =>
  v => maybe(f(v))

maybe.lift2 = f =>
  (m1, m2) => maybe(
    m1.bind(v1 =>
      m2.bind(v2 =>
        f(v1, v2))));

let one = maybe('1').map(Number);
log(one); // [Maybe 1]

let none = maybe().map(Number);
log(none); // [Maybe undefined]

let addOne = x => x + 1;
let two       = one.bind(maybe.lift(addOne));
let stillnone = none.bind(maybe.lift(addOne));

for (let val of two()) log(val); // 2
for (let val of stillnone()) log(val); // does not run

let add = (x, y) => x + y;
let addMaybe = maybe.lift2(add);

let three         = addMaybe(one, two);
let evenstillnone = addMaybe(one, none);
for (let val of three()) log(val); // 3
for (let val of evenstillnone()) log(val); // does not run