Maybe monad via ES6 generators (via Traceur)

maybe.js

var Nothing = function Nothing() {};
var Just = function Just(x) { this.value = x; };

function runMaybe(f) {
  var result = f.next();

  while (!result.done) {
    var value = result.value;
    switch (value.constructor) {
      case Nothing:
        return result.value;
      case Just:
        result = f.next(value.value);
        break;
      default: // fmap fallback behavior
        result = f.next(value);
        break;
    }
  }

  return result.value;
}

maybe_example.js

function tryDouble(x) {
  return new Just(2*x);
}

function lookup(obj, key) {
  if (key in obj) {
    return new Just(obj[key]);
  } else {
    return new Nothing();
  }
}

function* test(obj) {
  var x = yield lookup(obj, "stuff");
  var y = yield tryDouble(x);
  return new Just(x + y);
}


// runMaybe(test({stuff: 42}))
//=> Just {value: 126}
//
// runMaybe(test({notstuff: 42}))
//=> Nothing {}

Just having some fun with generators. I don't think you can do a List monad like in Haskell, since you'd need to be able to run everything after the yield multiple times, so generators can't help with monads that need to do that kind of thing...