a Maybe Monad

maybe.js

function Maybe() {
	Object.freeze(this);
}

function Just (x) {
  this.toString = function () { return "Just " + x.toString(); };
  this.just = x;
  Object.freeze(this);
}
Just.prototype = new Maybe();
Just.prototype.constructor = Just;

function Nothing () {
  this.toString = function () { return "Nothing"; };
  Object.freeze(this);
}
Nothing.prototype = new Maybe();
Nothing.prototype.constructor = Nothing;

Maybe.unit = function (x) {
	return new Just(x);
};
Maybe.bind = function (f) {
	return function (a) {
		if ( a instanceof Just ) {
			return f( a.just );
		} else if (a instanceof Nothing) {
			return new Nothing();
		} else {
			throw Error("argument is not a Maybe instance.");
		}
	};
};
Maybe.lift = function (f) {
	return function (x) {
		try {
			return new Just( f(x) );
		} catch(e) {
			return new Nothing();
		}
	};
};
Maybe.do = function (m) {
	var fns = Array.prototype.slice.call(arguments, 1);
	fns.forEach(function(fn) {
		m = Maybe.bind( fn )(m);
	});
	return m;
};