Some fun with Monads

monad.js

function Maybe(value) {
  this.value = value;
}

Maybe.prototype.map = function _map(fn, ctx) {
  ctx = ctx || {};
  return this.value === null ? new Maybe(this.value) : new Maybe(fn.call(ctx, this.value));
};

function Either(left, right) {
  this.left = left;
  this.right = right;
}

Either.prototype.map = function _map(fn, ctx) {
  ctx = ctx || {};
  return this.right !== null ? 
    new Either(this.left, fn.call(ctx, this.right)) :
    new Either(fn.call(ctx, this.left), null);
};

function Promise(val) {
  this.value = val;
}

Promise.prototype.map = function _map(fn, ctx) {
  ctx = ctx || {};
  var self = this;
  setTimeout(function() {
    fn(self.value);
  }, 1000);
  return this;
};

function map(val, fn, ctx) {
  ctx = ctx || {};
  return val.map ? val.map(fn, ctx) : fn(val, ctx);
}

function greet(name) {
  console.log('Hello ' + name + '!');
}

map(new Maybe('Bob'), greet);
map(new Maybe(null), greet);
map(new Either('Anonymous', 'Bob'), greet);
map(new Either('Anonymous', null), greet);
map(new Promise('Bob'), greet);