Functors and monads

monads.js

var _Container = function(val) { this.val = val }
var Container = function(val) { return new _Container(val); }
Container.prototype.map = function(f) {
  return new Container(f(this.val));
}
Container.prototype.flatMap = function(f) {
  return f(this.val);
}
var c = Container(2);
// functor
c.map(function(x) { return x+2});
// monad
c.flatMap(function(x){ return Container(x+2)}).flatMap(function(x) {return Container(x*-1)});

Looking great! Just two things:

1. No need for new keyword in front of Container (that's what the extra Container method is for)
2. flatMap expects a Container returned. That's the magic behind monads:
   c.flatMap(function(x){ return Container(x+2) }).flatMap(function(x) {return Container(x* -1) });

Try it out with some https://github.com/folktale or https://github.com/fantasyland monads to see different behaviors via different "Containers", but remember they call flatMap chain. Good stuff!

Updated! I evidently didn't have alerts for comments set up. Thanks, Brian!