foo.js

// data Maybe a = Just a | Nothing

var Maybe = function(){}

// type: reserved keyword in haskell
Maybe.type = {
  Just: function(_0){this._0 = _0;},
  Nothing: function(){}
}

Maybe.type.Just.prototype             = Object.create(Maybe.prototype);
Maybe.type.Just.prototype.constructor = Maybe.type.Just;

Maybe.type.Nothing.prototype             = Object.create(Maybe.prototype);
Maybe.type.Nothing.prototype.constructor = Maybe.type.Nothing;

// class method:
//   * unqualified: data constructor
//   * qualified:   instance method(first argument /= self type)
Maybe.Just    = function(a){return new Maybe.type.Just(a)};
Maybe.Nothing = new Maybe.type.Nothing;

// instance method: instance method(first argument == self type)

// class Show a where
//   show [toString] :: a -> String
//
// instance Show a => Show (Maybe a) where
//   show (Just a) = "Just " ++ show a;
//   show _        = "Nothing"
Maybe.prototype.toString = function(){
  if (this instanceof Maybe.type.Just) {
    var a = this._0;
    return "Just " + a.toString();
  } else {
    return "Nothing";
  }
}

// class Monad m where
//   (>>=) "bind" :: m a -> (a -> m b) -> m b
//   return :: a -> m a
//
// instance Monad Maybe where
//   (>>=) (Just a) f = f a
//   return _       f = Nothing
Maybe.prototype.Prelude$Monad$bind = function(f){
  if(this instanceof Maybe.type.Just){
    return f(this._0);
  } else {
    return Maybe.Nothing;
  }
}

Maybe.Prelude$Monad$return = Maybe.Just;

/*
a = do
  a <- return 3
  b <- Nothing
  c <- return (b + 3)
  return (c * 2)
*/

var a = (function($){
return $.Prelude$Monad.Prelude$Monad$return(3)
  .Prelude$Monad$bind(function(a){
  return Maybe.Nothing;
}).Prelude$Monad$bind(function(a){
  return $.Prelude$Monad.Prelude$Monad$return(a + 3);
}).Prelude$Monad$bind(function(a){
  return $.Prelude$Monad.Prelude$Monad$return(a * 2);
});
}({Prelude$Monad: Maybe, Prelude$Read: parseInt}));

console.log(a.toString())