// These need to be registered from general to specific.
// The alternative I can see is to use a prototype-based solutions as done in:
// https://github.com/loop-recur/typeclasses/blob/master/functor.js

Functor(Array, {fmap: function(f, arr){
    return arr.map(function(x){return f(x);});
}});
 
Functor(Function, {fmap: function(f, g) {
    return compose(f, g);
}});
 
Functor(Maybe, {fmap: function(f, maybe) {
    return (maybe.val == null) ? maybe : Maybe(f(maybe.val));
}});
 
Functor(Either, {fmap: function(f, either) {
    return (either.right == null) ? Either(f(either.left), null) : Either(either.left, f(either.right));
}});

var plus1 = function(n) {return n + 1;};
var times2 = function(n) {return n * 2;};

console.log(fmap(plus1, [2, 4, 6, 8])); //=> [3, 5, 7, 9]
console.log(fmap(plus1, Maybe(5))); //=> Maybe(6)
console.log(fmap(plus1, Maybe(null))); //=> Maybe(null)
console.log(fmap(plus1, times2)(3)); //=> 7 (=  3 * 2 + 1)
console.log(fmap(plus1, Either(10, 20))); //=> Either(10, 21)
console.log(fmap(plus1, Either(10, null))); //=> Either(11, null)