This is just a quick note while I was reading below articles.

Functor?

Functor f maps a concrete type to a concrete type(* -> *), and it defines fmap function. fmap : (a -> b) -> f a -> f b

class Functor f where  
    fmap :: (a -> b) -> f a -> f b

list as a functor.

It maps a concrete type a to a concrete type [a], and it defines fmap function.

instance Functor [] where
    fmap = map

Maybe as a functor.

instance Functor Maybe where  
    fmap foo (Just x) = Just (foo x)  
    fmap foo Nothing = Nothing

Here, fmap: (a -> b) -> Maybe a -> Maybe b

Applicative Functor?

let a = fmap (*) [1,2,3,4]   
-- a = [(1*),(2*),(3*),(4*)]
-- * is `int -> int -> int`.  or `int -> (int -> int)`

fmap (\f -> f 9) a  
-- results in [9, 18, 27, 36]