The (->)a functor

Here's fmap's type signature on the functor (->)a:

fmap :: (b -> c) -> ((->)a b) -> ((->)a c)

Or with a more readable syntax:

fmap :: (b -> c) -> (a -> b) -> (a -> c)

Applied with b and c equal to Int and a equal to [Int]:

λ> (fmap (-1 +) length) [0,1,2,3,4]
4

So fmap f sends g to f . g.