An implementation of length function in terms of Const functor

Length.hs

import Prelude hiding (length)

newtype Const a b = Const a

unConst :: Const c a -> c
unConst (Const x) = x

length' :: [a] -> Const Int a
length' [] = Const 0
length' (x:xs) = Const (1 + unConst (length' xs))

length :: [a] -> Int
length xs = unConst (length' xs)