data Nil a = Nil

infixr 6 `Cons`
data Cons fs a = Cons a (fs a)

data Unary fs a = Z (fs (fs a)) | S (Unary (Cons fs) a)

type Square = Unary Nil

zero :: Square a
zero = Z Nil

one :: Square Int
one = S . Z $ (1 `Cons` Nil) `Cons` Nil

two :: Square Int
two = S . S . Z $      (1 `Cons` 2 `Cons` Nil)
                `Cons` (3 `Cons` 4 `Cons` Nil)
                `Cons` Nil

size' :: Unary fs a -> Int
size' (Z _) = 0
size' (S s) = 1 + size' s

size :: Square a -> Int
size = size'