data Zero
data Succ n

type One = Succ Zero
type Two = Succ One
type Four = Succ (Succ Two)
type Six = Succ (Succ Four)
type Eight = Succ (Succ Six)

class Nat n where
  toInt :: n -> Int

instance Nat Zero where
  toInt _ = 0

instance (Nat n) => Nat (Succ n) where
  toInt _ = 1 + toInt (undefined::n)