Nat1.hs

{-# LANGUAGE ScopedTypeVariables #-}

data Zero
data Suc n
  
type One   = Suc Zero
type Two   = Suc One
type Three = Suc Two

class Nat n where
  fromNat :: Num a => n -> a

instance Nat Zero where
  fromNat _ = 0

instance Nat n => Nat (Suc n) where
  fromNat _ = fromNat (undefined :: n) + 1