Type-Naturals and GADTs

{-# LANGUAGE TypeNaturals, GADTs #-}

import GHC.TypeNats

data Vec :: Nat -> * -> * where
  Nil  :: Vec 0 a
  Cons :: a -> Vec n a -> Vec (n + 1) a

cat :: Vec m a -> Vec n a -> Vec (m + n) a 
cat Nil ys          = ys
cat (Cons x xs) ys  = Cons x (cat xs ys)

data UNat :: Nat -> * where
  Zero :: UNat 0
  Succ :: UNat n -> UNat (n + 1)

split :: UNat m -> Vec (m + n) a -> (Vec m a, Vec n a)
split Zero xs = (Nil, xs)
split (Succ n) (Cons x xs)  = case split n xs of
                                (as,bs) -> (Cons x as, bs)

instance Show a => Show (Vec n a) where
  show Nil = "[]"
  show (Cons x xs) = show x ++ " : " ++ show xs