data Foo a
         b
  = Foo a b

data a :-> b = Arrow (a -> b)

data (f :* g) a = f a :* g a

data (f
      :+
      g)
  a = L (f a)
    | R (g a)

data a `Arrow` b = Arrow' (a -> b)

data (f `Product` g) a = f a `Product` g a

data (f
      `Sum`
      g)
  a = L' (f a)
    | R' (g a)