Foo.hs

data EitherDup a = MyLeft a
                 | MyRight a

class HasEndo t where
  -- | prop>  endo . endo == id
  endo :: t -> t
  
instance HasEndo (EitherDup a) where
  endo (MyLeft a) = MyRight a
  endo (MyRight a) = MyLeft a

data Exp = Lit Int
         | Exp :+: Exp
         | Exp :*: Exp
     deriving HasEndo Exp as pattern a :+: a = MyLeft (a,a)
                             pattern a :*: a = MyRight (a,a)