{-# LANGUAGE TupleSections #-}

-- Follow the type and the properties 

f :: Either a a -> (Bool, a)
f = either (False,) (True,)

g :: (Bool, a) -> Either a a
g (False, a) = Left a
g (True, a) = Right a

g' :: (Bool, a) -> Either a a
g' = uncurry $ cata Right Left
    where cata :: a -> a -> Bool -> a
          cata a _ True = a
          cata _ a False = a

-- |
-- prop> (f . g) x == x
-- prop> (g . f) x == x