data Prod a b = Par a b
  deriving Show

pi_a :: Prod a b -> a
pi_a (Par x y) = x

pi_b :: Prod a b -> b
pi_b (Par x y) = y

instance (Eq a, Eq b) => Eq (Prod a b)
  where
    (Par x y) == (Par z w) = (x == z) && (y == w)

data Prod' a b = Par' { pi'_a :: a, pi'_b :: b }
  deriving (Show, Eq)

x' = Par' {pi'_a = 1, pi'_b = 2}