data MyNum = MyInteger Integer
           | MyDouble Double
           | MyRational Rational


instance Eq MyNum where
  MyInteger _  == MyInteger _  = True
  MyInteger _  == _            = False
  MyDouble _   == MyDouble _   = True
  MyDouble _   == _            = False
  MyRational _ == MyRational _ = True
  MyRational _ == _            = False

-- any (\x -> x == MyDouble 42) [MyInteger 1, MyInteger 2, MyDouble 3] == True