data MyNum = MyInteger Integer
           | MyDouble Double
           | MyRational Rational


data MyNumType = MyIntegerType | MyDoubleType | MyRationalType deriving (Eq)

typeOf :: MyNum -> MyNumType
typeOf (MyInteger _)  = MyIntegerType
typeOf (MyDouble _)   = MyDoubleType
typeOf (MyRational _) = MyRationalType


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