I want to apply a function to every element in a list (map) but the elements may have different types but all implement the same function (here "putOut") like an interface. However I cannot create a list of this "interface" type (here "Outputable").

How do I map a list of different types implementing the same function?

main :: IO ()
main = do
 map putOut lst
 putStrLn "end"
 where
  lst :: [Outputable] -- ERROR: Class "Outputable" used as a type
  lst = [(Out1 1),(Out2 1 2)]

class Outputable a where
 putOut :: a -> IO ()

-- user defined:

data Out1 = Out1 Int deriving (Show)
data Out2 = Out2 Int deriving (Show)

instance Outputable Out1 where
 putOut out1 = putStrLn $ show out1

instance Outputable Out2 where
 putOut out2 = putStrLn $ show out2

I cannot define it this way:

data Out = Out1 Int | Out2 Int Int

putOut Out1 = ...
putOut Out2 = ...

because this is a library and users should be able to extend Out with their own types