class Message a

data UserMessage = UserMessage deriving(Show)
data SystemMessage = SystemMessage deriving (Show)
data Something = Something deriving (Show)

instance Message UserMessage
instance Message SystemMessage

data Msg = forall t. (Show t, Message t) => M t
instance Show Msg where
  show (M t) = show t

messages :: [Msg]
messages = [
    M $ UserMessage
  , M $ SystemMessage
  --, M $ Something
  ]

main = print $ messages