Hello everybody!
I'm soliciting once again your help!
It's been several days I'm blocked by this problem:

{-# LANGUAGE DeriveDataTypeable #-}

import Data.Typeable

class (Typeable e) => Event e

data Player = Player Int deriving (Typeable)
data Message m = Message String deriving (Typeable)

instance Event Player
instance (Typeable m) => Event (Message m)

viewEvent :: (Event e) => e -> IO ()
viewEvent event = do
case cast event of
Just (Player a) -> putStrLn $ show a
Nothing -> return ()
case cast event of
Just (Message s) -> putStrLn $ show s
Nothing -> return ()

gives me a:
    Ambiguous type variable `t0' in the constraint:
      (Typeable t0) arising from a use of `cast'
    Probable fix: add a type signature that fixes these type variable(s)
    In the expression: cast event
    In the expression:
      case cast event of {
        Just (Message s) -> putStrLn $ show s
        Nothing -> return () }

This is because Message has a type variable, while Player has not...
How to get this to work? I tried everything, existential types, scoped type variables etc. without success...

Thanks!!
Corentin