[Haskell-cafe] type variable in class instance

10 Sep 2012

      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

[Haskell-cafe] type variable in class instance

Corentin Dupont