On 11-09-12 08:53, oleg@okmij.org wrote:
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 () Indeed the overloaded function cast needs to know the target type --
Corentin Dupon wrote about essentially the read-show problem: the type to cast to. In case of Player, the pattern (Player a) uniquely determines the type of the desired value: Player. This is not so for Message: the pattern (Message s) may correspond to the type Message (), Message Int, etc.
To avoid the problem, just specify the desired type explicitly I had the same idea, but it doesn't work. Fixing m to () causes the cast to fail for any other type, so viewEvent (Message "yes" :: Message ()) will work, but viewEvent (Message "no" :: Message Char) won't.
Putting viewEvent in the Event class though, like Ryan suggested, seems to be an elegant solution that stays close to the original source. Cheers, Martijn
case cast event of Just (Message s::Message ()) -> putStrLn $ show s Nothing -> return () (ScopedTypeVariables extension is needed). The exact type of the message doesn't matter, so I chose Message ().
BTW, if the set of events is closed, GADTs seem a better fit
data Player data Message s
data Event e where Player :: Int -> Event Player Message :: String -> Event (Message s)
viewEvent :: Event e -> IO () viewEvent (Player a) = putStrLn $ show a viewEvent (Message s) = putStrLn $ show s
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