[Haskell-cafe] type variable in class instance

[Sean Leather]

On Tue, Sep 11, 2012 at 3:39 PM, Corentin Dupontwrote:

> @Oleg: Yes the set of events is closed and I would be much happier with a
> GADT! But no matter how hard I tried I couldn't manage.
> Here is the full problem:
>
> *{-# LANGUAGE ExistentialQuantification, TypeFamilies, DeriveDataTypeable
> #-}
>
> import Data.Typeable
>
> -- | Define the events and their related data
> class (Eq e, Typeable e, Show e) => Event e where
>     data EventData e
>
> -- | Groups of events
> data PlayerEvent = Arrive | Leave deriving (Typeable, Show, Eq)
>
> -- | events types
> data Player          = Player PlayerEvent deriving (Typeable, Show, Eq)
> data Message m  = Message String     deriving (Typeable, Show, Eq)
>
> -- | event instances
> instance Event Player                                      where data
> EventData Player             = PlayerData {playerData :: Int}
> instance (Typeable m) => Event (Message m)   where data EventData (Message
> m)   = MessageData {messageData :: m}
>
> -- | structure to store an event
> data EventHandler = forall e . (Event e) =>
>      EH {eventNumber :: Int,
>          event :: e,
>          handler :: (EventData e) -> IO ()} deriving Typeable
>
> -- store a new event with its handler in the list
> addEvent :: (Event e) => e -> (EventData e -> IO ()) -> [EventHandler] ->
> [EventHandler]
> addEvent event handler ehs = undefined
>
> -- trigger all the corresponding events in the list, passing the **data
> to the handlers
> triggerEvent :: (Event e) => e -> (EventData e) -> [EventHandler] -> IO ()
> triggerEvent event edata ehs = undefined
>
> --Examples:
> msg :: Message Int
> msg = Message "give me a number"
> myList = addEvent msg (\(MessageData n) -> putStrLn $ "Your number is: "
> ++ show n) []
> trigger = triggerEvent msg (MessageData 1) **myList --Yelds "Your number
> is: 1"*
>
> Has you can see this allows me to associate an arbitrary data type to each
> event with the type family "EventData". Furthermore some events like
> "Message" let the user choose the data type using the type parameter. This
> way I have nice signatures for the functions "addEvent" and "triggerEvent".
> The right types for the handlers and the data passed is enforced at
> compilation time.
> But I couldn't find any way to convert this into a GATD and get rid of the
> "Event" class......
>

Would this work?

data Player = Arrive | Leave
data Message m = Message String

data Data a where
  PlayerData  :: Int -> Data Player
  MessageData :: m -> Data (Message m)

data Handler where
  Handler :: Int -> e -> (Data e -> IO ()) -> Handler

addEvent :: e -> (Data e -> IO ()) -> [Handler] -> [Handler]
addEvent = undefined

triggerEvent :: e -> Data e -> [Handler] -> IO ()
triggerEvent = undefined

Regards,
Sean
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