[Haskell-cafe] event handler

[Alexander Solla]

On Thu, Jun 14, 2012 at 2:04 PM, Corentin Dupont
<corentin.dupont at gmail.com>wrote:

> That look really nice!
> Unfortunately I need to have an heterogeneous list of all events with
> their handlers.
> With this test code it won't compile:
>
> test1 = addEvent (New :: Event Player) (H (undefined::(Player -> IO ())))
> []
> test2 = addEvent (New :: Event Rule) (H (undefined::(Rule -> IO ()))) test1
>
>
Right, okay.  Heterogenous lists are tricky, but I think we can get away
with using ExistentialQuantification, since you seem to only want to
dispatch over the heterogenous types.  The assumption I made is a big deal!
 It means you can't extract the d value.  You can only apply properly typed
functions (your handlers) on it.


*
{-# LANGUAGE ExistentialQuantification #-}
*

*
type Player = Int
*

*
type Rule = Int
*

*
data Event d = New d
*
*
*

*class Handled data where -- Together with EventHandler, corresponds to
your "Data" type*
*
*

*data EventHandler = forall d . (Handled d) => EH (Event d) (d -> IO ()) --
EventHandler takes the place of your (Event d, Handler d) pairs without
referring to d.*
*
*

*instance Handled Player*

*instance Handled Rule*


*addEvent :: (Handled d) => Event d -> Handler d -> [EventHandler] ->
[EventHandler] -- Every [EventHandler] made using addEvent will be of
"correct" types (i.e., preserve the typing invariants you want), but YOU
must ensure that only [EventHandler]s made in this way are used.  This can
be done statically with another type and an explicit export list.  We can
talk about that later, if this works in principle.*




*triggerEvent :: (Handled d) => Event d -> d -> [EventHandler] -> IO ()*
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