Re: [Haskell-cafe] event handler

15 Jun 2012

      I made some modifications based on your suggestions (see below).
I made a two parameters class:
*class (Typeable e, Typeable d) => Handled e d *
Because after all what I want is to associate an event with its type
parameters.
I don't know why I cannot implement you suggestion to restrict the
instances of Event:
*data **(Handled e d) => **Event e = Event deriving (Typeable, Eq)
*gives me a
*Not in scope: type variable `d'*

But apart from that it works very well! It's quite a nice interface!
Also just to know, is there a way of getting ride of all these "Typeable"?

{-# LANGUAGE ExistentialQuantification, DeriveDataTypeable,
MultiParamTypeClasses #-}

*module Events (addEvent, newPlayer, newRule) where

import Control.Monad
import Data.List
import Data.Typeable

newtype Player = P Int deriving Typeable
newtype Rule = R Int deriving Typeable
data Event e = Event deriving (Typeable, Eq)

data NewPlayer deriving Typeable
data NewRule deriving Typeable

newPlayer :: Event NewPlayer
newPlayer = Event
newRule :: Event NewRule
newRule = Event

class (Typeable e, Typeable d) => Handled e d
instance Handled NewPlayer Player
instance Handled NewRule Rule

data EventHandler = forall e d . (Handled e d) => EH (Event e) (d -> IO ())

addEvent :: (Handled e d) => Event e -> (d -> IO ()) -> [EventHandler] ->
[EventHandler]
addEvent e h ehs = (EH e h):ehs

triggerEvent :: (Handled e d) => Event e -> d -> [EventHandler] -> IO ()
triggerEvent e d ehs = do
    let r = find (\(EH myEvent _) -> cast e == Just myEvent) ehs
    case r of
       Nothing -> return ()
       Just (EH _ h) -> case cast h of
        Just castedH -> castedH d
        Nothing -> return ()

-- TESTS
h1 :: Player -> IO ()
h1 (P a) = putStrLn $ "Welcome Player " ++ (show a) ++ "!"
h2 :: Rule -> IO ()
h2 (R a) = putStrLn $ "New Rule " ++ (show a)
eventList1 = addEvent newPlayer h1 []
eventList2 = addEvent newRule h2 eventList1

trigger1 = triggerEvent newPlayer (P 1) eventList2 --Yelds "Welcome Player
1!"
trigger2 = triggerEvent newRule (R 2) eventList2 --Yelds "New Rule 2" *

On Fri, Jun 15, 2012 at 4:53 PM, Alexander Solla wrote:
...
On Fri, Jun 15, 2012 at 6:38 AM, Corentin Dupont <
corentin.dupont@gmail.com> wrote:
...
It just bothers me a little that I'm not able to enumerate the events,
and also that the user is able to create events with wrong types (like New
:: Event String), even if they won't be able to register them.
This can be solved with an explicit export list/smart constructors.
newPlayer :: Event Player
newRule    :: Event Rule
(hide the New constructor)
In any case, my thinking was that your original
data Event = *NewPlayer | NewRule*
*
*
was basically trying to "join" the semantics of "new things" with Player
and Rule.  But the original approach ran into the problem you mention below
-- it is difficult to maintain invariants, since the types want to
"multiply".  So formally, I factored:
data Event = NewPlayer | NewRule ==>
data Event = New (Player | Rule)    ==>
data Event d = New -- (since the original event didn't want a Player or
Rule value.  It witnessed the type relation)
On the other hand, if you want to make sure that a type must be "Handled"
before you can issue an Event, you can do:
data (Handled d) => Evend d = New
I'm pretty sure the compiler will complain if you try to make a (New ::
Event String).  I like this idea better than smart constructors for events,
if only because you get to use ScopedTypeVariables.
...
I also have several unrelated events that use the same type of data, so
this would be a problem.
Can you clarify?
I mean that I have events like:
Message String
UserEvent String
That have a "data" of the same type, but they are not related.
...
...
Adding more events like
*data Event d = NewPlayer | NewRule deriving (Typeable, Eq)*
is not correct because I can add wrong events like:
addEvent (NewPlayer :: Event Rule) (H(undefined::(Rule -> IO()))) []
**
Also one question: I don't understand the "where" clause in your class.
If I remove it, it works the same...
Yes, unnecessary where clauses are optional.
Here is my code:
...
*newtype Player = P Int deriving Typeable
newtype Rule = R Int deriving Typeable
data Event d = New deriving (Typeable, Eq)
class (Typeable d) => Handled d where
data Handler d = H (d -> IO ())
data EventHandler = forall d . (Handled d) => EH (Event d) (Handler d)
instance Handled Player
instance Handled Rule
addEvent :: (Handled d) => Event d -> Handler d -> [EventHandler] ->
[EventHandler]
 addEvent e h ehs = (EH e h):ehs
triggerEvent :: (Handled d) => Event d -> d -> [EventHandler] -> IO ()
triggerEvent e d ehs = do
    let r = find (\(EH myEvent _) -> cast e == Just myEvent) ehs
case r of
       Nothing -> return ()
       Just (EH _ (H h)) -> case cast h of
        Just castedH -> castedH d
        Nothing -> return ()
h1 :: Player -> IO ()
h1 (P a) = putStrLn $ "Welcome Player " ++ (show a) ++ "!"
h2 :: Rule -> IO ()
h2 (R a) = putStrLn $ "New Rule " ++ (show a)
eventList1 = addEvent (New :: Event Player) (H h1) []
eventList2 = addEvent (New :: Event Rule) (H h2) eventList1
trigger1 = triggerEvent (New :: Event Player) (P 1) eventList2 -- yelds
"Welcome Player 1!"
trigger2 = triggerEvent (New :: Event Rule) (R 2) eventList2 --yelds "New
Rule* 2"
Best,
Corentin
On Fri, Jun 15, 2012 at 12:40 AM, Alexander Solla wrote:
...
On Thu, Jun 14, 2012 at 2:04 PM, Corentin Dupont <
corentin.dupont@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 ()*