Hi,
I designed my event engine like this:

-- | events types
data Player = Arrive | Leave deriving (Typeable, Show, Eq)
data RuleEvent = Proposed | Activated | Rejected | Added | Modified | Deleted deriving (Typeable, Show, Eq)
data Time           deriving Typeable
data InputChoice c deriving Typeable
(...)

-- | events names
data Event a where
    Player      :: Player ->     Event Player
    RuleEv      :: RuleEvent -> Event RuleEvent
    Time        :: UTCTime ->    Event Time
    InputChoice :: (Eq c, Show c) => PlayerNumber -> String -> [c] -> c -> Event (InputChoice c)
(...)

-- | data associated with each events
data EventData a where
    PlayerData      :: {playerData :: PlayerInfo}    -> EventData Player
    RuleData        :: {ruleData :: Rule}            -> EventData RuleEvent
    TimeData        :: {timeData :: UTCTime}         -> EventData Time
    InputChoiceData :: (Show c, Read c, Typeable c) => {inputChoiceData :: c}        -> EventData (InputChoice c)
(...)

-- associate an event with an handler
data EventHandler where
    EH :: (Typeable e, Show e, Eq e) =>
        {eventNumber :: EventNumber,
         event       :: Event e,
         handler     :: (EventNumber, EventData e) -> Exp ()} -> EventHandler

--execute all the handlers of the specified event with the given data
triggerEvent :: (Typeable a, Show a, Eq a) => Event a -> EventData a -> [EventHandler] -> State Game ()


I use a type parameter "e" on Event and EventData to be sure that the right data is shuffled to the right event handler.
It worked well until now. But now I'm hitting a wall with the GUI, because the data sent back for InputChoice can only be a String.
So, I need to call triggerEvent with: Event(InputChoice String) and EventData (InputChoice String)...
Which doesn't work obviously because the types are not the same than initially (for example, the event was built with Event(InputChoice Bool)).

Cheers,
C

On Wed, Oct 24, 2012 at 7:25 PM, Stephen Tetley <stephen.tetley@gmail.com> wrote:

Hi Corentin

It looks like you are writing the event handler on the server side. If
so, the range of events you can handle is fixed to just those you
implement handlers for - having an openly extensible event type is
useless if this is the case.

Ignoring client/server for a moment, a function (State -> State) would
be the most "extensible" API you could allow for clients. You don't
need to worry about an open set of Event types, a client knows the
state exactly and doesn't need extensibility.

Client/Server operation won't allow a state transformer API as Haskell
can't readily serialize functions. But you can implement a "command
language" enumerating state changing operations. The second Quickcheck
paper gives a very good example of how to implement such a command
language.

Testing Monadic Code with QuickCheck (2002)
Koen Claessen , John Hughes
www.cse.chalmers.se/~rjmh/Papers/QuickCheckST.ps

Or Citeseer if you need a PDF:
http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.19.9275

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