Hi Cafe!! For my game Nomyx, I am using events that the player can program. For example, the player can register a callback that will be triggered when a new player arrives. He can also program some forms (with buttons, checkboxes, textboxes...) to appear on the Web GUI. The problem is those events are not composable: he has to create and handle them one by one. So I'm thinking of making those events composable by making them an instance of Applicative and Alternative. For Applicative, this makes events composable very much like in Applicative-Functors and Reform. I can build neat composed events such as (full program below): onInputMyRecord :: Event MyRecord onInputMyRecord = MyRecord <$> onInputText <*> onInputCheckbox For Alternative, I haven't seen any example of it on the net. The idea is that the first event that fires is used to build the alternative: onInputMyAlternative :: Event Bool onInputMyAlternative = (const True <$> onInputButton) <|> (const False <$> onInputButton) Here is an example program: {-# LANGUAGE GADTs #-} module ComposableEvents where import Control.Applicative import Data.Time import Data.Traversable type PlayerNumber = Int data Event a where OnInputText :: PlayerNumber -> Event String -- A textbox will be created for the player. When filled, this event will fire and return the result OnInputCheckbox :: PlayerNumber -> Event Bool -- Idem with a checkbox OnInputButton :: PlayerNumber -> Event () -- Idem with a button OnTime :: UTCTime -> Event () -- time event EventSum :: Event a -> Event a -> Event a -- The first event to fire will be returned EventProduct :: Event (a -> b) -> Event a -> Event b -- both events should fire, and then the result is returned Fmap :: (a -> b) -> Event a -> Event b -- transforms the value returned by an event. Pure :: a -> Event a -- Create a fake event. The result is useable with no delay. Empty :: Event a -- An event that is never fired. instance Functor Event where fmap = Fmap instance Applicative Event where pure = Pure (<*>) = EventProduct instance Alternative Event where (<|>) = EventSum empty = Empty onInputText = OnInputText onInputCheckbox = OnInputCheckbox onInputButton = OnInputButton onTime = OnTime -- A product type data MyRecord = MyRecord String Bool -- A sum type data MyAlternative = A | B -- Using the Applicative instance, we can build a product type from two separate event results. -- The event callback should be called only when all two events have fired. onInputMyRecord :: Event MyRecord onInputMyRecord = MyRecord <$> onInputText 1 <*> onInputCheckbox 1 -- other possible implementation (given a monad instance) -- onInputMyRecord' = do -- s <- onInputText -- b <- onInputCheckbox -- return $ MyRecord s b -- Using the Alternative instance, we build a sum type. -- The event callback should be called when the first event have fired. onInputMyAlternative :: Event MyAlternative onInputMyAlternative = (const A <$> onInputButton 1) <|> (const B <$> onInputButton 1) allPlayers = [1 .. 10] -- Now complex events can be created, such as voting systems: voteEvent :: UTCTime -> Event ([Maybe Bool]) voteEvent time = sequenceA $ map (singleVote time) allPlayers singleVote :: UTCTime -> PlayerNumber -> Event (Maybe Bool) singleVote timeLimit pn = (Just <$> onInputCheckbox pn) <|> (const Nothing <$> onTime timeLimit) vote :: UTCTime -> Event Bool vote timeLimit = unanimity <$> (voteEvent timeLimit) unanimity :: [Maybe Bool] -> Bool unanimity = all (== Just True) --Evaluation --evalEvent :: Event a -> State Game a --evalEvent = undefined With this DSL, I can create complex events such as time limited votes very neatly... There is much left to do for a full implem: the way to register callbacks on complex events, the evaluator and the event manager. Have you heard about a similar implementation? It seems pretty useful. Maybe in FRP frameworks? Thanks a lot!! Corentin PS: I copied this example also in https://github.com/cdupont/Nomyx-design/blob/master/ComposableEvents.hs.