Corentin,

As far as I understand, it is similar in implementation to what I have in my simulation library Aivika [1]. Please correct me if I am wrong:

-- | Execute the specified computations in parallel within

-- the current computation and return their results. The cancellation

-- of any of the nested computations affects the current computation.

-- The exception raised in any of the nested computations is propagated

-- to the current computation as well.

processParallel :: [Process a] -> Process [a]

It looks like that my Process type is an equivalent of your type Event in some sense. Only here we would have to cancel (or, interrupt in your terms) all other Process computations right after we receive a final result.

So, if it is true then it is possible to write a new function based on the stated above so that it would be similar to the ShortcutEvents function:

shortcutProcesses: [Process a] -> ([Maybe a] -> Maybe b) -> Process b

Probably, I should add such a function to my library too.

Thanks,

David 

[1] http://hackage.haskell.org/package/aivika

08 сент. 2014 г., в 0:23, Corentin Dupont <corentin.dupont@gmail.com> написал(а):

Hi guys,

thanks for the nice answers!

I'll give you a little bit more context: I'm designing an event engine. I have instances for Applicative, Alternative, Monad, MonadPlus.

It's like that:

-- | Composable events
data Event a where
   SumEvent       :: Event a -> Event a -> Event a            -- The first event to fire will be returned.
   AppEvent       :: Event (a -> b) -> Event a -> Event b     -- Both events should fire, and then the result is returned.
   PureEvent      :: a -> Event a                             -- Create a fake event. The result is useable with no delay.
   EmptyEvent     :: Event a                                  -- An event that is never fired.
   BindEvent      :: Event a -> (a -> Event b) -> Event b     -- The first event should fire, then a second event is created using the result.
   BaseEvent      :: BaseEvent a -> Event a                   -- Embed a base event.
   ShortcutEvents :: [Event a] -> ([Maybe a] -> Maybe b) -> Event b -- The function is called each time an event fires, as soon as the result can be computed from the available data, it is returned, dismissing the events that haven't fired yet.


instance Functor Event where
   fmap f a = pure f <*> a

instance Applicative Event where
   pure = PureEvent
   (<*>) = AppEvent

instance Alternative Event where
   (<|>) = SumEvent
   empty = EmptyEvent

instance Monad Event where
   (>>=) = BindEvent
   return = PureEvent

instance MonadPlus Event where
   mplus = SumEvent
   mzero = EmptyEvent


The Applicative instance is good if you have two events and you want both of them to fire ("and"). The Alternative instance is good if you have two events and you need only one to fire ("or").
But what if you have several events, but you need only a part of them to fire in order to construct a final result? Say you have 10 events, but the 5 first to fire will give you enough data to construct a result.
You cannot do that with Applicative/Alternative because with Applicative, you need *all* events results, with Alternative you need *only one*.

That's why I added this primitive "ShortcutEvents" in my DSL, but I'm not convinced by it. So my questions are:

1. is ShortcutEvents expressible in term of Applicative/Alternative/Monad/MonadPlus?

2. if not is their a well known typeclass that covers this case?

3. if not is their a better way to write it? I especially don't like the list of Event, I'd prefer a more generic writing. What if I want a structure containing the events, instead of a list? What if I want event of various types (say a pair (Event a, Event b) for example)?

Note that I'm not working with streams of events (like in traditional FRP frameworks): just with single events (the "BaseEvents") that I want to combine with each other. Those "BaseEvents" will fire only once. The final result of the combination of events will trigger a callback.

Cheers,

Corentin