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