Type of union is insufficiently general about reactive-banana HOT 2 CLOSED

The very first version of reactive-banana provided two functions with different names

union :: Event a -> Event a -> Event a
merge :: Event a -> Event b -> Event (Either a b)

Later, I have dropped the merge function because it turned out to be way less useful than union. (That's also why the MonadPlus and Alternative classes don't provide something like merge).

The reason is that its usually more convenient to apply different pattern matches right away instead of building a direct sum first and then deconstructing it again. In other words, the result of merge is usually used with a pattern match. The prototypical example

either f g (e1 `merge` e2) = (f <$> e1) `union` (g <$> e2)

shows that union is actually more convenient for this application, since it dispenses with the intermediate Either type and easily generalizes to severeal unions.

In your example, we have the special situation that the return type is ignored, i.e.

eQuit = () <$ (eWindowClose `merge` eCtrlQ `merge` eSIGINT)

and the pattern match disappears entirely, that's why it looks a bit more convenient. However, using the function

Control.Monad.void :: Functor f => f a -> f ()

you can drop the parentheses and make the whole thing a bit nicer

eQuit = void eWindowClose `union` void eCtrlQ `union` void eSIGINT

I think that this is a good compromise.

Summary

• I'm against changing the type of union.
• I'm don't like introducing the merge function again, because I feel that it clutters the interface.
• I think that the void function is enough to make your example reasonably nice.

Let me know what you think.

PS: I've changed your post to read newUnion :: FRP f => Event f a -> Event f b -> Event f (Either a b) because that's what you probably meant.

from reactive-banana.

Oh, silly me; I had assumed that Control.Monad.void would only work on monads, due to the module name. Yes, I agree with your conclusion; I think this thought was a bit premature.

from reactive-banana.