Multiple Simultaneous Events (Issue #25)

Discussion

Main discussion thread: https://github.com/HeinrichApfelmus/reactive-banana/issues/25

The main reason for disallowing multiple simultaneous events is the intricate interaction with accumE and apply. See this comment.

Conclusion

The compromise that is going to be implemented is

1. Add a function

    unionWith :: (a -> a -> a) -> Event a -> Event a -> Event a
    unionWith f e1 e2 = foldr1 f <$> collect (e1 `union` e2)
   

   Example

    unionWith (+) [ [1,2], [3] ] [ [4], [] ] = [ [7], [3] ]
   

2. Remove general Monoid instance for Event. Introduce Monoid instance

    instance Monoid (Event (a -> a)) where
        mempty  = never
        mappend = unionWith (flip (.)) 
   

3. Provide a different type (proposed name CalmEvent) in a specialized module that offers the guarantee that it contains only one event at each point in time. Not sure yet whether to implemented it on top of the current semantics or as alternate semantics.

   See also https://github.com/HeinrichApfelmus/reactive-banana/issues/26.