'coincidence' combinator

[mitchellwrosen]

Here's another combinator in reflex missing that's from reactive-banana:

coincidence :: Event (Event a) -> Event a

[ocharles]

Isn't this switchE, but without the monad enforcing that we don't leak?

[HeinrichApfelmus]

A semantic specification would be nice, so that we know what exactly, if anything, to implement. 😄

[mitchellwrosen]

Whoops :)

The returned event emits whenever the outer and inner events emit simultaneously.

[mitchellwrosen]

The semantics are

type Event a = [Maybe a] -- infinite

coincidence :: Event (Event a) -> Event a
coincidence (x0 : xs) =
  case x0 of
    Just (Just x : _) -> Just x : ys
    _ -> Nothing : ys
  where
    ys = coincidence (map (drop 1) xs)

[ocharles]

Presumably case x0 of Just (Nothing : _) -> Nothing : ys too?

[mitchellwrosen]

Yeah, whoops! Editing