How to fire an event on the condition that a couple of other events have fired at least once?

[rubenmoor]

Let's say my website is waiting for two web requests:

evRequest1Done :: Event t () and evRequest2Done :: Event t ()

Now I want an event

evAllRequestsDone :: Event t ()
evAllRequestsDone = waitForAll [evRequest1Done, evRequest2Done]

that fires once all events in the list have fired at least once.

What is the best way to go about this?

---

I can offer two solutions, both with drawbacks:

Solution 1

EDIT: record instead of sumtype with Semigroup instance

data Done = Done
  { done1 :: Bool
  , done2 :: Bool
  }

setDone1 :: Done -> Done
setDone1 done = done { done1 = True }

setDone2 :: Done -> Done
setDone2 done = done { done2 = True }

waitForAll event1 event2 =
    mapAccumMaybe_ accFunc (Done False False) $
       mergeWith (.) [event1 $> setDone1, event2 $> setDone2]
  where
    accFunc done func =
        let done'@(Done d1 d2) = func done
        in  (Just done', if d1 && d2 then Just () else Nothing)

Disadvantages:

• record Once required
• bloated with helper functions (can me mitigated with lenses)
• gets more bloated for more then two events

Solution 2

waitFor :: Int -> Event t () -> Event t ()
waitFor n event =
    mapAccumMaybe_ accFunc 0 event
  where
    accFunc x y = let z = x + y in (Just z, if z == n then Just () else Nothing)

-- use it like this
waitFor 2 $ mergeWith (+) $ fmap ($> 1) events

Disadvantages:

• Doesn't actually solve the stated problem, when one of the events in the list can fire more then once