How do you remove repeating events in reactive-banana

Question

In reactive-banana I've got an event stream that produces a series of numbers, some of which repeat several times in a row (I'm not concerned with all duplicates, just sequential duplicates). How can I modify that event stream to contain only non-sequential duplicates?

I tried using changes to convert it to a behavior thinking the behavior would only "change" when the event was a new number but instead the behavior triggers a change event every time a new input event is received.

Answer 1

Note that the changes function should only be used for binding to GUI toolkits and thelike, it should not be used for regular programming with events and behaviors.

A function that supresses duplicate event occurrences can be expressed in terms of the mapAccum and filterJust combinators as follows

skipEqual :: Eq a => Event t a -> Event t a
skipEqual = filterJust . fst . mapAccum Nothing . fmap f
    where
    f y (Just x) = if x == y then (Nothing,Just x) else (Just y,Just y)
    f y Nothing  = (Just y, Just y)

test = interpretModel skipEqual $ map (:[]) [1 :: Int,1,2,3,3,2]

Running test gives

*Main> test
[[1],[],[2],[3],[],[2]]

as desired.

In other words, you can simply imagine Event as a list of occurrences and then apply your beloved "list" combinators to that.