How to implement reaction-banana behaviors that recursively depend on themselves?

Question

I have a behavior whose value I want to change depending on the occurrence of the event and the current value of the behavior. In the example below, I have two counters that are updated depending on whether the Boolean behavior is True or False. Since this code crashes with an exception <<loop>> , but I'm not sure how to restructure it to work or how else to approach this problem.

 {-# LANGUAGE ScopedTypeVariables #-} import Reactive.Banana import Reactive.Banana.Frameworks import Control.Arrow import Control.Concurrent import Control.Monad import Control.Monad.Fix counter :: Bool -> Event t Int -> Behavior t Bool -> (Behavior t Int, Event t (Bool -> Bool)) counter b input active = (result, whenE ((b/=) <$> active) (fmap (const not) input)) where result = accumB 0 (fmap (+) evt') evt' = whenE ((b==) <$> active) input alternater :: Event t Int -> Behavior t Bool -> (Behavior t (Bool, (Int, Int)), Event t (Bool -> Bool)) alternater input active = ((,) <$> active <*> ((,) <$> fst t1 <*> fst t2), snd t1 `union` snd t2) where t1 = counter True input active t2 = counter False input active main :: IO () main = do (inputHandler, fireInput) <- newAddHandler let network :: forall t . Frameworks t => Moment t () network = do eInput <- fromAddHandler inputHandler let ui :: Behavior t (Bool, (Int, Int)) -> Moment t (Behavior t (Bool, (Int, Int))) ui b = do let (behavior, evt) = alternater eInput (fst <$> b) return $ stepper id (fmap (***id) evt) <*> behavior output <- changes =<< mfix ui reactimate $ putStrLn . show <$> output forkIO $ actuate =<< compile network forever $ getLine >>= fireInput . read 

Answer 1

The exception is correct; you define the behavior directly in terms of yourself.

 alternater .. active = (.. <$> active <*> .. , ..) ui b = do let (behavior, ..) = alternater .. (.. <$> b) return $ .. <*> behavior ... mfix ui 

This code means that the current value of the result result in ui will depend on it in a circular way.

Recursion always requires a little delay in order to be clearly defined. The most convenient way to do this is to use mutual recursion between the event and the behavior built with stepper or accumB . See also this answer .