Create new behaviors and switch between them.

DynamicEventSwitching.hs

{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE ScopedTypeVariables #-}

import Data.Traversable (sequenceA)
import Reactive.Banana
import Reactive.Banana.Frameworks

newtype Wrapper = Wrapper (forall t. Moment t (Event t () -> Behavior t Integer))

main :: IO ()
main = do
    (a, h) <- newAddHandler
    let networkDescription :: forall t. Frameworks t => Moment t ()
        networkDescription = do
            e     <- fromAddHandler a
            eTick <- trimE $ () <$ e

            let
                e1 :: Event t (AnyMoment Behavior Integer)
                e1 = (\(Wrapper m) -> anyMoment $ m <*> now eTick) <$> e
                
                -- Trim the behaviors so that they can accumulate state.
                --
                -- The following is indented to simply read
                --
                --     observe (trimB <$> e1)
                --
                -- but the lack of support for imperative polymorphism
                -- forces us to be a little more verbose.
                e1b = observeE $ (\mb -> anyMoment $ do
                        b <- now mb
                        Identity <$> trimB b) <$> e1
                
                e2 :: Event t [AnyMoment Behavior Integer]
                e2 = accumE [] $ (:) <$> e1b
                
                e3 :: Event t (AnyMoment Behavior [Integer])
                e3 = sequenceA <$> e2
                
                b :: Behavior t [Integer]
                b = pure [] `switchB` e3

            reactimate $ print <$> (b <@ e)

    network <- compile networkDescription
    actuate network
    sequence_ . replicate 4 $ h $ Wrapper $ return $ \e -> accumB 0 $ (+ 1) <$ e