reactive-banana Events, Behaviors and Zipping with +

ZipSum.hs

import Reactive.Banana
import Control.Concurrent

main = do
  -- Create handles for pushing values into events
  (addHandler1, push1) <- newAddHandler
  (addHandler2, push2) <- newAddHandler
  -- Create event network
  network <- compile $ do
    -- Actual events from AddHandlers
    e1 <- fromAddHandler addHandler1
    e2 <- fromAddHandler addHandler2

    -- Behaviors from Events
    let d1 = stepperD 0 e1 :: Discrete Int
    let d2 = stepperD 0 e2 :: Discrete Int

    -- Sum Behavior
    let sumD = (+) <$> d1 <*> d2
    -- Back to Event
    let sumE = changes sumD

    -- Assign side-effects
    reactimate $ fmap (prefixPrint "e1=") e1
    reactimate $ fmap (prefixPrint "e2=") e2
    reactimate $ fmap (prefixPrint "sum=") sumE

  -- Fire it up
  actuate network
  -- Push data
  push1 10
  push2 20

-- Helper for printing values
prefixPrint p thing = putStrLn $ p ++ (show thing)

This does not really work: the sumE event always gets an out-of-date value. Can you fix it?

Finally fixed it! Thanks to John L! http://stackoverflow.com/questions/8826970/whats-wrong-with-my-sum-event-in-reactive-banana