haskellfrpreactive-banana

How to look back to the previous moment

I am reading a button's state (whether being pressed or not) every moment:

readButton :: IO Boolean
readButton = ...

main = do
    (add, fire) <- newAddHandler
    network <- compile (desc add)
    actuate network
    forever $ do
        buttonState <- readButton
        fire buttonState

desc addButtonEvent = do
    eButtonState <- fromAddHandler addButtonEvent
    ...

All the read states are stored into eButtonState in the network description desc.

The button is considered to be newly pressed when the current moment's state is 1 with the previous moment's being 0. So, if the event sequence was a list, the function would be written like this:

f :: [Bool] -> Bool
f (True:False:_) = True
f _              = False

I want to apply this function to eButtonState so I would know whether the button is newly pressed or not in the moment.

Is it ever possible? How would you do it? I would appreciate if there is a better or more common idea or method to achieve this goal.

Solution

  • Here is one way (this is a runnable demo):

    import Reactive.Banana
import Reactive.Banana.Frameworks
import Control.Monad
import Control.Applicative -- Needed if you aren't on GHC 7.10.

desc addDriver = do
    -- Refreshes the button state. Presumably fired by external IO.
    eButtonDriver <- fromAddHandler addDriver
    let -- Canonical repersentation of the button state.
        bButtonState = stepper False eButtonDriver
        -- Observes the button just before changing its state.
        ePreviousState = bButtonState <@ eButtonDriver
        -- Performs the test your f function would do.
        newlyPressed :: Bool -> Bool -> Bool
        newlyPressed previous current = not previous && current
        -- Applies the test. This works because eButtonDriver and
        -- ePreviousState are fired simultaneously.
        eNewlyPressed = unionWith newlyPressed
            ePreviousState eButtonDriver
        -- The same but more compactly, without needing ePreviousState.
        {-
        eNewlyPressed = newlyPressed <$> bButtonState <@> eButtonDriver
        -}
    reactimate (print <$> eNewlyPressed)

main = do
    (addDriver, fireDriver) <- newAddHandler
    network <- compile (desc addDriver)
    actuate network
    -- Demo: enter y to turn the button on, and any other string to
    -- turn it off.
    forever $ do
        buttonState <- (== "y") <$> getLine
        fireDriver buttonState

    Notes: