haskell - 如何回顾前一刻

Question

我每时每刻都在读取按钮的状态(无论是否按下):

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
    ...

所有读取状态都存储在网络描述desc中的eButtonState中。

当当前时刻的状态为1，而前一时刻的状态为0时，按钮被认为是新按下的。所以，如果事件序列是一个列表，函数应该这样写:

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

我想把这个函数应用到eButtonState 上，这样我就可以知道按钮是否被按下了。

有可能吗？你会怎么做？如果有更好或更通用的想法或方法来实现这一目标，我将不胜感激。

Answer 1

这是一种方式(这是一个可运行的演示):

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

注意事项:

• 事件是短暂的，行为是永久的 是决定您是否需要行为或事件流的通用规则。在这种情况下，您需要查看按钮在更新之前的状态，以确定它是否是新更新的。那么，自然要做的事情就是用行为 (bButtonState) 表示按钮状态，该行为由外部触发的事件 (eButtonDriver) 更新。
• 有关组合器正在做什么的详细信息，请参阅 Reactive.Banana.Combinators .
• 有关 reactive-banana 中事件和行为更新时间的细则，请参阅 this question .
• 根据您要执行的操作，changes功能可能会有用。请注意文档中提到的与之相关的注意事项。