Haskell 中的软件事务内存：无法将预期的类型 STM a0 与实际类型 IO () 匹配

Question

我有一个小程序来定义帐户，提取功能并尝试从该帐户中提取。但是，它不会编译到应付并丢弃以下错误：

Couldn't match expected type ‘(STM a0 -> IO a0)
                                    -> STM () -> IO ()’
                  with actual type ‘IO ()’

似乎编译器无法识别从STM到IO的转换。任何指针都会很棒。

import System.IO
import Control.Concurrent.STM
type Account = TVar Int
withdraw :: Account -> Int -> STM ()
withdraw acc amount = do
    bal <- readTVar acc    
    writeTVar acc (bal - amount)
good :: Account -> IO ()
good acc = do
    hPutStr stdout "Withdrawing..."
    {-hi-}atomically{-/hi-} (withdraw acc 10)
main = do
    acc <- atomically (newTVar 200)
    good acc 
    hPutStr stdout "nDone!n"

Answer 1

{-hi-}和 {-/hi-}注释结果"缩进" automically，因此，您写了 hPutStr stdout "Withdrawing..." atomically (withdraw acc 10)。例如，如果您写：

good :: Account -> IO ()
good acc = do
        hPutStr stdout "Withdrawing..."
 {-hi-} atomically (withdraw acc 10)

它可以正常工作，因为"噪声"({-hi-}注释(不会导致atomically函数。

该评论确实没有语义上没有效果，但是您可以认为它被空间取代。