Question

如果STM事务失败并重试，是否重新执行对writeTChan的调用，以便最终进行两次写操作，或者如果事务提交，STM是否只实际执行写操作？即，睡眠理发师问题的解决方案是否有效，或者如果enterShop中的交易第一次失败，客户可能会获得两次折扣吗？

import Control.Monad
import Control.Concurrent
import Control.Concurrent.STM
import System.Random
import Text.Printf

runBarber :: TChan Int -> TVar Int -> IO ()
runBarber haircutRequestChan seatsLeftVar = forever $ do
  customerId <- atomically $ readTChan haircutRequestChan
  atomically $ do
    seatsLeft <- readTVar seatsLeftVar
    writeTVar seatsLeftVar $ seatsLeft + 1
  putStrLn $ printf "%d started cutting" customerId
  delay <- randomRIO (1,700)
  threadDelay delay
  putStrLn $ printf "%d finished cutting" customerId

enterShop :: TChan Int -> TVar Int -> Int -> IO ()
enterShop haircutRequestChan seatsLeftVar customerId = do
  putStrLn $ printf "%d entering shop" customerId
  hasEmptySeat <- atomically $ do
    seatsLeft <- readTVar seatsLeftVar
    let hasEmptySeat = seatsLeft > 0
    when hasEmptySeat $ do
      writeTVar seatsLeftVar $ seatsLeft - 1
      writeTChan haircutRequestChan customerId
    return hasEmptySeat
  when (not hasEmptySeat) $ do
    putStrLn $ printf "%d turned away" customerId    

main = do
  seatsLeftVar <- newTVarIO 3
  haircutRequestChan <- newTChanIO
  forkIO $ runBarber haircutRequestChan seatsLeftVar

  forM_ [1..20] $ \customerId -> do
    delay <- randomRIO (1,3)
    threadDelay delay
    forkIO $ enterShop haircutRequestChan seatsLeftVar customerId

更新直到事实上上述hairRequestChan无论如何都不是交易的一部分之后我才注意到。我可以使用常规Chan并在writeChan if atomically enterShop块{{1}}块中{{1}}后执行{{1}}。但是，做出这样的改进会破坏提出问题的全部理由，所以我会把它留在这里。

Answer 1

提交事务时执行

TChan操作，就像其他STM操作一样，因此无论您的事务重试多少次，您总会得到一次写操作。否则他们会变得毫无用处。

为了说服自己，试试这个例子：

import Control.Concurrent
import Control.Concurrent.STM
import Control.Concurrent.STM.TChan

main = do
  ch <- atomically newTChan
  forkIO $ reader ch >>= putStrLn
  writer ch

reader = atomically . readTChan
writer ch = atomically $ writeTChan ch "hi!" >> retry

这将抛出一个异常，抱怨该事务被无限期阻止。如果writeTChan在事务提交之前导致写入，则程序将打印“hi！”在抛出异常之前。

TChan是否已将其集成到Haskell STM中？

1 个答案: