Haskell の STM に慣れるために、食事の哲学者の問題に対する次の解決策を書きました。
import Control.Concurrent
import Control.Concurrent.STM
import Control.Monad
import System.Random
type Fork = TVar Bool
type StringBuffer = TChan String
philosopherNames :: [String]
philosopherNames = map show ([1..] :: [Int])
logThinking :: String -> StringBuffer -> STM ()
logThinking name buffer = writeTChan buffer $ name ++ " is thinking..."
logEating :: String -> StringBuffer -> STM ()
logEating name buffer = writeTChan buffer $ name ++ " is eating..."
firstLogEntry :: StringBuffer -> STM String
firstLogEntry buffer = do empty <- isEmptyTChan buffer
if empty then retry
else readTChan buffer
takeForks :: Fork -> Fork -> STM ()
takeForks left right = do leftUsed <- readTVar left
rightUsed <- readTVar right
if leftUsed || rightUsed
then retry
else do writeTVar left True
writeTVar right True
putForks :: Fork -> Fork -> STM ()
putForks left right = do writeTVar left False
writeTVar right False
philosopher :: String -> StringBuffer -> Fork -> Fork -> IO ()
philosopher name out left right = do atomically $ logThinking name out
randomDelay
atomically $ takeForks left right
atomically $ logEating name out
randomDelay
atomically $ putForks left right
randomDelay :: IO ()
randomDelay = do delay <- getStdRandom(randomR (1,3))
threadDelay (delay * 1000000)
main :: IO ()
main = do let n = 8
forks <- replicateM n $ newTVarIO False
buffer <- newTChanIO
forM_ [0 .. n - 1] $ \i ->
do let left = forks !! i
right = forks !! ((i + 1) `mod` n)
name = philosopherNames !! i
forkIO $ forever $ philosopher name buffer left right
forever $ do str <- atomically $ firstLogEntry buffer
putStrLn str
ソリューションをコンパイルして実行すると、明らかな並行性の問題はないように見えます。各哲学者は最終的に食べてしまい、どの哲学者も支持されないようです。ただし、randomDelayステートメントを から削除しphilosopher、コンパイルして実行すると、プログラムの出力は次のようになります。
1 is thinking...
1 is eating...
1 is thinking...
1 is eating...
2 is thinking...
2 is eating...
2 is thinking...
2 is eating...
2 is thinking...
2 is eating...
2 is thinking...
About 2500 lines later...
2 is thinking...
2 is eating...
2 is thinking...
3 is thinking...
3 is eating...
3 is thinking...
3 is eating...
And so on...
この場合、何が起こっていますか？