Haskell STM : 在子线程完成执行之前，主线程不会退出

Question

在下面的程序中，我希望主线程在其所有子线程执行完毕后才退出。请注意，我已经使用 bang 模式来评估 Fibonacci 调用，因此它 returns 是对主线程的评估 thunk。

{-# LANGUAGE BangPatterns #-}

module Main
where

import Control.Concurrent.STM 
import Control.Concurrent
import System.IO

nfib :: Int -> Int
nfib n | n <= 2 = 1
       | otherwise = (n1 + n2 )
                     where n1 = nfib (n-1)
                           n2 = nfib (n-2)

type TInt = TVar Int

updateNum :: TInt -> Int -> STM()
updateNum n v = do writeTVar n v

updateTransaction :: TInt -> Int -> IO ()
updateTransaction n v = do 
        atomically $ do
             updateNum n v

main :: IO ()
main = do 
    n <- newTVarIO 10

    forkIO $ do             
        let v = 30
        let !x = nfib v
        updateTransaction n x

    forkIO $ do             
        let v = 15
        let !x = nfib v
        updateTransaction n x

    forkIO $ do             
        let v = 25
        let !x = nfib v
        updateTransaction n x   

    nv <- readTVarIO n
    putStrLn ("Fib number of "  ++ " = " ++ (show nv))  

    nv <-  readTVarIO n
    putStrLn ("Fib number of "  ++ " = " ++ (show nv))

    nv <-  readTVarIO n
    putStrLn ("Fib number of "  ++ " = " ++ (show nv))

我已经厌倦了按照 [link] () 解决这个问题。我不知道方法是否正确，并且在尝试打印 TMVar 的值时也出现错误。这是代码：-（nfib 与上面相同）

type TMInt = TMVar Int

updateNum1 :: TMInt -> Int -> STM()
updateNum1 n v = do putTMVar n v

updateTransaction1 :: TMInt -> Int -> IO ()
updateTransaction1 n v = do 
        atomically $ do
            updateNum1 n v

main1 :: IO ()
main1 = do 
    n <- newTMVarIO 0
    forkIO $ do             
        let v = 30
        let !x = nfib v
        updateTransaction1 n x

    forkIO $ do             
        let v = 15
        let !x = nfib v
        updateTransaction1 n x

    forkIO $ do             
        let v = 25
        let !x = nfib v
        updateTransaction1 n x  

    -- t <- takeTMVar n
    -- putStrLn( "result: " ++ (show t))

** 错误如下：-

Couldn't match type `STM' with `IO'
Expected type: IO Int
     Actual type: STM Int
In the return type of a call of `takeTMVar'
In a stmt of a 'do' block: t <- takeTMVar n

请帮忙。谢谢。

Answer 1

main1 在 IO 但 takeTMVar returns 在 STM Int。您需要运行交易：

t <- atomically $ takeTMVar n
putStrLn( "result: " ++ (show t))

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