Can I ensure that Haskell performs atomic IO?

Question

I have two threads in haskell that perform IO. (They print only). Something like the following:

thread1 :: IO () thread1 = putStrLn "One"  thread2 :: IO () thread2 = putStrLn "Two" 

I am currently getting results like the following:

OnTwoe OTnweo 

How can I ensure that each thread completes its IO atomically?

Answer 1

Use a synchronization variable to ensure atomic access to the resource. A simple way is with an MVar:

main = do    lock <- newMVar ()    forkIO $ ... lock     forkIO $ ... lock 

Now, to do IO without interleaving, each thread takes the lock:

thread1 lock = do       withMVar lock $ \_ -> putStrLn "foo"  thread2 lock = do       withMVar lock $ \_ -> putStrLn "bar" 

An alternate design is to have a dedicated worker thread that does all the putStrLns, and you send messages to print out over a Chan.