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.