Can I ensure that Haskell performs atomic IO?

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.

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