Tight (non-allocating) loop freezes the scheduler
Here is a program that spawns a thread from the main thread, which tries to constantly write out a message to the console.
module Main where import Control.Concurrent (forkIO) main :: IO () main = do _ <- forkIO $ runForever $ putStrLn "Hey" runForever $ return () runForever :: IO () -> IO () runForever action = action >> runForever action
If you compile it with 'ghc main', it works correctly - it prints out the message continuously, and you can terminate it by pressing Ctrl-C. However, if you compile it with 'ghc -O main' (or -O2, or -O3...), it doesn't print out anything, and the only way to exit is to kill the process from Task Manager.
This was reproducable with GHC 7.10.1, on a Windows 7 x64 machine, with an AMD A4-5300 APU.
''EDIT: As it turns out, using "yield" instead of "return ()" solves the problem. It seems I misunderstood how forkIO works. However, I'm not sure if the current working is intentional or not, so I think I should leave this ticket open - just to be on the safe side.''