{- Hi, Possible Concurrent Haskell bug Here is an attempt at concurrent, data-driven assertions for Haskell. The code has been simplified to just work for type Char: -} import Concurrent import IOExts(unsafePerformIO) assert :: (Char -> Bool) -> Char -> Char assert p x = unsafePerformIO $ do mv <- newEmptyMVar forkIO $ check p (listen mv) return (demand mv x) check :: (a -> Bool) -> a -> IO () check p x | p x = return () | otherwise = putStrLn "Assertion failed!" demand :: MVar Char -> Char -> Char demand mv c = unsafePerformIO $ do putMVar mv c; yield; return c listen :: MVar Char -> Char listen mv = unsafePerformIO $ takeMVar mv main :: IO () main = putChar (assert (isLower) 'z') {- When main is evaluated, it causes evaluation of assert, which returns the data it was called on, but also forks a "check" IO action to check predicate p on the data. This check is synchronised using "listen" which waits on an MVar into which the data will be placed when available. The data is placed here by evaluation of "demand" (which is returned by assert). Demand yields after putting the data, to allow the check to be carried out before computation is continued. In this example, the data is immediately available so the result should simply be 'z'. However, the actual output is: Thread raised exception: no more threads (deadlock?) which must then be interrupted with Ctrl-C. If listen is replaced with the following version, the printout "takeMVar: 'z'" occurs after the message about the exception, but execution still halts. listen mv = unsafePerformIO $ do val <- takeMVar mv putStrLn ("takeMVar: " ++ show val) return val I've tried the code in GHC and it works fine. Any ideas? Cheers, Dan -}