Hello. I'm trying to get some threads that I can stop and get last values that was computed (and that values are IO values, in fact). Here is my first approach: module Main where import Control.Concurrent (MVar, threadDelay, forkIO, newMVar, putMVar, readMVar) tick :: Int -> IO Int tick v = return $ v + 1 loop :: MVar Bool -> a -> (a -> IO a) -> IO a loop var init loopfun = do next <- loopfun init shouldStop <- readMVar var case shouldStop of True -> return next False -> loop var next loopfun runLoop :: Int -> IO () runLoop timeout = do var <- newMVar False forkIO $ threadDelay timeout >> putMVar var True value <- loop var 0 tick print value main :: IO () main = runLoop 30000000 The problem is that it looks a little messy and what's worse it leaks memory. So I'm wondering if there is a better approach to do so or some fix to memory leak.