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copied!<p>Implementing your desired <code>timed</code> on top of <code>System.Timeout.timeout</code> is easy:</p> <pre><code>import System.Timeout (timeout) timed :: Int -> IO a -> b -> IO (Either b a) timed us act def = liftM (maybe (Left def) Right) (timeout us act) </code></pre> <p>By the way, the common implementation of <code>timeout</code> is closer to this: (<code>$!</code> = <code>seq</code> to try to force evaluation of the returned value in the thread rather than only returning a thunk):</p> <pre><code>import Control.Concurrent (forkIO, threadDelay, killThread) import Control.Concurrent.MVar (newEmptyMVar, putMVar, takeMVar) import System.IO (hPrint, stderr) timeout :: Int -> IO a -> IO (Maybe a) timeout us act = do mvar <- newEmptyMVar tid1 <- forkIO $ (putMVar mvar . Just $!) =<< act tid2 <- forkIO $ threadDelay us >> putMVar mvar Nothing res <- takeMVar mvar killThread (maybe tid1 (const tid2) res) `catch` hPrint stderr return res </code></pre> <p>The implementation of <code>System.Timeout.timeout</code> in the libraries is a little more complex, handling more exceptional cases.</p> <pre><code>import Control.Concurrent (forkIO, threadDelay, myThreadId, killThread) import Control.Exception (Exception, handleJust, throwTo, bracket) import Data.Typeable import Data.Unique (Unique, newUnique) data Timeout = Timeout Unique deriving Eq timeoutTc :: TyCon timeoutTc = mkTyCon "Timeout" instance Typeable Timeout where { typeOf _ = mkTyConApp timeoutTc [] } instance Show Timeout where show _ = "<<timeout>>" instance Exception Timeout timeout n f | n < 0 = fmap Just f | n == 0 = return Nothing | otherwise = do pid <- myThreadId ex <- fmap Timeout newUnique handleJust (\e -> if e == ex then Just () else Nothing) (\_ -> return Nothing) (bracket (forkIO (threadDelay n >> throwTo pid ex)) (killThread) (\_ -> fmap Just f)) </code></pre>

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