The correct way to implement timeout in GHC 5.00 is below. This should really be in a library somewhere. This implementation works for GHC's current "blocking" semantics for throwTo, but if we change the semantics of throwTo to match the asynchronous exceptions paper (http://www.haskell.org/~simonmar/papers/async.ps.gz) then a different implementation of timeout will be needed. Cheers, Simon ------------------------------------------------------------------------ ----- import Dynamic import Unique import Exception import Concurrent data TimeOut = TimeOut Unique timeOutTc = mkTyCon "TimeOut"; instance Typeable TimeOut where { typeOf _ = mkAppTy timeOutTc [] } timeout secs on_timeout action = do { parent <- myThreadId; i <- newUnique; block (do timeout <- forkIO (timeout_thread secs parent i); Exception.catchDyn ( unblock ( do { result <- action; killThread timeout; return result; } ) ) ( \exception -> case exception of TimeOut u | u == i -> unblock on_timeout other -> do { killThread timeout; throwDyn exception } ) ) } timeout_thread secs parent i = do { threadDelay (secs * 1000000); throwTo parent (DynException (toDyn (TimeOut i))) }