unbounded-delays- Unbounded thread delays and timeouts

Copyright2011 Bas van Dijk & Roel van Dijk
LicenseBSD3 (see the file LICENSE)
MaintainerBas van Dijk <v.dijk.bas@gmail.com> , Roel van Dijk <vandijk.roel@gmail.com>
Safe HaskellSafe



Wait arbitrarily long for an IO computation to finish.



timeout :: Integer -> IO α -> IO (Maybe α) #

Like timeoutWithPred, but does not expose the Timeout exception to the called action.

timeoutWithPred :: Integer -> ((Timeout -> Bool) -> IO α) -> IO (Maybe α) #

Like System.Timeout.timeout, but not bounded by an Int. (..) Wrap an IO computation to time out and return Nothing in case no result is available within n microseconds (1/10^6 seconds). In case a result is available before the timeout expires, Just a is returned. A negative timeout interval means "wait indefinitely".

If the computation has not terminated after n microseconds, it is interrupted by an asynchronous exception. The function passed to f can be used to detect whether it was interrupted by this timeout or some other exception.

The design of this combinator was guided by the objective that timeout n (const f) should behave exactly the same as f as long as f doesn't time out. This means that f has the same myThreadId it would have without the timeout wrapper. Any exceptions f might throw cancel the timeout and propagate further up. It also possible for f to receive exceptions thrown to it by another thread.

A tricky implementation detail is the question of how to abort an IO computation. This combinator relies on asynchronous exceptions internally. The technique works very well for computations executing inside of the Haskell runtime system, but it doesn't work at all for non-Haskell code. Foreign function calls, for example, cannot be timed out with this combinator simply because an arbitrary C function cannot receive asynchronous exceptions. When timeout is used to wrap an FFI call that blocks, no timeout event can be delivered until the FFI call returns, which pretty much negates the purpose of the combinator. In practice, however, this limitation is less severe than it may sound. Standard I/O functions like hGetBuf, hPutBuf, Network.Socket.accept, or hWaitForInput appear to be blocking, but they really don't because the runtime system uses scheduling mechanisms like select(2) to perform asynchronous I/O, so it is possible to interrupt standard socket I/O or file I/O using this combinator.