Control.Timeout

This module provides generalized sleep and timeout functions.

Example:

 module Main where

 import Control.Timeout (timeout, sleep)

 main :: IO ()
 main = do
     timeout 1 $ sleep 2  -- Will return IO Nothing
     timeout 2 $ sleep 1  -- Will return IO (Just ())
     return ()

Documentation

data NominalDiffTime

This is a length of time, as measured by UTC. Conversion functions will treat it as seconds. It has a precision of 10^-12 s. It ignores leap-seconds, so it's not necessarily a fixed amount of clock time. For instance, 23:00 UTC + 2 hours of NominalDiffTime = 01:00 UTC (+ 1 day), regardless of whether a leap-second intervened.

Instances

Enum NominalDiffTime

Eq NominalDiffTime

Fractional NominalDiffTime

Data NominalDiffTime

Num NominalDiffTime

Ord NominalDiffTime

Real NominalDiffTime

RealFrac NominalDiffTime

Show NominalDiffTime

Typeable NominalDiffTime

NFData NominalDiffTime

newtype Timeout

Exception used for timeout handling

Constructors

Timeout Unique

Instances

Eq Timeout

Show Timeout

Typeable Timeout

Exception Timeout

timeout :: (MonadMask m, MonadIO m) => NominalDiffTime -> m a -> m (Maybe a)

Wrap an MonadIO computation to time out and return Nothing in case no result is available within n seconds. In case a result is available before the timeout expires, Just a is returned. A negative timeout interval means "timeout immediately".

The design of this combinator was guided by the objective that timeout n 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.

sleep :: MonadIO m => NominalDiffTime -> m ()

Sleep for NominalDiffTime, example:

 sleep 5  -- Will sleep for 5 seconds