1 -------------------------------------------------------------------------------
3 -- Module : System.Timeout
4 -- Copyright : (c) The University of Glasgow 2007
5 -- License : BSD-style (see the file libraries/base/LICENSE)
7 -- Maintainer : libraries@haskell.org
8 -- Stability : experimental
9 -- Portability : non-portable
11 -- Attach a timeout event to arbitrary 'IO' computations.
13 -------------------------------------------------------------------------------
15 module System.Timeout ( timeout ) where
18 timeout :: Int -> IO a -> IO (Maybe a)
19 timeout n f = fmap Just f
22 import Prelude (Show(show), IO, Ord((<)), Eq((==)), Int,
24 import Data.Maybe (Maybe(..))
25 import Control.Monad (Monad(..), guard)
26 import Control.Concurrent (forkIO, threadDelay, myThreadId, killThread)
27 import Control.Exception (Exception, handleJust, throwTo, bracket)
28 import Data.Dynamic (Typeable, fromDynamic)
29 import Data.Unique (Unique, newUnique)
31 -- An internal type that is thrown as a dynamic exception to
32 -- interrupt the running IO computation when the timeout has
35 data Timeout = Timeout Unique deriving (Eq, Typeable)
37 instance Show Timeout where
38 show _ = "<<timeout>>"
40 instance Exception Timeout
42 -- |Wrap an 'IO' computation to time out and return @Nothing@ in case no result
43 -- is available within @n@ microseconds (@1\/10^6@ seconds). In case a result
44 -- is available before the timeout expires, @Just a@ is returned. A negative
45 -- timeout interval means \"wait indefinitely\". When specifying long timeouts,
46 -- be careful not to exceed @maxBound :: Int@.
48 -- The design of this combinator was guided by the objective that @timeout n f@
49 -- should behave exactly the same as @f@ as long as @f@ doesn't time out. This
50 -- means that @f@ has the same 'myThreadId' it would have without the timeout
51 -- wrapper. Any exceptions @f@ might throw cancel the timeout and propagate
52 -- further up. It also possible for @f@ to receive exceptions thrown to it by
55 -- A tricky implementation detail is the question of how to abort an @IO@
56 -- computation. This combinator relies on asynchronous exceptions internally.
57 -- The technique works very well for computations executing inside of the
58 -- Haskell runtime system, but it doesn't work at all for non-Haskell code.
59 -- Foreign function calls, for example, cannot be timed out with this
60 -- combinator simply because an arbitrary C function cannot receive
61 -- asynchronous exceptions. When @timeout@ is used to wrap an FFI call that
62 -- blocks, no timeout event can be delivered until the FFI call returns, which
63 -- pretty much negates the purpose of the combinator. In practice, however,
64 -- this limitation is less severe than it may sound. Standard I\/O functions
65 -- like 'System.IO.hGetBuf', 'System.IO.hPutBuf', Network.Socket.accept, or
66 -- 'System.IO.hWaitForInput' appear to be blocking, but they really don't
67 -- because the runtime system uses scheduling mechanisms like @select(2)@ to
68 -- perform asynchronous I\/O, so it is possible to interrupt standard socket
69 -- I\/O or file I\/O using this combinator.
71 timeout :: Int -> IO a -> IO (Maybe a)
74 | n == 0 = return Nothing
77 ex <- fmap Timeout newUnique
78 handleJust (\e -> if e == ex then Just () else Nothing)
79 (\_ -> return Nothing)
80 (bracket (forkIO (threadDelay n >> throwTo pid ex))