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 -------------------------------------------------------------------------------
17 module System.Timeout ( timeout ) where
20 timeout :: Int -> IO a -> IO (Maybe a)
21 timeout n f = fmap Just f
24 import Prelude (Show(show), IO, Ord((<)), Eq((==)), Int,
26 import Data.Maybe (Maybe(..))
27 import Control.Monad (Monad(..), guard)
28 import Control.Concurrent (forkIO, threadDelay, myThreadId, killThread)
29 import Control.Exception.Base (Exception, handleJust, throwTo, bracket)
30 import Data.Dynamic (Typeable, fromDynamic)
32 import Data.Unique (Unique, newUnique)
34 -- An internal type that is thrown as a dynamic exception to
35 -- interrupt the running IO computation when the timeout has
38 data Timeout = Timeout Unique deriving Eq
39 INSTANCE_TYPEABLE0(Timeout,timeoutTc,"Timeout")
41 instance Show Timeout where
42 show _ = "<<timeout>>"
44 instance Exception Timeout
46 -- |Wrap an 'IO' computation to time out and return @Nothing@ in case no result
47 -- is available within @n@ microseconds (@1\/10^6@ seconds). In case a result
48 -- is available before the timeout expires, @Just a@ is returned. A negative
49 -- timeout interval means \"wait indefinitely\". When specifying long timeouts,
50 -- be careful not to exceed @maxBound :: Int@.
52 -- The design of this combinator was guided by the objective that @timeout n f@
53 -- should behave exactly the same as @f@ as long as @f@ doesn't time out. This
54 -- means that @f@ has the same 'myThreadId' it would have without the timeout
55 -- wrapper. Any exceptions @f@ might throw cancel the timeout and propagate
56 -- further up. It also possible for @f@ to receive exceptions thrown to it by
59 -- A tricky implementation detail is the question of how to abort an @IO@
60 -- computation. This combinator relies on asynchronous exceptions internally.
61 -- The technique works very well for computations executing inside of the
62 -- Haskell runtime system, but it doesn't work at all for non-Haskell code.
63 -- Foreign function calls, for example, cannot be timed out with this
64 -- combinator simply because an arbitrary C function cannot receive
65 -- asynchronous exceptions. When @timeout@ is used to wrap an FFI call that
66 -- blocks, no timeout event can be delivered until the FFI call returns, which
67 -- pretty much negates the purpose of the combinator. In practice, however,
68 -- this limitation is less severe than it may sound. Standard I\/O functions
69 -- like 'System.IO.hGetBuf', 'System.IO.hPutBuf', Network.Socket.accept, or
70 -- 'System.IO.hWaitForInput' appear to be blocking, but they really don't
71 -- because the runtime system uses scheduling mechanisms like @select(2)@ to
72 -- perform asynchronous I\/O, so it is possible to interrupt standard socket
73 -- I\/O or file I\/O using this combinator.
75 timeout :: Int -> IO a -> IO (Maybe a)
78 | n == 0 = return Nothing
81 ex <- fmap Timeout newUnique
82 handleJust (\e -> if e == ex then Just () else Nothing)
83 (\_ -> return Nothing)
84 (bracket (forkIO (threadDelay n >> throwTo pid ex))