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