add a dummy implementation of System.Timeout.timeout for nhc98
[packages/base.git] / System / Timeout.hs
1 {-# OPTIONS -fglasgow-exts #-}
2 -------------------------------------------------------------------------------
3 -- |
4 -- Module : System.Timeout
5 -- Copyright : (c) The University of Glasgow 2007
6 -- License : BSD-style (see the file libraries/base/LICENSE)
7 --
8 -- Maintainer : libraries@haskell.org
9 -- Stability : experimental
10 -- Portability : non-portable
11 --
12 -- Attach a timeout event to arbitrary 'IO' computations.
13 --
14 -------------------------------------------------------------------------------
15
16 module System.Timeout ( timeout ) where
17
18 #if __NHC__
19 timeout :: Int -> IO a -> IO (Maybe a)
20 timeout n f = fmap Just f
21 #else
22
23 import Prelude (IO, Ord((<)), Eq((==)), Int, (.), otherwise, fmap)
24 import Data.Maybe (Maybe(..))
25 import Control.Monad (Monad(..), guard)
26 import Control.Concurrent (forkIO, threadDelay, myThreadId, killThread)
27 import Control.Exception (handleJust, throwDynTo, dynExceptions, bracket)
28 import Data.Dynamic (Typeable, fromDynamic)
29 import Data.Unique (Unique, newUnique)
30
31 -- An internal type that is thrown as a dynamic exception to
32 -- interrupt the running IO computation when the timeout has
33 -- expired.
34
35 data Timeout = Timeout Unique deriving (Eq, Typeable)
36
37 -- |Wrap an 'IO' computation to time out and return @Nothing@ in case no result
38 -- is available within @n@ microseconds (@1\/10^6@ seconds). In case a result
39 -- is available before the timeout expires, @Just a@ is returned. A negative
40 -- timeout interval means \"wait indefinitely\". When specifying long timeouts,
41 -- be careful not to exceed @maxBound :: Int@.
42 --
43 -- The design of this combinator was guided by the objective that @timeout n f@
44 -- should behave exactly the same as @f@ as long as @f@ doesn't time out. This
45 -- means that @f@ has the same 'myThreadId' it would have without the timeout
46 -- wrapper. Any exceptions @f@ might throw cancel the timeout and propagate
47 -- further up. It also possible for @f@ to receive exceptions thrown to it by
48 -- another thread.
49 --
50 -- A tricky implementation detail is the question of how to abort an @IO@
51 -- computation. This combinator relies on asynchronous exceptions internally.
52 -- The technique works very well for computations executing inside of the
53 -- Haskell runtime system, but it doesn't work at all for non-Haskell code.
54 -- Foreign function calls, for example, cannot be timed out with this
55 -- combinator simply because an arbitrary C function cannot receive
56 -- asynchronous exceptions. When @timeout@ is used to wrap an FFI call that
57 -- blocks, no timeout event can be delivered until the FFI call returns, which
58 -- pretty much negates the purpose of the combinator. In practice, however,
59 -- this limitation is less severe than it may sound. Standard I\/O functions
60 -- like 'System.IO.hGetBuf', 'System.IO.hPutBuf', 'Network.Socket.accept', or
61 -- 'System.IO.hWaitForInput' appear to be blocking, but they really don't
62 -- because the runtime system uses scheduling mechanisms like @select(2)@ to
63 -- perform asynchronous I\/O, so it is possible to interrupt standard socket
64 -- I\/O or file I\/O using this combinator.
65
66 timeout :: Int -> IO a -> IO (Maybe a)
67 timeout n f
68 | n < 0 = fmap Just f
69 | n == 0 = return Nothing
70 | otherwise = do
71 pid <- myThreadId
72 ex <- fmap Timeout newUnique
73 handleJust (\e -> dynExceptions e >>= fromDynamic >>= guard . (ex ==))
74 (\_ -> return Nothing)
75 (bracket (forkIO (threadDelay n >> throwDynTo pid ex))
76 (killThread)
77 (\_ -> fmap Just f))
78 #endif