1{-# LANGUAGE CPP, DeriveDataTypeable, NoImplicitPrelude #-} 2 3#if __GLASGOW_HASKELL__ >= 704 4{-# LANGUAGE Safe #-} 5#endif 6 7-- | Wait arbitrarily long for an IO computation to finish. 8module Control.Concurrent.Timeout ( timeout, Timeout, timeoutWithPred ) where 9 10 11------------------------------------------------------------------------------- 12-- Imports 13------------------------------------------------------------------------------- 14 15-- from base: 16import Control.Concurrent ( forkIOWithUnmask, myThreadId, throwTo, killThread ) 17import Control.Exception ( Exception, bracket, handleJust ) 18import Control.Monad ( return, (>>), fmap ) 19import Data.Bool ( Bool(False), otherwise ) 20import Data.Eq ( Eq, (==) ) 21import Data.Function ( (.), const) 22import Data.Maybe ( Maybe(Nothing, Just) ) 23import Data.Ord ( (<) ) 24import Data.Typeable ( Typeable ) 25import Data.Unique ( Unique, newUnique ) 26import Prelude ( Integer ) 27import System.IO ( IO ) 28import Text.Show ( Show, show ) 29 30#if __GLASGOW_HASKELL__ < 700 31import Prelude ( fromInteger ) 32import Control.Monad ( (>>=), fail ) 33#endif 34 35#ifdef __HADDOCK_VERSION__ 36import Data.Int ( Int ) 37import System.IO ( hGetBuf, hPutBuf, hWaitForInput ) 38import qualified System.Timeout ( timeout ) 39#endif 40 41-- from unbounded-delays (this package): 42import Control.Concurrent.Thread.Delay ( delay ) 43 44 45------------------------------------------------------------------------------- 46-- Long delays and timeouts 47------------------------------------------------------------------------------- 48 49{- 50The following code was mostly copied from the module System.Timeout in the 51package base-4.2.0.0. 52 53(c) The University of Glasgow 2007 54-} 55 56newtype Timeout = Timeout Unique deriving (Eq, Typeable) 57 58instance Show Timeout where 59 show _ = "<<timeout>>" 60 61instance Exception Timeout 62 63{-| 64Like @System.Timeout.'System.Timeout.timeout'@, but not bounded by an 'Int'. 65(..) 66Wrap an 'IO' computation to time out and return 'Nothing' in case no result is 67available within @n@ microseconds (@1\/10^6@ seconds). In case a result is 68available before the timeout expires, 'Just' @a@ is returned. A negative timeout 69interval means \"wait indefinitely\". 70 71If the computation has not terminated after @n@ microseconds, it is interrupted 72by an asynchronous exception. The function passed to @f@ can be used to detect 73whether it was interrupted by this timeout or some other exception. 74 75The design of this combinator was guided by the objective that @timeout n (const f)@ 76should behave exactly the same as @f@ as long as @f@ doesn't time out. This 77means that @f@ has the same 'myThreadId' it would have without the timeout 78wrapper. Any exceptions @f@ might throw cancel the timeout and propagate further 79up. It also possible for @f@ to receive exceptions thrown to it by another 80thread. 81 82A tricky implementation detail is the question of how to abort an 'IO' 83computation. This combinator relies on asynchronous exceptions internally. The 84technique works very well for computations executing inside of the Haskell 85runtime system, but it doesn't work at all for non-Haskell code. Foreign 86function calls, for example, cannot be timed out with this combinator simply 87because an arbitrary C function cannot receive asynchronous exceptions. When 88@timeout@ is used to wrap an FFI call that blocks, no timeout event can be 89delivered until the FFI call returns, which pretty much negates the purpose of 90the combinator. In practice, however, this limitation is less severe than it may 91sound. Standard I\/O functions like 'System.IO.hGetBuf', 'System.IO.hPutBuf', 92Network.Socket.accept, or 'System.IO.hWaitForInput' appear to be blocking, but 93they really don't because the runtime system uses scheduling mechanisms like 94@select(2)@ to perform asynchronous I\/O, so it is possible to interrupt 95standard socket I\/O or file I\/O using this combinator. 96-} 97timeoutWithPred :: Integer -> ((Timeout -> Bool) -> IO α) -> IO (Maybe α) 98timeoutWithPred n f 99 | n < 0 = fmap Just (f (const False)) 100 | n == 0 = return Nothing 101 | otherwise = do 102 pid <- myThreadId 103 ex <- fmap Timeout newUnique 104 handleJust (\e -> if e == ex then Just () else Nothing) 105 (\_ -> return Nothing) 106 (bracket (forkIOWithUnmask (\unmask -> unmask (delay n >> throwTo pid ex))) 107 (killThread) 108 (\_ -> fmap Just (f (==ex))) 109 ) 110 111{-| 112Like 'timeoutWithPred', but does not expose the 'Timeout' exception to the called action. 113-} 114timeout :: Integer -> IO α -> IO (Maybe α) 115timeout n = timeoutWithPred n . const 116