|Copyright||(c) The University of Glasgow 2007|
|License||BSD-style (see the file libraries/base/LICENSE)|
Attach a timeout event to arbitrary
IO computation to time out and return
Nothing in case no result is available within
n microseconds (
1/10^6 seconds). In case a result is available before the timeout expires,
Just a is returned. A negative timeout interval means "wait indefinitely". When specifying long timeouts, be careful not to exceed
maxBound :: Int.
>>> timeout 1000000 (threadDelay 1000 *> pure "finished on time") Just "finished on time"
>>> timeout 10000 (threadDelay 100000 *> pure "finished on time") Nothing
The design of this combinator was guided by the objective that
timeout n f should behave exactly the same as
f as long as
f doesn't time out. This means that
f has the same
myThreadId it would have without the timeout wrapper. Any exceptions
f might throw cancel the timeout and propagate further up. It also possible for
f to receive exceptions thrown to it by another thread.
A tricky implementation detail is the question of how to abort an
IO computation. This combinator relies on asynchronous exceptions internally. The technique works very well for computations executing inside of the Haskell runtime system, but it doesn't work at all for non-Haskell code. Foreign function calls, for example, cannot be timed out with this combinator simply because an arbitrary C function cannot receive asynchronous exceptions. When
timeout is used to wrap an FFI call that blocks, no timeout event can be delivered until the FFI call returns, which pretty much negates the purpose of the combinator. In practice, however, this limitation is less severe than it may sound. Standard I/O functions like
hPutBuf, Network.Socket.accept, or
hWaitForInput appear to be blocking, but they really don't because the runtime system uses scheduling mechanisms like
select(2) to perform asynchronous I/O, so it is possible to interrupt standard socket I/O or file I/O using this combinator.
© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).