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| Description |
This module handles timeouts by using a (single) thread sitting in threadDelay
and the STM. One can request an IO action be performed after some number of
seconds and later cancel that request if need be.
The number of threads used is constant.
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| Synopsis |
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| Documentation |
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| addTimeout |
| :: Float | the number of seconds in the future to perform the action
| | -> IO () | the action to perform
| | -> IO TimeoutTag | | | Add an action to be performed at some point in the future. The action will
occur inside a thread which is dedicated to performing them so it should
run quickly and certainly should not block on IO etc.
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| addTimeoutAtomic |
| :: Float | the number of seconds in the future to perform the action
| | -> IO (IO () -> STM TimeoutTag) | an action to add the timeout and return the tag
| Similar in function to addTimeout above, this call splits the IO and STM
parts of the process so that a timeout can be added atomically. Consider
the following code:
- We add a timeout with an action which reads from a global TVar
- We add the TimeoutTag (in case we wish to handle the timeout) and
some bookkeeping data to the global TVar and trigger some external
action (i.e. a network request)
In this case, the timeout could occur before the bookkeeping is added. Now
the timeout code won't find the correct state. If we switch the two actions
then we don't have the TimeoutTag to add to the bookkeeping structure and we
would need another TVar, or some such, to fill in later.
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| cancelTimeout |
| :: TimeoutTag | the tag returned by addTimeout
| | -> STM Bool | returns False if the timeout didn't exist
| Remove a timeout. This function never fails, but will return False if the
given timeout couldn't be found. This may be because cancelTimeout has
already been called with this tag, or because the timeout has already
fired. Note that, since timeouts are IO actions, they don't run atomically.
Thus it's possible that this call returns False and that the timeout is
currently in the process of running.
Note that one should never call cancelTimeout twice with the same tag since
it's possible that the tag will be reused and thus the second call could
cancel a different timeout.
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| data TimeoutTag |
| This is an opaque type of timeouts. A value of this type is returned
when creating a timeout and can be used to cancel the same timeout.
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| Produced by Haddock version 2.7.2 |
