Control.Concurrent.AlarmClock

Device for running an action at (i.e. shortly after) a certain time, which can be used to implement things like time-based cache expiry.

This implementation avoids the use of polling and leans on Haskell's scheduler to achieve low-latency without lots of computational overhead.

The alarm can be set multiple times, and in this case the alarm will go off at the earliest requested time. If the alarm is set in the past, the action will run immediately. When the action runs, it clears all future alarms; the action can itself return the time at which it should run again.

To perform time-based cache expiry, create an AlarmClock whose action flushes any stale entries from the cache and returns the next time that an entry will expire. If the cache contains no entries that will expire, return Nothing from the alarm action. When expiring entries are added to the cache, call setAlarm to ensure that they will expire in a timely fashion.

Documentation

data AlarmClock

An AlarmClock is a device for running an action at (or shortly after) a certain time.

newAlarmClock

Arguments

:: IO (Maybe UTCTime)

Action to run when the alarm goes off. The return value, if Just, is used as the next wakeup time. If Nothing, the alarm will not wake up again until setAlarm or setAlarmNow is called, even if setAlarm has previously been called with a time that is still in the future.

-> IO AlarmClock

Create a new AlarmClock that runs the given action. Initially, there is no wakeup time set: you must call setAlarm for anything else to happen.

destroyAlarmClock :: AlarmClock -> IO ()

Destroy the AlarmClock so no further alarms will occur. If a wakeup is in progress then it will run to completion.

setAlarm :: AlarmClock -> UTCTime -> IO ()

Make the AlarmClock go off at (or shortly after) the given time. This can be called more than once; in which case, the alarm will go off at the earliest given time.

setAlarmNow :: AlarmClock -> IO ()

Make the AlarmClock go off right now.