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 then calls setAlarm for the next time that an entry will expire (if there are any). 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

:: (AlarmClock -> IO ())

Action to run when the alarm goes off. The action is provided the alarm clock so it can set a new alarm if desired. Note that setAlarm must be called once the alarm has gone off to cause it to go off again.

-> 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 the alarm is currently going off then this will block until the action is finished.

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.

setAlarmSTM :: AlarmClock -> UTCTime -> STM ()

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.

isAlarmSet :: AlarmClock -> IO Bool

Is the alarm set - i.e. will it go off at some point in the future even if setAlarm is not called?

isAlarmSetSTM :: AlarmClock -> STM Bool

Is the alarm set - i.e. will it go off at some point in the future even if setAlarm is not called?