efficiently run periodic, on-demand actions

 A common problem is the desire to have an action run at a scheduled interval,
 but only if it is needed. For example, instead of having every web request
 result in a new getCurrentTime call, we'd like to have a single worker
 thread run every second, updating an IORef. However, if the request
 frequency is less than once per second, this is a pessimization, and worse,
 kills idle GC.
 .
 This library allows you to define actions which will either be performed by a
 dedicated thread or, in times of low volume, will be executed by the calling
 thread.
 .
 This package provides a library for the Haskell programming language.
 See http://www.haskell.org/ for more information on Haskell.