A typeclass abstraction over much of Control.Concurrent (and some extras!). If you're looking for a general introduction to Haskell concurrency, you should check out the excellent Parallel and Concurrent Programming in Haskell, by Simon Marlow. If you are already familiar with concurrent Haskell, just change all the imports from Control.Concurrent.* to Control.Concurrent.Classy.* and fix the type errors.