Hello, As of stm-2.2.0.1, the documentation for the isEmptyTMVar function [1] reads isEmptyTMVar :: TMVar a -> STM BoolSource Check whether a given TMVar is empty. Notice that the boolean value returned is just a snapshot of the state of the TMVar. By the time you get to react on its result, the TMVar may have been filled (or emptied) - so be extremely careful when using this operation. Use tryTakeTMVar instead if possible. This being software transactional memory, where everything is wrapped into a call to the atomically function, I believe that the second paragraph is now completely obsolete, to the point of being misleading (it sure scared me). I would be relieved if someone could remove it. Most likely, it was originally copied directly from the corresponding documentation of Control.Concurrent.MVar . [1]: http://hackage.haskell.org/packages/archive/stm/2.2.0.1/doc/html/Control-Con... Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com