Software Transactional Memory: a modular composable concurrency abstraction. See

• Composable memory transactions, by Tim Harris, Simon Marlow, Simon Peyton Jones, and Maurice Herlihy, in ACM Conference on Principles and Practice of Parallel Programming 2005. http://research.microsoft.com/Users/simonpj/papers/stm/index.htm

This module only defines the STM monad; you probably want to import Control.Concurrent.STM (which exports Control.Monad.STM).