ttrie (hackage): A contention-free STM hash map. "Contention-free" means that the map will never cause spurious conflicts. A transaction operating on the map will only ever have to retry if another transaction is operating on the same key at the same time.  This is an implementation of the transactional trie, which is basically a lock-free concurrent hash trie lifted into STM. For a detailed discussion, including an evaluation of its performance, see Chapter 4 of my master's thesis.
Keywords: concurrency|library|mit