I recently had to implement a thread-safe wrapper to fcntl locks, which involved implementing my own inter-thread locking code. Because POSIX is horrible. http://git-annex.branchable.com/devblog/day_286-287__rotten_locks/

Take a look at this implementation: http://source.git-annex.branchable.com/?p=source.git;a=blob;f=Utility/LockPool/STM.hs

• First we have the unsafePerformIO of doom on line 46. (Actually safe in this application.)
• But then, look at how nice the implementation of tryTakeLock is! (line 70) It uses waitTakeLock and converts it from a blocking wait to a one-time try, by simply using the orElse combinator included in Haskell's STM implementation.

STM is awesome, and the orElse combinator is an awesome improvement on top of it, described in beautiful concurrency:

In particular, blocking (retry) and choice (orElse), which are fundamentally non-modular when expressed using locks, are fully modular in STM.