[Haskell-cafe] Shared/Exclusive Locks

Hello, I have the need for a locking object that can provide shared and exclusive locks in much the same manner as the POSIX flock() function. A thread that acquires an exclusive lock has a guarantee that no other thread holds any locks. A thread that acquires a shared lock has a guarantee that no other thread holds an exclusive lock, though any number of other threads hold shared locks. My intuition tells me that this could be implemented in terms of an MVar somehow. (At least, I've used MVars for simple locks for quite some time.) But I can't quite figure out how. Any ideas? Thanks, -- John