In the following section of the book "Parallel and Concurrent Programming in Haskell" http://chimera.labs.oreilly.com/books/1230000000929/ch07.html#sec_conc-phone... Simon Marlow explains that MVars can be used to implement something like locks on shared functional state: "To acquire the lock, we take the MVar, whereas, to update the variable and release the lock, we put the MVar." Am I right in thinking this only holds if we are careful to ensure both those operations happen in the given order? For example, if I take the MVar (lock) and I am about to put something back in (unlock) but someone else puts something in before I do, then the lock system becomes broken, doesn't it? So if we want to be sure we have a robust lock system we have to wrap up MVars in another abstraction? Tom