Re: [Haskell-cafe] readMVar and the devils

at a guess the magic take put is:

block ( do a <- takeMVar x putMVar x a ) return a

This doesn't prevent the race condition Conor mentioned. It only prevents the thread executing the above code from being interrupted by an asynchronous exception (i.e., Control-C, or another thread killing this one). The only way to prevents race conditions is through discipline (take then put is one example). A good way to enforce this discipline is to use withX-style combinators, and treat the put/take group of operations as primitives. A