Hi folks I'm having a play with Concurrent Haskell, and came across this in the library. readMVar :: MVar a -> IO a This is a combination of takeMVar and putMVar; ie. it takes the value from the MVar, puts it back, and also returns it. I was just wondering if I understand things correctly. Suppose myMVar is nonempty and I have one thread executing do x <- takeMVar myMVar -- (A) putMVar myMVar x -- (B) return x and another executing putMVar myMVar y -- (C) Questions (1) Is it possible that an evil scheduler will execute (A) then (C) and block on (B)? (2) Is it the case that if myMVar is nonempty and readMVar is chosen for execution, that readMVar myMVar takes the value from myMVar, guaranteed that the _same_ value will be put back immediately, and also returned? (3) If yes to both, what combination of takeMVar and putMVar manages to maintain the lock in between the two? I'm only asking, because I'm trying to cook up some kind of partial evaluator for programs which are being simultaneously edited by fiercely argumentative devils. Kind of `demonic laziness', where the computations decide when _they_ feel like running (eg sometime after the program turns up), rather than the `angelic laziness' we're used to. I'd like to use MVars in a write-once-read-many style, so that once someone fills in a bit of program, it stays put. I'd hate for another devil to sneak in a replacement program just in the twinkling of an eye between taking something out to read and putting it back to be read again. I guess I could use some kind of extra semaphore MVar to ensure that the reader has the lock on the program MVar, but that's more like hard work. Is readMVar what I want? Another more general question would be `is this an utterly stupid thing to do in practice?'. I mean, just consider an evaluator for some simple language (eg arithmetic expressions), implemented by forking a thread for every subcomputation, and using MVars to transmit the values. Is that diabolically slow compared to a normal evaluator? I guess I'll try it, but I was wondering if anyone had any useful experience to share? Cheers Conor