Hello everyone, I'm currently in the process of wrapping a C API, and I've run across an interesting possibility. Basically, the C API exposes non-blocking versions of some potentially long-running operations, and will invoke a callback to indicate that the long running operation has finished. For instance, I have something like: int longRunningReadOperation(int length, byte * buf, void (*callback)()) When callback is called, then there are length bytes in buf. What I'd like to do is wrap this in Haskell function like: longRunningReadOperation :: Int -> IO [Byte] such that the function returns immediately, and only blocks when someone pulls on the results if the callback hasn't been triggered yet. I figure I'm going to need unsafeInterleaveIO, but I'm not sure what to do in the computation I pass to it. I had a small hope that if the callback put the results into an MVar (say var), and I returned (unsafeInterleaveIO (readMVar var)) that might work, but it seems like if the readMVar blocks then the callback doesn't execute either. This doesn't surprise me, but it does leave me needing another option. I don't see a way to get the callback into a new Haskell thread, so that it can do the putMVar from there... am I missing something obvious? Thanks in advance! /g