On 03 August 2005 11:51, Robert van Herk wrote:
I think I've read somewhere that STM doesn't like unsafePerformIO. However, I would like to use a global STM variable. Something like this:
*module* Main *where* import GHC.Conc import System.IO.Unsafe
tSid = unsafePerformIO (atomically (newTVar 0))
tickSessionID :: STM Int tickSessionID = *do* sid <- readTVar tSid writeTVar tSid (sid + 1) return sid
main = atomically tickSessionID
But, when I try this, the evaluation of main causes a segmentation fault. Is there a workaround for this bug?
You're ok as long as the global variable doesn't get evaluated inside another atomically. The problem is that nested atomically isn't supported; the RTS gets confused. For example, you could say main = do evaluate tSid ... to force evaluation of tSid before doing anything else. Cheers, Simon