STM and unsafePerformIO

Remi Turk rturk at
Wed Aug 3 07:05:25 EDT 2005

On Wed, Aug 03, 2005 at 12:50:54PM +0200, Robert van Herk wrote:
> Hello All,
> 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?
> Regards,
> Robert

It probably dies not because of unsafePerformIO per se, but
because STM doesn't understand nested transactions, and
unsafePerformIO here results in a nested transaction. Using the
following main works for me, as it forces both "atomically"'s to
be evaluated sequentially:

main = tSid `seq` atomically tickSessionID

See also

Happy hacking,

P.S. Could you find out (and fix) what inserts those spurious *'s in your code?

Nobody can be exactly like me. Even I have trouble doing it.

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