STM and unsafePerformIO

[Robert van Herk]

Oh I see! Your sollutions are indeed a lot better than implementating 
all my code using MVars :-).

Thanks,

Robert

Remi Turk wrote:
>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
>http://haskell.org/pipermail/glasgow-haskell-users/2005-June/008615.html
>and
>http://sourceforge.net/tracker/index.php?func=detail&aid=1235728&group_id=8032&atid=108032
>
>Happy hacking,
>Remi
>
>P.S. Could you find out (and fix) what inserts those spurious *'s in your code?
>
>