STM and unsafePerformIO

[Simon Marlow]

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