[Haskell-beginners] theStdGen unsafePerformIO

[Brent Yorgey]

On Sun, Jan 09, 2011 at 05:55:41PM +0200, John Smith wrote:
> Why does theStdGen require unsafePerformIO? I recompiled the Random module with
> 
> theStdGen :: IO (IORef StdGen)
> theStdGen  = do
>                 rng <- mkStdRNG 0
>                 newIORef rng
> 
> The implementations of a few functions needed to change slightly (to
> extract the IORef from IO), but no other type signatures needed
> changing, and nothing blew up.

With your definition, theStdGen is a computation that gives you a
*new*, *different* IORef (containing yet another generator initialized
with a seed of zero) every time you call it.  The original definition
with unsafePerformIO generates a single, global IORef which is
accessed by every subsequent call to getStdGen.

-Brent