[Haskell] IO == ST RealWorld

[Ross Paterson]

On Tue, Jan 24, 2006 at 06:40:38PM +0000, Ben Rudiak-Gould wrote:
> Ross Paterson wrote:
> >But IO isn't a state monad: others are concurrently changing the world
> >without waiting for my Haskell program to terminate.
> 
> I think that closed-world behavior should be treated as a property of 
> runST, not of the ST monad operations. Otherwise your IORef = STRef 
> IORegion proposal doesn't work either, because e.g.
> 
>     myId :: STRef s a -> a -> ST s a
>     myId r x = writeSTRef r x >> readSTRef r
> 
> doesn't necessarily return its second argument when lifted to IO, in the 
> presence of multithreading.

Ouch!  Being able to do that kind of reasoning in ST is really important.
I'd rather sacrifice stToIO than lose that.