Now that RealWorld, formerly an implementation trick inside GHC, has become part of the public interface:

stToIO :: ST RealWorld a -> IO a

(to make stToIO sound), I think it needs a better name. The IO monad does not in fact give you exclusive access to the world, but to a region of the heap separate from those used by invocations of runST. (That's not all IO does, of course, but it's the relevant part for stToIO.) I propose IORegion.

Fine by me too. Cheers, Simon