Underlying ST is: GHC.Prim.State# s -> (# GHC.Prim.State# s, a #) Underlying IO is: GHC.Prim.State# GHC.Prim.RealWorld -> (# GHC.Prim.State# GHC.Prim.RealWorld, a #) Based on the (very helpful!) conversation I had on the #ghc IRC channel, it seems to me that the mechanism for preventing things like reordering operations or spurious sharing is shared in common between ST and IO via State#. Is this accurate? I believe the exception is how RealWorld is used w/ IO but we can put that off for this question. If anyone could confirm this understanding that would be helpful. If anyone could point out exceptions to this notion, I'd very much appreciate that as well. Thanks again for everyone's time. -- Chris Allen Currently working on http://haskellbook.com