| > I could be a bit out of date but I think Hugs uses different implementations | > because IO has concurrency and exceptions and ST doesn't. Apart from effort | > required, I don't know if there is any cost in switching. | | I think IORef and STRef have both been MUTVAR as far back as I can see. | The IO and (strict) ST monads used to be very different, and no longer | are (so they both use the same state primitives now), but I don't think | this affects the IORef = STRef IOState question. I'm a bit confused here. Is the proposal that the library design exposes to the programmer the following type synonyms: type IO a = ST IORegion a type IORef a = STRef IORegion a I think it is sound to do so. The advantage would be that newIORef and newSTRef would be the same (newRef, perhaps); ditto readRef, writeRef etc. I think this would be fine for GHC. It might not be fine for Hugs because they implement IO and ST differently I think. No idea about NHC. Or, alternatively, is the question something to do with the internals of the library, of interest to implementers but not to users of the library? Or is it just to do with the type of stToIO, which should be stToIO :: ST IORegion a -> IO a Simon