On Thu, Sep 18, 2003 at 08:56:40AM +0100, Simon Peyton-Jones wrote:
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
The proposal is to expose type IORef = STRef IORegion type IOArray = STArray IORegion type IOUArray = STUArray IORegion (modulo the name of IORegion) but to keep IO opaque. I think that while IO includes state, it is not itself a state monad (because the "state" there can be changed by other agents at the same time).
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.
If IO is opaque, then we have newIORef = stToIO . newSTRef etc. It's not quite as generic as your version, but it is a conceptual simplification, and it does allow generic classes like class MonadST s m | m -> s where liftST :: Control.Monad.ST.ST s a -> m a with instances for IO, both ST's and various constructed monads, so we could have the likes of readSTRef' :: MonadST s m => STRef s a -> m a readSTRef' r = liftST (Data.STRef.readSTRef r) My problem with exposing the GHC implementation of IO is that I don't think it's the Truth, and Haskell has a record of punishing untruths sooner or later.
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.
NHC doesn't have ST (rank-2 types). In Hugs, they are now newtype IO a = IO ((a -> IOResult) -> IOResult) newtype ST s a = ST (forall r. (a -> r) -> r) primitive newIORef "newRef" :: a -> IO (IORef a) primitive newSTRef "newRef" :: a -> ST s (STRef s a) But I'm not proposing that this be exposed.
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
It's more a matter of making IORegion work a bit harder since we have to put it in the interface anyway.