At 2002-02-05 15:10, John Hughes wrote:

Oh no, please don't do this! I use the RefMonad class, but *without* the dependency r -> m. Why not? Because I want to manipulate (for example) STRefs in monads built on top of the ST monad via monad transformers. So I use the same reference type with *many different* monads! Your change would make this impossible.

I don't think so. Provided you have a class that represents your monads, I don't see why you can't do this: class (Monad m) => MyMonad m where ... myIntRef :: (MyMonad m) => Ref m Int; ...except that with Simon's suggested change you wouldn't be able to define your own Refs. I believe the best solution to the problem involves leaving the existing functions as is but adding something like this: data Ref m a = MkRef { get :: m a, set :: a -> m (), modify :: (a -> a) -> m () }; mkRef g s = MkRef g s (\map -> do { val <- g; s (map val); }); refBind :: (Monad m) => (m a) -> (a -> Ref m b) -> Ref m b; refBind ma arb = MkRef ( ma >>= (\a -> get (arb a) )) (\b -> ma >>= (\a -> set (arb a) b )) (\map -> ma >>= (\a -> modify (arb a) map )); class (Monad m) => RefMonad m where { newRef :: a -> m (Ref m a); -- "standard" ref for this monad }; instance RefMonad IO where { newRef a = do { r <- newIORef a; return MkRef (readIORef r) (writeIORef r) (modifyIORef r) }; }; instance RefMonad (ST s) where etc. The point is that the m -> r dependency is also unnecessary, except when you want a new "standard" ref for a monad. -- Ashley Yakeley, Seattle WA