Implementing RefMonads in Haskell without ST,IO

[Tim Sweeney]

Given the common definition of RefMonad:   class Monad m => RefMonad m r | m -> r where     newRef :: a -> m (r a)     readRef :: r a -> m a     writeRef :: r a -> a -> m ()   Is it possible to actually implement a working instance of RefMonad in Haskell, without making use of a built-in monad like IO or ST?  If so, I'd love any tips -- I've been making good use of monads for a while, but can't figure this one out.   The Java programmer in me wants to implement RefMonad by passing around a function from integers (think of them as pointers or heap indices) to "objects", and in readRef, "cast" the "object" to the appropriate type "t".    If it's not possible to implement a typesafe RefMonad instance directly in Haskell, without making use of built-in imperative features like IO, then doesn't this refute the claims that monads implement imperative features functionally?   -Tim