[Haskell-cafe] Destructive updates to plain ADTs

Hi all, is there any way to perform a destructive update on a plain ADT? Imagine I have a simple data Tree a = Nil | Node a (Tree a) (Tree a) I would like to be able to modify right subtree of an existing tree. I can do that for example when using IORefs by changing the datatype to data Tree a = Nil | Node a (IORef (Tree a)) (IORef (Tree a)) and use unsafePerformIO + writeIORef. But the IORefs cause additional complexity when working with the data type. At the moment I am interested in any GHC solution, be it non-portable or version specific. I would like just to run some benchmarks and see the results. Cheers, Milan