2008/10/23 Andreas-Christoph Bernstein
apfelmus wrote:
But what i need is [(0,"a"),(1,"a"),(2,"a"),(1,"a"),(0,"a")]
So state changes should affect only their subtree, not the rest of the tree to the right.
It seems to me that you are looking for the Reader monad. Try the following: import Control.Monad.Reader t :: (a -> b -> b) -> BTree a -> Reader b (BTree b) t f (Leaf x) = do s <- ask return (Leaf (f x s)) t f (Fork x l r) = do s <- ask l' <- local (f x) (t f l) r' <- local (f x) (t f r) return (Fork (f x s) l' r') new = runReader (t modState sampleTree) globalState Then, flattenTree new gives you [(0,"a"),(1,"a"),(2,"a"),(1,"a"),(0,"a")] I think that the Reader monad is a standard way to this. When you want the state to affect also the rest of the tree then use the State monad. Sincerely, jan.