Hi Didier I think I've simplified what you are saying to remove the dependency on wxHaskell. Recursing a widget hierarchy to all get a list of all widgets seems a bit strange, though demo1 does this. Generally I'd expect you would want to recursively process the widget hierarchy collecting some attribute of each widget - demo2 does this. I've put the recursive functions into the IO monad to model wxHaskell where widgets are in some monad. code follows... -- Model widgets as a simple binary tree - labels and interior -- nodes both have names. -- data Tree = Leaf String | Node Tree String Tree deriving (Show) -- simulate the 'get children' on a widget - -- the function is in the IO monad as an analogy -- to wxHaskell being in some monad. -- children :: Tree -> IO [Tree] children (Leaf _) = return [] children (Node l _ r) = return [l,r] -- allTrees returns a list of trees - -- the current tree (t) plus trees of all children (kids_deep) -- Again in the IO monad to simulate wxHaskell being in some -- monad. -- allTrees :: Tree -> IO [Tree] allTrees t = do kids <- children t -- [Tree] kids_deep <- mapM allTrees kids -- [[Tree]] return (t : concat kids_deep) tree1 = Node (Leaf "a") "top" (Node (Leaf "c") "node_b" (Leaf "d")) demo1 = allTrees tree1 -- Simpler idiom just collecting an 'attribute' at each level -- of the tree - here the name. -- allNames :: Tree -> IO [String] allNames t = case t of Leaf s -> return [s] Node l s r -> do { ls <- allNames l ; rs <- allNames r ; return (s:ls ++ rs) } demo2 = allNames tree1