Hello, I have a gtk2hs application. I'd like to implement some kind of browser history for it. It should have Back and Forward buttons and a popup menu with radio items.History is a list of strings. There is a current position in this list, so you can move this position back and forward. Some kind of list zipper must be enough to implement this: data History a = Empty | Hist { histBefore :: [a], histCurrent :: a, histAfter :: [a] } back, forward:: History a -> History a back (Hist (b:bs) c as) = Hist bs b (c:as) forward (Hist bs c (a:as)) = Hist (c:bs) a as add :: a -> History a -> History a add x (Hist bs c _) = Hist (c:bs) x [] --- When you add something, future part of history becomes blank. add x Empty = Hist [] x [] start :: History a -> History a start (Hist bs c as) = Hist [] h (hs ++ [c] ++ as) where (h:hs) = reverse bs But there is another requirement. There must be some "points" in the history to store in popup menu. When you select any point the history sould be rolled to this point. For example. data HistPoint a = HP Int current :: History a -> HistPoint a current (Hist bs _ _) = HP (length bs) goTo :: HistPoint a -> History a -> History a goTo (HP n) = foldr1 (.) (replicate n forward) . start I wonder is there more effective, more easy to implement data structure to represent History and HistPoint -- Victor Nazarov