So I am trying to traverse a tree in a specific order, but i have no idea where the things that i am looking for are located, and i want to avoid explicit backtracking. I was thinking i could do it with the continuation monad. Here is what i have module TestCont where import Control.Monad.Cont import Control.Monad.Identity import Control.Monad.State.Lazy --our stupid tree data Tree a = Tree [Tree a] | Leaf a --traverse all the branches search (Tree ts) next = do mapM_ (\ ti -> (callCC (search ti))) ts next $ () search tt@(Leaf a) next = do cur <- lift get case ((cur + 1) == a) of True -> do --the current leaf is what we want, update the state and return lift $ put a return $ () False -> do --the current leaf is not what we want, continue first, then try again next () search tt (\ _ -> error "fail") t1 = Leaf 1 t2 = Leaf 2 t3 = Tree [t1,t2] t4 = Leaf 3 t5::Tree Int = Tree [t4,t3] run = runIdentity (runStateT ((runContT $ callCC (search t5)) return) 0) it seems like next isn't quite doing what i want, because i don't think I ever try again after i call next $ () in the second clause. Any ideas? Thanks, Anatoly