On Fri, May 1, 2009 at 8:47 PM, Anatoly Yakovenko
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.
Though I don't fully understand what you are doing (specifically what you mean by "specific order"), but in a lazy language, traversals are usually simply encoded as lists. Just write a function which returns all the leaves as a list, and filter over it. traverse (Tree ts) = concatMap traverse ts traverse (Leaf x) = [x] I believe this simple definition has more overhead than necessary because of all the appends; a DList will be more efficient. import qualified Data.DList as DList traverse = DList.toList . traverse' where traverse' t (Tree ts) = DList.concat (map traverse' ts) traverse' t (Leaf x) = DList.singleton x (DList is on hackage) 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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe