> 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.
yea, i know, i am trying to learn how to use the Cont monad. or continuation in haskell. The idea is that while i am processing some data i may hit a point whree some dependency isn't met and i want to take a different branch via continuation. I expect that branch to furfill my dependency and when its done i want to continue down the original branch >> 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 this is where i succeed in my current branch, so i can just do my thing and exit >> lift $ put a >> return $ () >> False -> do --the current leaf is not what we want, continue first, >> then try again this is where i fail, so i want to take the "other" branch first expecting it to fulfill my dependency. >> 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) but i think next doesn't do exactly what i think it does _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe