Cont with success and failure isn't Cont; it's something else (albeit similar)
There's a great exposition of using something much like Cont to get
success and failure "for free" here:
http://www-ps.informatik.uni-kiel.de/~sebf/haskell/barefaced-pilferage-of-mo...
-- ryan
On Sat, May 2, 2009 at 2:13 AM, Anatoly Yakovenko
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