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
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