[Haskell-cafe] traversing a tree using monad.cont

[Ryan Ingram]

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-monadic-bind.lhs.html

  -- ryan

On Sat, May 2, 2009 at 2:13 AM, Anatoly Yakovenko <aeyakovenko at gmail.com> wrote:
>> 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
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