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

[Anatoly Yakovenko]

thanks, that looks promising, but will probably take me a week to understand :)

On Sun, May 3, 2009 at 2:40 PM, Ryan Ingram <ryani.spam at gmail.com> wrote:
> 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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