In particular, note that most of the monads in Control.Monad.* are "stackable" in that there is a version of the monad which you can stack on top of an existing monad. So for example, you could use ContT to stack the CPS monad on top of the State monad, or StateT to stack the State monad on top of the CPS monad.

On Nov 10, 2009, at 12:18 PM, jean-christophe mincke wrote:

Hello,

I would like to get some advice about state monad (or any other monad I guess) and CPS.

Let's take a simple exemple (see the code below)

'walk' is a function written in CPS that compute the number of nodes & leaves in a tree. It use a counter which is explicitly passed through calls.
'walk2' is does the same using the state monad but is not written in CPS

Is it possible to write a function 'walk3' written in CPS and using the state monad?

Thank you

Regards

J-C

module M where

import Control.Monad.State

data Node =
    Node (Node, Int, Node)
    |Leaf Int
    |Empty
    deriving (Show)

walk Empty acc k = k acc
walk (Leaf _) acc k = k (acc+1)
walk (Node (l, _, r)) acc k = let k1 acc = walk r acc k
                              in
                              walk l (acc+1) k1


nb = Node (Leaf 1, 2, Leaf 3)
nd = Node (nb, 4, Empty)

nh = Node (Empty, 8, Leaf 9)
ng = Node (Leaf 6, 7, nh)

ne = Node (nd, 5, ng)

r = walk ne 0 id

walk2 Empty = return ()
walk2 (Leaf _ ) = do acc <- get
                     put (acc+1)
                     return ()
walk2 (Node (l, _, r)) = do acc <- get
                            put (acc+1)
                            walk2 l
                            walk2 r
                            return ()


r2 = runState (walk2 ne) 0

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