[Haskell-cafe] (state) monad and CPS

jean-christophe mincke jeanchristophe.mincke at gmail.com
Tue Nov 10 15:18:34 EST 2009


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



module M where

import Control.Monad.State

data Node =
     Node (Node, Int, Node)
    |Leaf Int
    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
                              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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