Gregory Crosswhite gcross at phys.washington.edu
Tue Nov 10 15:49:20 EST 2009

```Yes;  check out the module "Control.Monad.Cont", which has a monad for
continuation passing style.

"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

Hope this helps,
Greg

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

> Hello,
>
> 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
>
> Thank you
>
> Regards
>
> J-C
>
>
> module M where
>
>
> 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
>
> _______________________________________________