[Haskell-cafe] (state) monad and CPS

[Gregory Crosswhite]

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

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.

Hope this helps,
Greg


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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