Ryan Ingram ryani.spam at gmail.com
Wed Nov 11 01:09:47 EST 2009

```Something like this should work:

newtype ContState r s a = ContState { runCS :: s -> (a -> s -> r) -> r }

instance Monad (ContState r s) where
return a = ContState \$ \s k -> k a s
m >>= f = ContState \$ \s0 k -> runCS m s \$ \a s1 -> runCS (f a) s1 k

instance MonadState s (ContState r s) where
get = ContState \$ \s k -> k s s
put s = ContState \$ \_ k -> k () s

instance MonadCont (ContState r s) where
callCC f = ContState \$ \s0 ka -> runCS (f \$ \a -> ContState \$ \s1
kb -> ka a s1) s0 ka

There's a design choice as to whether the inner continuation should be
called with s0 or s1; it depends if you want the continuation from
callCC to abort any state changes or preserve them up to that point.

-- ryan

On Tue, Nov 10, 2009 at 12:18 PM, jean-christophe mincke
<jeanchristophe.mincke at gmail.com> wrote:
> Hello,
>
> 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
>
> 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
>
>
> _______________________________________________