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
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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