State monad using Cont

Cont.hs

{-# LANGUAGE NoMonomorphismRestriction, Rank2Types #-}
import Control.Monad.Cont

type State s v = s -> (s, v)

set :: a -> State a ()
set x = const (x, ())

get :: State a a
get = \s -> (s, s)

bind :: State a b -> (b -> State a c) -> State a c
bind ma f = \s -> let (s', v) = ma s in f v s'

ret :: a -> State x a
ret a = \s -> (s, a)

run :: s -> State s a -> (s, a)
run s f = f s

foo = get `bind` \s -> set (s + 42) `bind` \_ -> ret "foo" 

c x = cont (\fred -> x `bind` fred)

cget :: CState a a
cget = c get

cset :: a -> CState a ()
cset = c . set

type CState s v = forall a . Cont (State s a) v

bar :: CState Int Int
bar = do
  x <- cget
  cset (x + 42)
  cget

crun x f = run x $ runCont f ret 

main :: IO ()
main = do
   print $ crun 1000 bar
   print $ run 1000 foo