shift/reset via double CPS transform

DoubleCont.hs

module DoubleCont where

import Control.Monad.Cont

reset :: ContT a (ContT r m) a -> ContT b (ContT r m) a
reset e = ContT $ \k -> 
              ContT $ \m -> 
                  runContT (runContT e return) 
                           (\r -> runContT (k r) m)

shift :: ((a -> ContT x (ContT r m) x) -> ContT x (ContT r m) x) 
         -> ContT x (ContT r m) a
shift e = ContT $ \k -> 
             ContT $ runContT (
                      runContT (
                            e (\v -> ContT $ \k' -> 
                                        ContT $ \m'' -> 
                                           runContT (k v) 
                                                    (\w -> runContT (k' w) m'')
                            )
                         ) 
                      return
                     )