import Control.Monad.Trans.Cont
import Control.Monad.Trans
import Control.Monad(void)
data Val = MkVal
data RetC = MkRetC
data Other = MkOther

needsCallback :: (Val -> IO RetC) -> IO RetC
needsCallback cb = cb MkVal

returnsValM :: ContT RetC IO Val
returnsValM = ContT needsCallback

usesValM :: ContT RetC IO Other
usesValM = do
  val <- returnsValM
  lift $ putStrLn "i am using Val as if it were a value"
  seq val $ pure MkOther 
  -- here you can put anything that uses your Val value
  -- the seq is just to show that we can force the val as if we had it
  
callbackM :: Other -> IO RetC
callbackM MkOther = pure MkRetC
main :: IO ()
main = do
  putStrLn "before needsCallback"
  void $ runContT usesValM callbackM
  putStrLn "after needsCallback"