The Continuation monad represents computations in continuation-passing style (CPS). In continuation-passing style function result is not returned, but instead is passed to another function, received as a parameter (continuation). Computations are built up from sequences of nested continuations, terminated by a final continuation (often id) which produces the final result. Since continuations are functions which represent the future of a computation, manipulation of the continuation functions can achieve complex manipulations of the future of the computation, such as interrupting a computation in the middle, aborting a portion of a computation, restarting a computation, and interleaving execution of computations. The Continuation monad adapts CPS to the structure of a monad.
Before using the Continuation monad, be sure that you have a firm understanding of continuation-passing style and that continuations represent the best solution to your particular design problem. Many algorithms which require continuations in other languages do not require them in Haskell, due to Haskell's lazy semantics. Abuse of the Continuation monad can produce code that is impossible to understand and maintain.
|class Monad m => MonadCont m where|
|newtype Cont r a|
|mapCont :: (r -> r) -> Cont r a -> Cont r a|
|withCont :: ((b -> r) -> a -> r) -> Cont r a -> Cont r b|
|newtype ContT r m a|
|mapContT :: (m r -> m r) -> ContT r m a -> ContT r m a|
|withContT :: ((b -> m r) -> a -> m r) -> ContT r m a -> ContT r m b|
|Example 1: Simple Continuation Usage|
Calculating length of a list continuation-style:
calculateLength :: [a] -> Cont r Int calculateLength l = return (length l)
Here we use calculateLength by making it to pass its result to print:
main = do runCont (calculateLength "123") print -- result: 3
It is possible to chain Cont blocks with >>=.
double :: Int -> Cont r Int double n = return (n * 2) main = do runCont (calculateLength "123" >>= double) print -- result: 6
|Example 2: Using callCC|
This example gives a taste of how escape continuations work, shows a typical pattern for their usage.
-- Returns a string depending on the length of the name parameter. -- If the provided string is empty, returns an error. -- Otherwise, returns a welcome message. whatsYourName :: String -> String whatsYourName name = (`runCont` id) $ do -- 1 response <- callCC $ \exit -> do -- 2 validateName name exit -- 3 return $ "Welcome, " ++ name ++ "!" -- 4 return response -- 5 validateName name exit = do when (null name) (exit "You forgot to tell me your name!")
Here is what this example does:
|Example 3: Using ContT Monad Transformer|
ContT can be used to add continuation handling to other monads. Here is an example how to combine it with IO monad:
import Control.Monad.Cont import System.IO main = do hSetBuffering stdout NoBuffering runContT (callCC askString) reportResult askString :: (String -> ContT () IO String) -> ContT () IO String askString next = do liftIO $ putStrLn "Please enter a string" s <- liftIO $ getLine next s reportResult :: String -> IO () reportResult s = do putStrLn ("You entered: " ++ s)
Action askString requests user to enter a string, and passes it to the continuation. askString takes as a parameter a continuation taking a string parameter, and returning IO (). Compare its signature to runContT definition.
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