Control.Monad.Cont (original) (raw)

Description

Computation type:

Computations which can be interrupted and resumed.

Binding strategy:

Binding a function to a monadic value creates a new continuation which uses the function as the continuation of the monadic computation.

Useful for:

Complex control structures, error handling, and creating co-routines.

Zero and plus:

None.

Example type:

`Cont` r a

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.

MonadCont class

class Monad m => MonadCont (m :: Type -> Type) where Source #

Methods

callCC :: ((a -> m b) -> m a) -> m a Source #

callCC (call-with-current-continuation) calls a function with the current continuation as its argument. Provides an escape continuation mechanism for use with Continuation monads. Escape continuations allow to abort the current computation and return a value immediately. They achieve a similar effect to [throwError](Control-Monad-Error-Class.html#v:throwError "Control.Monad.Error.Class") and [catchError](Control-Monad-Error-Class.html#v:catchError "Control.Monad.Error.Class") within an [Except](Control-Monad-Except.html#v:Except "Control.Monad.Except") monad. Advantage of this function over calling return is that it makes the continuation explicit, allowing more flexibility and better control (see examples in Control.Monad.Cont).

The standard idiom used with callCC is to provide a lambda-expression to name the continuation. Then calling the named continuation anywhere within its scope will escape from the computation, even if it is many layers deep within nested computations.

label :: MonadCont m => a -> m (a -> m b, a) Source #

Introduces a recursive binding to the continuation. Due to the use of callCC, calling the continuation will interrupt execution of the current block creating an effect similar to goto/setjmp in C.

Since: 2.3.1

type Cont r = ContT r Identity #

Continuation monad.Cont r a is a CPS ("continuation-passing style") computation that produces an intermediate result of type a within a CPS computation whose final result type is r.

The return function simply creates a continuation which passes the value on.

The >>= operator adds the bound function into the continuation chain.

cont :: ((a -> r) -> r) -> Cont r a #

Construct a continuation-passing computation from a function. (The inverse of [runCont](Control-Monad-Cont.html#v:runCont "Control.Monad.Cont"))

runCont #

Arguments

:: Cont r a continuation computation (Cont).
-> (a -> r) the final continuation, which produces the final result (often id).
-> r

The result of running a CPS computation with a given final continuation. (The inverse of [cont](Control-Monad-Cont.html#v:cont "Control.Monad.Cont"))

evalCont :: Cont r r -> r #

The result of running a CPS computation with the identity as the final continuation.

The ContT monad transformer

newtype ContT (r :: k) (m :: k -> Type) a #

The continuation monad transformer. Can be used to add continuation handling to any type constructor: the [Monad](/package/base-4.16.3.0/docs/Control-Monad.html#t:Monad "Control.Monad") instance and most of the operations do not require m to be a monad.

[ContT](Control-Monad-Cont.html#t:ContT "Control.Monad.Cont") is not a functor on the category of monads, and many operations cannot be lifted through it.

Constructors

| ContT ((a -> m r) -> m r) | | | ------------------------- | |

mapContT :: forall {k} m (r :: k) a. (m r -> m r) -> ContT r m a -> ContT r m a #

Apply a function to transform the result of a continuation-passing computation. This has a more restricted type than the map operations for other monad transformers, because [ContT](Control-Monad-Cont.html#t:ContT "Control.Monad.Cont") does not define a functor in the category of monads.

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:

  1. Runs an anonymous Cont block and extracts value from it with(`runCont` id). Here id is the continuation, passed to the Cont block.
  2. Binds response to the result of the following [callCC](Control-Monad-Cont-Class.html#v:callCC "Control.Monad.Cont.Class") block, binds exit to the continuation.
  3. Validates name. This approach illustrates advantage of using [callCC](Control-Monad-Cont-Class.html#v:callCC "Control.Monad.Cont.Class") over return. We pass the continuation to validateName, and interrupt execution of the Cont block from inside of validateName.
  4. Returns the welcome message from the [callCC](Control-Monad-Cont-Class.html#v:callCC "Control.Monad.Cont.Class") block. This line is not executed if validateName fails.
  5. Returns from the Cont block.

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.

Example 4: Using label

The early exit behavior of [callCC](Control-Monad-Cont-Class.html#v:callCC "Control.Monad.Cont.Class") can be leveraged to produce other idioms:

whatsYourNameLabel :: IO () whatsYourNameLabel = evalContT $ do (beginning, attempts) <- label (0 :: Int) liftIO $ putStrLn $ "Attempt #" <> show attempts liftIO $ putStrLn $ "What's your name?" name <- liftIO getLine when (null name) $ beginning (attempts + 1) liftIO $ putStrLn $ "Welcome, " ++ name ++ "!"

Calling beggining will interrupt execution of the block, skipping the welcome message, which will be printed only once at the very end of the loop.