Portability non-portable (type families) Stability experimental Maintainer libraries@haskell.org
 Contents MonadCont class The Cont monad The ContT monad transformer Example 1: Simple Continuation Usage Example 2: Using callCC Example 3: Using ContT Monad Transformer
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.

Synopsis
 callCC :: ((a -> m b) -> m a) -> m a
type Cont r = ContT r Identity
runCont :: Cont r a -> (a -> r) -> r
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 = ContT {
 runContT :: (a -> m r) -> m r
}
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
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 Control.Monad.Error.throwError and Control.Monad.Error.catchError within an Control.Monad.Error.Error 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.

Instances
 type Cont r = ContT r Identity Source

Continuation monad. Cont r a is a CPS 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.

 runCont Source
 :: => Cont r a the final continuation, which produces the final result (often id). -> a -> r -> r Runs a CPS computation, returns its result after applying the final continuation to it.
 mapCont :: (r -> r) -> Cont r a -> Cont r a Source
 withCont :: ((b -> r) -> a -> r) -> Cont r a -> Cont r b Source
 newtype ContT r m a Source
Constructors
ContT
 runContT :: (a -> m r) -> m r
Instances
 mapContT :: (m r -> m r) -> ContT r m a -> ContT r m a Source
 withContT :: ((b -> m r) -> a -> m r) -> ContT r m a -> ContT r m b Source
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 block, binds exit to the continuation.
3. Validates name. This approach illustrates advantage of using callCC 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 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

askString :: (String -> ContT () IO String) -> ContT () IO String