Control.Monad.Cont

mtl-2.2.1: Monad classes, using functional dependencies

Copyright | (c) The University of Glasgow 2001, (c) Jeff Newbern 2003-2007, (c) Andriy Palamarchuk 2007
License | BSD-style (see the file LICENSE)
Maintainer | libraries@haskell.org
Stability | experimental
Portability | portable
Safe Haskell | Safe
Language | Haskell98

Control.Monad.Cont

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

- class Monad m => MonadCont m where
  - callCC :: ((a -> m b) -> m a) -> m a
- type Cont r = ContT r Identity
- cont :: ((a -> r) -> r) -> Cont r a
- 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
  }
- runContT :: ContT r m a -> (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
- module Control.Monad
- module Control.Monad.Trans

# MonadCont class

class Monad m => MonadCont m where

Methods

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

`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` and `catchError` within an `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

MonadCont m => MonadCont (MaybeT m) |
MonadCont m => MonadCont (ListT m) |
MonadCont m => MonadCont (IdentityT m) |
(Monoid w, MonadCont m) => MonadCont (WriterT w m) |
(Monoid w, MonadCont m) => MonadCont (WriterT w m) |
(Error e, MonadCont m) => MonadCont (ErrorT e m) |
MonadCont m => MonadCont (ExceptT e m) |
MonadCont m => MonadCont (StateT s m) |
MonadCont m => MonadCont (StateT s m) |
MonadCont m => MonadCont (ReaderT r m) |
MonadCont (ContT r m) |
(Monoid w, MonadCont m) => MonadCont (RWST r w s m) |
(Monoid w, MonadCont m) => MonadCont (RWST r w s m) |

# The Cont monad

type Cont r = ContT r Identity

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.

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

Construct a continuation-passing computation from a function. (The inverse of `runCont`)

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`)

mapCont :: (r -> r) -> Cont r a -> Cont r a

Apply a function to transform the result of a continuation-passing computation.

-     runCont (mapCont f m) = f . runCont m

withCont :: ((b -> r) -> a -> r) -> Cont r a -> Cont r b

Apply a function to transform the continuation passed to a CPS computation.

-     runCont (withCont f m) = runCont m . f

# The ContT monad transformer

newtype ContT r m a :: * -> (* -> *) -> * -> *

The continuation monad transformer. Can be used to add continuation handling to other monads.

Constructors

ContT |
Fields runContT :: (a -> m r) -> m r

Instances

MonadState s m => MonadState s (ContT r m) |
MonadReader r' m => MonadReader r' (ContT r m) |
MonadTrans (ContT r) |
Monad (ContT r m) |
Functor (ContT r m) |
Applicative (ContT r m) |
MonadIO m => MonadIO (ContT r m) |
MonadCont (ContT r m) |

runContT :: ContT r m a -> (a -> m r) -> m r

mapContT :: (m r -> m r) -> ContT r m a -> ContT r m a

Apply a function to transform the result of a continuation-passing computation.

-     runContT (mapContT f m) = f . runContT m

withContT :: ((b -> m r) -> a -> m r) -> ContT r m a -> ContT r m b

Apply a function to transform the continuation passed to a CPS computation.

-     runContT (withContT f m) = runContT m . f

module Control.Monad

module Control.Monad.Trans

# 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
      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.