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Control.Monad.Cont.Class

Portability | non-portable (multi-parameter type classes)
Stability | experimental
Maintainer | libraries@haskell.org

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.

Documentation

class Monad m => MonadCont m 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 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

MonadCont m => MonadCont (ListT m)
MonadCont (Cont r)
(Error e, MonadCont m) => MonadCont (ErrorT e m)
MonadCont m => MonadCont (ReaderT r m)
MonadCont m => MonadCont (StateT s m)
MonadCont m => MonadCont (StateT s m)
(Monoid w, MonadCont m) => MonadCont (WriterT w m)
(Monoid w, MonadCont m) => MonadCont (WriterT w m)
Monad 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)

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