layers-0.1: Modular type class machinery for monad transformer stacks.

Safe HaskellNone



This module exports:

  1. The MonadCont type class and its operation callCC.
  2. An instance of MonadCont for the ContT monad transformer from the transformers package.
  3. A universal pass-through instance of MonadCont for any existing MonadCont wrapped by a MonadLayerControl.



class Monad m => MonadCont m whereSource

The MonadCont interface 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 MonadCont interface adapts CPS to the structure of a monad.

Before using the MonadCont interface, 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 MonadCont interface can produce code that is impossible to understand and maintain.


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

callCC (call-with-current-continuation) calls a function with the current continuation as its argument. Provides an escape continuation mechanism for use with instances of MonadCont. Escape continuations allow to abort the current computation and return a value immediately. They achieve a similar effect to throw and catch from the MonadException interface. Advantage of this function over calling return is that it makes the continuation explicit, allowing more flexibility and better control.

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.