Cont package:monads-tf

• 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.

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.

The continuation monad transformer. Can be used to add continuation handling to any type constructor: the Monad instance and most of the operations do not require m to be a monad. ContT is not a functor on the category of monads, and many operations cannot be lifted through it.

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

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

The result of running a CPS computation with a given final continuation. (The inverse of cont)

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

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