- 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.