Portability | portable |
---|---|

Stability | experimental |

Maintainer | libraries@haskell.org |

Continuation monads.

- 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

- 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
- callCC :: ((a -> ContT r m b) -> ContT r m a) -> ContT r m a
- liftLocal :: Monad m => m r' -> ((r' -> r') -> m r -> m r) -> (r' -> r') -> ContT r m a -> ContT r m a

# The Cont monad

type Cont r = ContT r IdentitySource

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 aSource

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

.)

:: Cont r a | continuation computation ( |

-> (a -> r) | the final continuation, which produces
the final result (often |

-> r |

Runs a CPS computation, returns its result after applying the final
continuation to it.
(The inverse of `cont`

.)

# The ContT monad transformer

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

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

`callCC`

(call-with-current-continuation) calls its argument
function, passing it the current continuation. It provides
an escape continuation mechanism for use with continuation
monads. Escape continuations one allow to abort the current
computation and return a value immediately. They achieve a
similar effect to `Control.Monad.Trans.Error.throwError`

and `Control.Monad.Trans.Error.catchError`

within an
`Control.Monad.Trans.Error.ErrorT`

monad. The 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.