Copyright | (c) The University of Glasgow 2001 |
---|---|

License | BSD-style (see the file LICENSE) |

Maintainer | [email protected] |

Stability | experimental |

Portability | portable |

Safe Haskell | Safe |

Language | Haskell98 |

Continuation monads.

Delimited continuation operators are taken from Kenichi Asai and Oleg Kiselyov's tutorial at CW 2011, "Introduction to programming with shift and reset" (http://okmij.org/ftp/continuations/#tutorial).

## Synopsis

- type Cont r = ContT r Identity
- cont :: ((a -> r) -> r) -> Cont r a
- runCont :: Cont r a -> (a -> r) -> r
- evalCont :: Cont r r -> r
- mapCont :: (r -> r) -> Cont r a -> Cont r a
- withCont :: ((b -> r) -> a -> r) -> Cont r a -> Cont r b
- reset :: Cont r r -> Cont r' r
- shift :: ((a -> r) -> Cont r r) -> Cont r a
- newtype ContT r m a = ContT {
- runContT :: (a -> m r) -> m r

- evalContT :: Monad m => ContT r 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
- resetT :: Monad m => ContT r m r -> ContT r' m r
- shiftT :: Monad m => ((a -> m r) -> ContT r m r) -> 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 Identity Source #

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.

cont :: ((a -> r) -> r) -> Cont r a Source #

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 |

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

)

## Delimited continuations

# The ContT monad transformer

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.

callCC :: ((a -> ContT r m b) -> ContT r m a) -> ContT r m a Source #

`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 `throwE`

and `catchE`

within an
`ExceptT`

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.