/Haskell 8

# Control.Monad.Trans.Cont

Copyright (c) The University of Glasgow 2001 BSD-style (see the file LICENSE) [email protected] experimental portable Safe Haskell98

#### Description

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).

## 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`)

#### Arguments

 :: Cont r a continuation computation (`Cont`). -> (a -> r) the final continuation, which produces the final result (often `id`). -> r

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

evalCont :: Cont r r -> r Source

The result of running a CPS computation with the identity as the final continuation.

• `evalCont (return x) = x`

mapCont :: (r -> r) -> Cont r a -> Cont r a Source

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

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

withCont :: ((b -> r) -> a -> r) -> Cont r a -> Cont r b Source

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

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

### Delimited continuations

reset :: Cont r r -> Cont r' r Source

`reset m` delimits the continuation of any `shift` inside `m`.

• `reset (return m) = return m`

shift :: ((a -> r) -> Cont r r) -> Cont r a Source

`shift f` captures the continuation up to the nearest enclosing `reset` and passes it to `f`:

• `reset (shift f >>= k) = reset (f (evalCont . k))`

## The ContT monad transformer

newtype ContT r m a Source

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.

ContT

#### Fields

##### Instances
Instances details
MonadTrans (ContT r)
Instance details

Defined in Control.Monad.Trans.Cont

#### Methods

lift :: Monad m => m a -> ContT r m a Source

Monad (ContT r m)
Instance details

Defined in Control.Monad.Trans.Cont

#### Methods

(>>=) :: ContT r m a -> (a -> ContT r m b) -> ContT r m b Source

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

return :: a -> ContT r m a Source

Functor (ContT r m)
Instance details

Defined in Control.Monad.Trans.Cont

#### Methods

fmap :: (a -> b) -> ContT r m a -> ContT r m b Source

(<\$) :: a -> ContT r m b -> ContT r m a Source

MonadFail m => MonadFail (ContT r m)
Instance details

Defined in Control.Monad.Trans.Cont

#### Methods

fail :: String -> ContT r m a Source

Applicative (ContT r m)
Instance details

Defined in Control.Monad.Trans.Cont

#### Methods

pure :: a -> ContT r m a Source

(<*>) :: ContT r m (a -> b) -> ContT r m a -> ContT r m b Source

liftA2 :: (a -> b -> c) -> ContT r m a -> ContT r m b -> ContT r m c Source

(*>) :: ContT r m a -> ContT r m b -> ContT r m b Source

(<*) :: ContT r m a -> ContT r m b -> ContT r m a Source

MonadIO m => MonadIO (ContT r m)
Instance details

Defined in Control.Monad.Trans.Cont

#### Methods

liftIO :: IO a -> ContT r m a Source

evalContT :: Monad m => ContT r m r -> m r Source

The result of running a CPS computation with `return` as the final continuation.

• `evalContT (lift m) = m`

mapContT :: (m r -> m r) -> ContT r m a -> ContT r m a Source

Apply a function to transform the result of a continuation-passing computation. This has a more restricted type than the `map` operations for other monad transformers, because `ContT` does not define a functor in the category of monads.

• `runContT (mapContT f m) = f . runContT m`

withContT :: ((b -> m r) -> a -> m r) -> ContT r m a -> ContT r m b Source

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

• `runContT (withContT f m) = runContT m . f`

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.

### Delimited continuations

resetT :: Monad m => ContT r m r -> ContT r' m r Source

`resetT m` delimits the continuation of any `shiftT` inside `m`.

• `resetT (lift m) = lift m`

shiftT :: Monad m => ((a -> m r) -> ContT r m r) -> ContT r m a Source

`shiftT f` captures the continuation up to the nearest enclosing `resetT` and passes it to `f`:

• `resetT (shiftT f >>= k) = resetT (f (evalContT . k))`

## Lifting other operations

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

`liftLocal ask local` yields a `local` function for `ContT r m`.

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.8.3/docs/html/libraries/transformers-0.5.6.2/Control-Monad-Trans-Cont.html