Portability	portable
Stability	experimental
Maintainer	ross@soi.city.ac.uk
Safe Haskell	Safe-Inferred

Control.Monad.Trans.Cont

Contents

The Cont monad
- Delimited continuations
The ContT monad transformer
- Delimited continuations
Lifting other operations

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).

Synopsis

The Cont monad

type Cont r = ContT r Identity Source

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)

runCont Source

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 -> rSource

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 aSource

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 bSource

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' rSource

reset m delimits the continuation of any shift inside m.

```
reset (return m) = return m
```

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

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 other monads.

Constructors

ContT ((a -> m r) -> m r)

Instances

MonadTrans (ContT r)
Monad (ContT r m)
Functor (ContT r m)
Applicative (ContT r m)
MonadIO m => MonadIO (ContT r m)

runContT :: ContT r m a -> (a -> m r) -> m rSource

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

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

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 aSource

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

runContT (mapContT f m) = f . runContT m

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

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 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 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 rSource

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 aSource

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 aSource

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