transformers-0.2.2.0: Concrete functor and monad transformersSource codeContentsIndex
Control.Monad.Trans.Cont
Portabilityportable
Stabilityexperimental
Maintainerross@soi.city.ac.uk
Contents
The Cont monad
The ContT monad transformer
Lifting other operations
Description
Continuation monads.
Synopsis
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.)
runContSource
:: Cont r acontinuation computation (Cont).
-> a -> rthe final continuation, which produces the final result (often id).
-> r
Runs a CPS computation, returns its result after applying the final continuation to it. (The inverse of cont.)
mapCont :: (r -> r) -> Cont r a -> Cont r aSource
withCont :: ((b -> r) -> a -> r) -> Cont r a -> Cont r bSource
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
runContT :: (a -> m r) -> m r
show/hide Instances
mapContT :: (m r -> m r) -> ContT r m a -> ContT r m aSource
withContT :: ((b -> m r) -> a -> m r) -> ContT r m a -> ContT r m bSource
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.

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.
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