transformers- Concrete monad transformers





Continuation monads.


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



:: Cont r a

continuation computation (Cont).

-> (a -> r)

the final continuation, which produces the final result (often id).

-> r 

Runs a CPS computation, returns its result after applying the final continuation to it.

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.




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


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

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.