Portability  portable 

Stability  experimental 
Maintainer  ross@soi.city.ac.uk 
 Computation type:
 Computations which can be interrupted and resumed.
 Binding strategy:
 Binding a function to a monadic value creates a new continuation which uses the function as the continuation of the monadic computation.
 Useful for:
 Complex control structures, error handling, and creating coroutines.
 Zero and plus:
 None.
 Example type:

Cont
r a
The Continuation monad represents computations in continuationpassing style
(CPS).
In continuationpassing style function result is not returned,
but instead is passed to another function,
received as a parameter (continuation).
Computations are built up from sequences
of nested continuations, terminated by a final continuation (often id
)
which produces the final result.
Since continuations are functions which represent the future of a computation,
manipulation of the continuation functions can achieve complex manipulations
of the future of the computation,
such as interrupting a computation in the middle, aborting a portion
of a computation, restarting a computation, and interleaving execution of
computations.
The Continuation monad adapts CPS to the structure of a monad.
Before using the Continuation monad, be sure that you have a firm understanding of continuationpassing style and that continuations represent the best solution to your particular design problem. Many algorithms which require continuations in other languages do not require them in Haskell, due to Haskell's lazy semantics. Abuse of the Continuation monad can produce code that is impossible to understand and maintain.
 class Monad m => MonadCont m where
 callCC :: ((a > m b) > m a) > m a
 type Cont r = ContT r Identity
 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
 module Control.Monad
 module Control.Monad.Trans
MonadCont class
class Monad m => MonadCont m where
callCC :: ((a > m b) > m a) > m a
callCC
(callwithcurrentcontinuation)
calls a function with the current continuation as its argument.
Provides an escape continuation mechanism for use with Continuation monads.
Escape continuations allow to abort the current computation and return
a value immediately.
They achieve a similar effect to Control.Monad.Error.throwError
and Control.Monad.Error.catchError
within an Control.Monad.Error.Error
monad.
Advantage of this function over calling return
is that it makes
the continuation explicit,
allowing more flexibility and better control
(see examples in Control.Monad.Cont).
The standard idiom used with callCC
is to provide a lambdaexpression
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.
MonadCont m => MonadCont (ListT m)  
MonadCont m => MonadCont (MaybeT m)  
MonadCont m => MonadCont (IdentityT m)  
MonadCont (ContT r m)  
(Error e, MonadCont m) => MonadCont (ErrorT e m)  
MonadCont m => MonadCont (ReaderT r m)  
MonadCont m => MonadCont (StateT s m)  
MonadCont m => MonadCont (StateT s m)  
(Monoid w, MonadCont m) => MonadCont (WriterT w m)  
(Monoid w, MonadCont m) => MonadCont (WriterT w m)  
(Monoid w, MonadCont m) => MonadCont (RWST r w s m)  
(Monoid w, MonadCont m) => MonadCont (RWST r w s m) 
The Cont monad
type Cont r = ContT r Identity
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 r a  continuation computation ( 
> (a > r)  the final continuation, which produces
the final result (often 
> r 
Runs a CPS computation, returns its result after applying the final
continuation to it.
(The inverse of cont
.)
The ContT monad transformer
newtype ContT r m a
The continuation monad transformer. Can be used to add continuation handling to other monads.
MonadReader r' m => MonadReader r' (ContT r m)  
MonadState s m => MonadState s (ContT r m)  
MonadTrans (ContT r)  
Monad (ContT r m)  
Functor (ContT r m)  
Applicative (ContT r m)  
MonadIO m => MonadIO (ContT r m)  
MonadCont (ContT r m) 
module Control.Monad
module Control.Monad.Trans
Example 1: Simple Continuation Usage
Calculating length of a list continuationstyle:
calculateLength :: [a] > Cont r Int calculateLength l = return (length l)
Here we use calculateLength
by making it to pass its result to print
:
main = do runCont (calculateLength "123") print  result: 3
It is possible to chain Cont
blocks with >>=
.
double :: Int > Cont r Int double n = return (n * 2) main = do runCont (calculateLength "123" >>= double) print  result: 6
Example 2: Using callCC
This example gives a taste of how escape continuations work, shows a typical pattern for their usage.
 Returns a string depending on the length of the name parameter.  If the provided string is empty, returns an error.  Otherwise, returns a welcome message. whatsYourName :: String > String whatsYourName name = (`runCont` id) $ do  1 response < callCC $ \exit > do  2 validateName name exit  3 return $ "Welcome, " ++ name ++ "!"  4 return response  5 validateName name exit = do when (null name) (exit "You forgot to tell me your name!")
Here is what this example does:
 Runs an anonymous
Cont
block and extracts value from it with(`runCont` id)
. Hereid
is the continuation, passed to theCont
block.  Binds
response
to the result of the followingcallCC
block, bindsexit
to the continuation.  Validates
name
. This approach illustrates advantage of usingcallCC
overreturn
. We pass the continuation tovalidateName
, and interrupt execution of theCont
block from inside ofvalidateName
.  Returns the welcome message from the
callCC
block. This line is not executed ifvalidateName
fails.  Returns from the
Cont
block.
Example 3: Using ContT
Monad Transformer
ContT
can be used to add continuation handling to other monads.
Here is an example how to combine it with IO
monad:
import Control.Monad.Cont import System.IO main = do hSetBuffering stdout NoBuffering runContT (callCC askString) reportResult askString :: (String > ContT () IO String) > ContT () IO String askString next = do liftIO $ putStrLn "Please enter a string" s < liftIO $ getLine next s reportResult :: String > IO () reportResult s = do putStrLn ("You entered: " ++ s)
Action askString
requests user to enter a string,
and passes it to the continuation.
askString
takes as a parameter a continuation taking a string parameter,
and returning IO ()
.
Compare its signature to runContT
definition.