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{- | | ||
Module : Control.Monad.Cont | ||
Copyright : (c) The University of Glasgow 2001, | ||
(c) Jeff Newbern 2003-2007, | ||
(c) Andriy Palamarchuk 2007 | ||
License : BSD-style (see the file LICENSE) | ||
Maintainer : libraries@haskell.org | ||
Stability : experimental | ||
Portability : portable | ||
[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 co-routines. | ||
[Zero and plus:] None. | ||
[Example type:] @'Cont' r a@ | ||
The Continuation monad represents computations in continuation-passing style | ||
(CPS). | ||
In continuation-passing 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 continuation-passing 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. | ||
-} | ||
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module Control.Monad.Cont ( | ||
-- * MonadCont class | ||
MonadCont(..), | ||
-- * The Cont monad | ||
Cont, | ||
cont, | ||
runCont, | ||
mapCont, | ||
withCont, | ||
-- * The ContT monad transformer | ||
ContT(..), | ||
mapContT, | ||
withContT, | ||
module Control.Monad, | ||
module Control.Monad.Trans, | ||
-- * Example 1: Simple Continuation Usage | ||
-- $simpleContExample | ||
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-- * Example 2: Using @callCC@ | ||
-- $callCCExample | ||
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-- * Example 3: Using @ContT@ Monad Transformer | ||
-- $ContTExample | ||
) where | ||
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import Control.Monad.Cont.Class | ||
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import Control.Monad.Trans | ||
import Control.Monad.Trans.Cont | ||
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import Control.Monad | ||
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{- $simpleContExample | ||
Calculating length of a list continuation-style: | ||
>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 | ||
-} | ||
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{- $callCCExample | ||
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: | ||
(1) Runs an anonymous 'Cont' block and extracts value from it with | ||
@(\`runCont\` id)@. Here @id@ is the continuation, passed to the @Cont@ block. | ||
(1) Binds @response@ to the result of the following 'Control.Monad.Cont.Class.callCC' block, | ||
binds @exit@ to the continuation. | ||
(1) Validates @name@. | ||
This approach illustrates advantage of using 'Control.Monad.Cont.Class.callCC' over @return@. | ||
We pass the continuation to @validateName@, | ||
and interrupt execution of the @Cont@ block from /inside/ of @validateName@. | ||
(1) Returns the welcome message from the 'Control.Monad.Cont.Class.callCC' block. | ||
This line is not executed if @validateName@ fails. | ||
(1) Returns from the @Cont@ block. | ||
-} | ||
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{-$ContTExample | ||
'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. | ||
-} |
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{- | | ||
Module : Control.Monad.Cont.Class | ||
Copyright : (c) The University of Glasgow 2001, | ||
(c) Jeff Newbern 2003-2007, | ||
(c) Andriy Palamarchuk 2007 | ||
License : BSD-style (see the file LICENSE) | ||
Maintainer : libraries@haskell.org | ||
Stability : experimental | ||
Portability : portable | ||
[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 co-routines. | ||
[Zero and plus:] None. | ||
[Example type:] @'Cont' r a@ | ||
The Continuation monad represents computations in continuation-passing style | ||
(CPS). | ||
In continuation-passing 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 continuation-passing 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. | ||
-} | ||
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module Control.Monad.Cont.Class ( | ||
MonadCont(..), | ||
) where | ||
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import Control.Monad.Trans.Cont (ContT) | ||
import qualified Control.Monad.Trans.Cont as ContT | ||
import Control.Monad.Trans.Error as Error | ||
import Control.Monad.Trans.Identity as Identity | ||
import Control.Monad.Trans.List as List | ||
import Control.Monad.Trans.Maybe as Maybe | ||
import Control.Monad.Trans.Reader as Reader | ||
import Control.Monad.Trans.RWS.Lazy as LazyRWS | ||
import Control.Monad.Trans.RWS.Strict as StrictRWS | ||
import Control.Monad.Trans.State.Lazy as LazyState | ||
import Control.Monad.Trans.State.Strict as StrictState | ||
import Control.Monad.Trans.Writer.Lazy as LazyWriter | ||
import Control.Monad.Trans.Writer.Strict as StrictWriter | ||
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import Control.Monad | ||
import Data.Monoid | ||
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class (Monad m) => MonadCont m where | ||
{- | @callCC@ (call-with-current-continuation) | ||
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 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. | ||
-} | ||
callCC :: ((a -> m b) -> m a) -> m a | ||
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instance MonadCont (ContT r m) where | ||
callCC = ContT.callCC | ||
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-- --------------------------------------------------------------------------- | ||
-- Instances for other mtl transformers | ||
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instance (Error e, MonadCont m) => MonadCont (ErrorT e m) where | ||
callCC = Error.liftCallCC callCC | ||
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instance (MonadCont m) => MonadCont (IdentityT m) where | ||
callCC = Identity.liftCallCC callCC | ||
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instance (MonadCont m) => MonadCont (ListT m) where | ||
callCC = List.liftCallCC callCC | ||
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instance (MonadCont m) => MonadCont (MaybeT m) where | ||
callCC = Maybe.liftCallCC callCC | ||
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instance (MonadCont m) => MonadCont (ReaderT r m) where | ||
callCC = Reader.liftCallCC callCC | ||
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instance (Monoid w, MonadCont m) => MonadCont (LazyRWS.RWST r w s m) where | ||
callCC = LazyRWS.liftCallCC' callCC | ||
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instance (Monoid w, MonadCont m) => MonadCont (StrictRWS.RWST r w s m) where | ||
callCC = StrictRWS.liftCallCC' callCC | ||
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instance (MonadCont m) => MonadCont (LazyState.StateT s m) where | ||
callCC = LazyState.liftCallCC' callCC | ||
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instance (MonadCont m) => MonadCont (StrictState.StateT s m) where | ||
callCC = StrictState.liftCallCC' callCC | ||
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instance (Monoid w, MonadCont m) => MonadCont (LazyWriter.WriterT w m) where | ||
callCC = LazyWriter.liftCallCC callCC | ||
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instance (Monoid w, MonadCont m) => MonadCont (StrictWriter.WriterT w m) where | ||
callCC = StrictWriter.liftCallCC callCC |
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