|Portability||non-portable (multi-parameter type classes)|
- 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
- Useful for:
- Complex control structures, error handling,
and creating co-routines.
- Zero and plus:
- Example type:
- Cont r a
The Continuation monad represents computations in continuation-passing style
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
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
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.
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.
|runCont :: ((a -> r) -> r)|
Runs a CPS computation, returns its result after applying
the final continuation to it.
- a continuation computation (Cont).
- the final continuation, which produces the final result (often id).
|The continuation monad transformer.
Can be used to add continuation handling to other monads.
|runContT :: ((a -> m r) -> m r)|
|Example 1: Simple Continuation Usage
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
|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). Here id is the continuation, passed to the Cont block.
- Binds response to the result of the following callCC block,
binds exit to the continuation.
- Validates name.
This approach illustrates advantage of using callCC over return.
We pass the continuation to validateName,
and interrupt execution of the Cont block from inside of validateName.
- Returns the welcome message from the callCC block.
This line is not executed if validateName 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:
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
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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