# monad-coroutine

This library, implemented by the `Control.Monad.Coroutine` module, provides a limited coroutine functionality in Haskell. The centerpiece of the approach is the monad transformer `Coroutine`, which transforms an arbitrary monadic computation into a suspendable and resumable one. The basic definition is simple:

newtype Coroutine s m r = Coroutine {resume :: m (Either (s (Coroutine s m r)) r)} instance (Functor s, Monad m) => Monad (Coroutine s m) where return = Coroutine . return . Right t >>= f = Coroutine (resume t >>= either (return . Left . fmap (>>= f)) (resume . f))

## Suspension Functors

The Coroutine transformer type is parameterized by a functor. The functor in question wraps the resumption of a suspended coroutine, and it can carry other information as well. Module `Control.Monad.Coroutine.SuspensionFunctors` exports some useful functors, one of which is `Yield`:

data Yield x y = Yield x y instance Functor (Yield x) where fmap f (Yield x y) = Yield x (f y)

A coroutine parameterized by this functor is a generator which yields a value every time it suspends. For example, the following function generates the program's command-line arguments:

genArgs :: Coroutine (Yield String) IO () genArgs = getArgs >>= mapM_ yield

The `Await` functor is dual to `Yield`; a coroutine that suspends using this functor is a consumer coroutine that on every suspension expects to be given a value before it resumes. The following example is a consumer coroutine that prints every received value to standard output:

printer :: Show x => Coroutine (Await x) IO () printer = await >>= print >> printer

While these two are the most obvious suspension functors, any functor whatsoever can be used as a coroutine suspension functor. See ListSuspension, for example.

## Running a coroutine

After a coroutine suspends, the suspension functor must be unpacked to get to the coroutine resumption. Here's an example of how the *printer* example could be run:

printerFeeder :: Show x => [x] -> Coroutine (Await x) IO () -> IO () printerFeeder [] _ = return () printerFeeder (head:tail) printer = do p <- resume printer case p of Left (Await p') -> printerFeeder tail (p' head) Right result -> return result

Alternatively, you can use the function `pogoStick` or `foldRun` to the same effect:

printerFeeder feed printer = liftM snd $ foldRun f feed printer where f (head:tail) (Await p) = (tail, p head) f [] _ = ([], return ())

## Links

- Download and API documentation is available at Hackage
- The current source code is in a Darcs repository at http://code.haskell.org/SCC/