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 ())