The monad-coroutine 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.
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 ())
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