Package 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))

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)