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 ())
```

## Links