I guess like minds think alike! See the very recent e-mail thread started by Ryan Ingram: http://thread.gmane.org/gmane.comp.lang.haskell.cafe/39155/focus=39159 Take a look at the code referenced in Luke Palmer's reply: http://luqui.org/git/?p=luqui-misc.git;a=blob;f=work/code/haskell/frp/Fregl/... A snippet follows:
class (Monad m) => MonadSuspend v m | m -> v where attempt :: m a -> m (Either a (v -> m a)) suspend :: m v
newtype SuspendT v m a = SuspendT { runSuspendT :: m (Either a (v -> SuspendT v m a)) }
Your (Coroutine m a) appears to be isomorphic to (SuspendT () m a) [so Coroutine m () = SuspendT () m ()] Your runCoroutineT appears to be isomorphic to a specialization of runSuspendT: runSuspendT' :: SuspendT () m () -> m (Either () (() -> SuspendT () m ())) Here the () -> a ~ a and Either () a ~ Maybe a Dan Joachim Breitner wrote:
Hi,
(for those who follow planet.haskell.org this is old news, but I thought I’d tell the others)
In http://www.joachim-breitner.de/blog/archives/291-Pausable-IO-actions-for-bet... I describe how I wrote a monad transformer that allows me to pause a computation from within by returning another computation that I can use to re-start the computation (or to throw it away if I want). I needed this for a long running drawing computation in a gtk2hs program that I want to pause at convenient points (to allow user interaction), and that I need to abort when what I’m drawing is not up-to-date anymore.
The API basically consists of the function
runCoroutineT :: Monad m => CoroutineT m () -> m (Maybe (CoroutineT m ())) which runs the pausable computation, any Maybe returns Just the resume action, and the function pause :: Monad m => CoroutineT m () to be used inside the computation, which pauses it.
I have put the complete module in the darcs repository that might later also contain the GUI program at http://darcs.nomeata.de/FrakView/
What do you think of CoroutineT? Could it have been easily implemented using the existing monad transformers? Is it useful enough so that it should be included somewhere, and where? Are there any problems with strictness or other tripping points that I have overlooked?
Greetings, Joachim
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