My try for a CoroutineT monad tranformer
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 -- Joachim Breitner e-Mail: mail@joachim-breitner.de Homepage: http://www.joachim-breitner.de ICQ#: 74513189 Jabber-ID: nomeata@joachim-breitner.de
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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Hi, Am Freitag, den 25.04.2008, 11:49 -0700 schrieb Dan Weston:
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
You are quite right, it really is the same thing. The implementation behind runCoroutineT is not just a specialization, but the exact same thing (with Left and Right switched). I just put the specialization there because I had no need for a return value in my use case. And interesting how Ryan and me had the same thoughts on the same day. Maybe the April 24th should be considered Suspend You Monadic Action Day. Greetings, Joachim -- Joachim Breitner e-Mail: mail@joachim-breitner.de Homepage: http://www.joachim-breitner.de ICQ#: 74513189 Jabber-ID: nomeata@joachim-breitner.de
Is there a Haskell Wiki page (or blog) on Monad Suspension? This looks like a nice paradigm that apfelmus points out "can be used to considerably shorten your code", but only if the rest of us learn how! If not, maybe someone can be persuaded to write one? Dan Joachim Breitner wrote:
Hi,
Am Freitag, den 25.04.2008, 11:49 -0700 schrieb Dan Weston:
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
You are quite right, it really is the same thing. The implementation behind runCoroutineT is not just a specialization, but the exact same thing (with Left and Right switched). I just put the specialization there because I had no need for a return value in my use case.
And interesting how Ryan and me had the same thoughts on the same day. Maybe the April 24th should be considered Suspend You Monadic Action Day.
Greetings, Joachim
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On Fri, Apr 25, 2008 at 3:45 PM, Dan Weston
Is there a Haskell Wiki page (or blog) on Monad Suspension? This looks like a nice paradigm that apfelmus points out "can be used to considerably shorten your code", but only if the rest of us learn how!
There are a few papers which deal with resumption monads, which appear
to be closely related.
You can also express CoroutineT (or something very much like it) using
a free monad.
data Term f a = Var a | Branch (f (Term f a))
instance Functor f => Monad (Term f) where
return = Var
Var a >>= f = f a
Branch as >>= f = Branch (fmap (>>= f) as)
lift :: (Functor f) => f a -> Term f a
lift m = Branch (fmap Var m)
runTerm :: (Monad m) => Term m () -> m (Maybe (Term m ()))
runTerm (Var ()) = return Nothing
runTerm (Branch m) = fmap Just m
pause :: (Monad m) => Term m ()
pause = Branch (return (Var ()))
Note that runTerm and pause really only require Applicative.
I believe Suspend can be implemented similarly. Note that "SuspendT v
m a" is isomorphic to "m (Term (ReaderT v m) a)".
--
Dave Menendez
participants (3)
-
Dan Weston -
David Menendez -
Joachim Breitner