Hi Conal,

Thank you for replying.

My aim is to find the simplest possible implementation of the semantics you 
describe in Push-pull FRP, so the denotational semantics are already in place. 
I guess what I am looking for is a simple translation of a denotational program 
into an imperative one. My intuition tells me that such a translation is 
possible, maybe even trivial, but I am not sure how to reason about 

While I like the idea of TCMs very much, they do not seem to be applicable for 
things that lack a denotation, such as IO. Maybe it is a question of how to 
relate denotational semantics to operational ones?


On 24 apr 2013, at 02:18, Conal Elliott wrote:

> Hi Hans,
> Do you have a denotation for your representation (a specification for your 
> implementation)? If so, it will likely guide you to exactly the right type 
> class instances, via the principle of type class morphisms (TCMs). If you 
> don't have a denotation, I wonder how you could decide what correctness means 
> for any aspect of your implementation.
> Good luck, and let me know if you want some help exploring the TCM process,
> -- Conal
> On Tue, Apr 23, 2013 at 6:22 AM, Hans Höglund <h...@hanshoglund.se> wrote:
> Hi everyone,
> I am experimenting with various implementation styles for classical FRP. My 
> current thoughts are on a continuation-style push implementation, which can 
> be summarized as follows.
> > newtype EventT m r a    = E { runE :: (a -> m r) -> m r -> m r }
> > newtype ReactiveT m r a = R { runR :: (m a -> m r) -> m r }
> > type Event    = EventT IO ()
> > type Reactive = ReactiveT IO ()
> The idea is that events allow subscription of handlers, which are 
> automatically unsubscribed after the continuation has finished, while 
> reactives allow observation of a shared state until the continuation has 
> finished.
> I managed to write the following Applicative instance
> > instance Applicative (ReactiveT m r) where
> >     pure a      = R $ \k -> k (pure a)
> >     R f <*> R a = R $ \k -> f (\f' -> a (\a' -> k $ f' <*> a'))
> But I am stuck on finding a suitable Monad instance. I notice the similarity 
> between my types and the ContT monad and have a feeling this similarity could 
> be used to clean up my instance code, but am not sure how to proceed. Does 
> anyone have an idea, or a pointer to suitable literature.
> Best regards,
> Hans
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