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 correctness. 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? Hans 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 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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If you are not looking for the full reactive formalism but to treat event driven applications in a procedural ,sequential, imperative way (whatever you may call it) by means o continuations, then this is a good paper in the context of web applications: inverting back the inversion of control http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.29.3112 2013/4/24 Conal Elliott

Hi Hans.

I'm delighted to hear that you have a precise denotation to define correctness of your implementation. So much of what gets called "FRP" these days abandons any denotational foundation, as well as continuous time, which have always been the two key properties of FRPhttp://stackoverflow.com/a/5878525/127335for me.

I like your goal of finding a provably correct (perhaps correct by construction/derivation) implementation of the simple denotational semantics. I'm happy to give feedback and pointers if you continue with this goal.

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

I suppose so, although I'd say it the other way around: things that lack denotation are not applicable for fulfilling denotational principles. Which suggests to me that IO will not get you to your goal. Instead, I recommend instead looking for a subset of imperative computation that suffices to implement the denotation you want, but is well-defined denotationally and tractable for reasoning. IO (general imperative computation) is neither, which is why we have denotative/functional programming in the first place.

Regards, - Conal

On Wed, Apr 24, 2013 at 8:31 AM, Hans Höglund wrote:

...

Hi Conal,

Thank you for replying.

My aim is to find the simplest possible implementation of the semantics you describe in Push-pull FRP http://conal.net/papers/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 correctness.

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?

Hans

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 morphismshttp://conal.net/papers/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 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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-- Alberto.

I first tried an imperative push-based FRP in 1998, and I had exactly the same experience as Heinrich mentions. The toughest two aspects of imperative implementation were sharing and event merge/union/mappend. -- Conal On Thu, Apr 25, 2013 at 5:16 AM, Heinrich Apfelmus < apfelmus@quantentunnel.de> wrote:

Hans Höglund wrote:

...

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.

In reactive-banana, the Reactive.Banana.Model module implements / defines the denotational semantics.

http://tinyurl.com/Reactive-**Banana-Modelhttp://tinyurl.com/Reactive-Banana-Model

It's very similar to Conal's semantics from the Push-Pull article.

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 correctness.

In my experience, the main problem with imperative implementations is that they have a hard time with the union combinator and with preserving sharing. Reactive-banana is the best I could come up with.

Best regards, Heinrich Apfelmus

-- http://apfelmus.nfshost.com

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