[Haskell-cafe] Instances for continuation-based FRP

Hans Höglund hans at hanshoglund.se
Tue Apr 23 15:22:37 CEST 2013

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,
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