FRP框架和IO

Question

I've just been investigating FRP in Haskell (mainly reactive-banana (testimonials!)), and I'm wondering is it common for them, and in case of reactive-banana what's the reason to operate in IO monad instead of being constrained to MonadIO , or rven better, any Monad (so that I could provide own stack)?

It seems to me that's because of focus on GUI programming, where you integrate it with some existing libraries that operate in IO ? Would it be feasible to change it so that I could integrate it with custom monad stack?

Answer 1

If you are asking why

reactimate :: Frameworks t => Event t (IO ()) -> Moment t ()

expects an event with values of type IO () instead of allowing for a custom monad M () with instance MonadIO M , then the answer is this:

In practice, custom monad stacks only add state/reader/writer effects to the base IO monad. However, in theory, it is entirely possible to add fancy control mechanism like coroutines or non-determinism. I don't know how to integrate the internal state arising from combinators accumE with these more general effects, and I have no idea what it means for an event to happen in a non-deterministic context. That's why reactimate is restricted to IO .

If you have a custom monad stack that is of the state/reader/writer family, then it is usually possible to map it to a pure IO computation and use this with reactimate . If you find that this doesn't work out (I do think that there might be a problem), I would need a more detailed description of the concrete situation to be able to help.