Both `reflex`[1] and `frpnow`[2] provide Monad instance for behavior. AFAIK only `reactive-banana` and arrowized FRP libraries don't have it. [1] https://github.com/reflex-frp/reflex/blob/develop/src/Reflex/Class.hs#L598 [2] https://hackage.haskell.org/package/frpnow-0.18/docs/Control-FRPNow-Core.htm... http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail Virus-free. www.avg.com http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> 2018-01-20 19:00 GMT+05:00 martin :

Hello all,

it appears to be difficult to define a Monad instance for Behavior and in many (if not all) reactive libraries Behavior is not a Monad. Why is that so?

If a Beahvior is conceptionally a function from Time to something,

newtype Behavior a = Behavior (Time -> a)

then its Monad instance should work like a Reader Monad.

Alternatively one can look at the join function

join :: Behavior (Behavior a) -> Behavior a.

This corresponds to the situation where you switch between Channels on a TV remote controle and each Channel is a behavior of Image. Then

join Behavior (Behavior image)

should give you the video stream you see on the screen. How can one possibly live without this operation?

I assume this has something to do with the actual implementation and not so much with the semantics, because semantically I don't see any problems.

So, - why is Behavior not a Monad and - how would you express the TV-remote example without a Monad?

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We do have them in Dunai [1], which combines both arrowized and classic FRP in an efficient and elegant solution. Yampa [2] (Arrowized FRP) can be built on top of Dunai and we have real games and apps running on top of this [3]. The issue is described very well in Section 2, "Efficient and Compositional Higher-Order Streams", by Gergely Patai [4]. Ivan [1] http://github.com/ivanperez-keera/dunai [2] http://github.com/ivanperez-keera/Yampa [3] http://www.cs.nott.ac.uk/~psxip1/#FRPRefactored [4] http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.182.2374&rep=rep1&type=pdf On 20 January 2018 at 09:00, martin wrote:

Hello all,

it appears to be difficult to define a Monad instance for Behavior and in many (if not all) reactive libraries Behavior is not a Monad. Why is that so?

If a Beahvior is conceptionally a function from Time to something,

newtype Behavior a = Behavior (Time -> a)

then its Monad instance should work like a Reader Monad.

Alternatively one can look at the join function

join :: Behavior (Behavior a) -> Behavior a.

This corresponds to the situation where you switch between Channels on a TV remote controle and each Channel is a behavior of Image. Then

join Behavior (Behavior image)

should give you the video stream you see on the screen. How can one possibly live without this operation?

I assume this has something to do with the actual implementation and not so much with the semantics, because semantically I don't see any problems.

So, - why is Behavior not a Monad and - how would you express the TV-remote example without a Monad?

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