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.html

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2018-01-20 19:00 GMT+05:00 martin <martin.drautzburg@web.de>:

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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