[Haskell-cafe] FRP: Why is Behavior not a Monad?

[Alexey Vagarenko]

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 at 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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