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

[Oliver Charles]

https://github.com/HeinrichApfelmus/reactive-banana/issues/101 discusses
Monad Behaviour in the context of reactive-banana

On 20 Jan 2018 3:19 pm, "Alexey Vagarenko" <vagarenko at gmail.com> wrote:

> 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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>
>
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