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

[Ivan Perez]

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 <martin.drautzburg at web.de> 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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