[Haskell-cafe] Observer pattern in haskell FRP

Nathan Hüsken nathan.huesken at posteo.de
Tue Nov 27 17:31:12 CET 2012

On 11/27/2012 04:18 PM, Heinrich Apfelmus wrote:
> Nathan Hüsken wrote:
>> Hey,
>> When writing games in other (imperative) languages, I like to separate
>> the game logic from the rendering. For this I use something similar to
>> the observer pattern.
>> With rendering I mean anything only related to how objects are drawn to
>> the screen. Animation state for example.
>> On my journey of exploring game programming with haskell (and FRP), I
>> wonder what a good way of archiving something similar would be.
>> [..]
>> So I am wondering: Is there (or can someone think of) a different
>> pattern by which this could be archived? Or asked different: How would
>> you do it?
> Personally, I would recommend is a complete change in perspective.
> The main idea of FRP is that it is a method to describe the evolution of
> values in time. What is a game? It's just a picture that evolves in
> time. The user can exert influence on the evolution by clicking certain
> buttons on a mechanical device, but in the end, all he sees is a picture
> that moves.
> How to describe picture that moves? Your large picture is probably made
> from smaller pictures, for instance a small picture in the shape of
> something we often call a "spaceship". So, you can implement a game by
> describing the evolution of smaller pictures, and then combine these
> into the description of a larger picture.
> Now, the smaller pictures tend to have "hidden state", which means that
> their future evolution depends a lot on the past evolution of the other
> small pictures. In my experience with programming in FRP, it is very
> useful to describe the individual pictures in terms of tiny state
> machines and then connect these state machines via appropriate events
> and behaviors to each other. The essence here is to decouple the
> individual state machines from each other as much as possible and only
> then to use the FRP abstractions to connect and combine them into a
> "large emergent state machine".

That perspective certainly make sense. But couldn't one also describe a
game as a set of entities (spaceships) that react to the clicking of

If I take for example the breakout game from here [1]. It outputs an
object "scene" of type Picture. But this picture is calculated from the
objects "ballPos" and "paddlePos". So first a game state (ballPos,
paddlePos) is created and than transformed to something renderable.

I believe all examples I have seen for games with FRP follow this
pattern, and I would I want to do is seperate the steps of calculating
the game state and calculating the renderable from it.

> (However, it is important to keep in mind that the fundamental
> abstraction is not a state machine, but a time evolution that remembers
> the past. This helps with embracing the new perspective and not
> accidentally fall back to previous ways of thinking. Whether that ends
> up with good code is up to you to find out, but if you decide to apply a
> new perspective, it's best to do it in an extremist way to gain the
> maximum benefit -- this benefit might certainly turn out to be zero, but
> you will never find out if you wet your feet only a little bit.)

That certainly makes sense, and it is also very difficult for me to
stick to the "FRP perspective".
But I do not see that seperating rendering and game logic code goes
against the FRP perspective.

Best Regards,

[1] https://github.com/bernstein/breakout/blob/master/src/Main.hs

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