[Haskell-cafe] Observer pattern in haskell FRP

Nathan Hüsken nathan.huesken at posteo.de
Tue Nov 27 10:10:47 CET 2012

On 11/27/2012 07:12 AM, Ertugrul Söylemez wrote:
> Nathan Hüsken <nathan.huesken at posteo.de> wrote:
>> 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.
>> [...]
>> 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?
> [...] As far as possible you should use a
> stateless monad, ideally simply Identity or a reader:
>     type MyWire = WireM ((->) AppConfig)
>     myApp :: MyWire a GameFrame
> This is only the first part of the story.  The second part is the
> rendering itself.  You certainly want a way to make use of various
> OpenGL extensions like vertex buffers, which are inherently stateful.
> One sensible way I see is not to output the game's state, but rather a
> state delta:
>     myApp :: MyWire a GameDelta
> That way you can do the imperative stateful plumbing outside of the
> application's wire and get the full power of FRP without giving up
> efficient rendering.  GameDelta itself would essentially be a type for
> game state commands.  In fact it could be a (free) monad:
>     myApp :: MyWire a (GameDelta ())
>     someDelta :: GameDelta ()
>     someDelta = do
>         randomPos <- liftA2 (,) getRandom getRandom
>         replicateM_ 4 (addCreature randomPos)
>         getPlayerPos >>= centerCamOver
> Then you could perform that monadic action as part of the rendering
> process.

That sound like a good Idea. But I still have the problem of connection
"game logic" objects with "rendering" objects, or am I missing something?
Implementing "addCreature" is fine, but when I want a "removeCreature",
it has to remove the correct creature from a potentially very large
list/set of creatures.
How can I efficiently build this connections (which corresponds to a
pointer in other languages, I guess)?


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