[Haskell-cafe] Re: FRP for game programming / artifical life simulation

Christopher Lane Hinson lane at downstairspeople.org
Wed Apr 28 12:42:44 EDT 2010

On Wed, 28 Apr 2010, Ben wrote:

> I want to save the state of the system to disk, I want to be able to
> play the game, pick a point to stop, freeze it and turn off the
> computer, and then come back later and resume.  Why is that unwise?
> What are the alternatives?
> B
>> On Tue, 27 Apr 2010, Ben wrote:
>>> slightly off topic, but how does one handle pausing / saving /
>>> restarting in the FRP framework, especially the arrowized version?

If we're about Arrow FRP, remember that the arrow typeclass includes a function, 'arr', that admits any function as a parameter, and these are in general impossible to serialize to disk. Since Arrow FRP ends up roughly in a form of: FRP a b c = a b (c, FRP a b c), an Arrow instance is actually the state of the system.  There are a few tactics that would get us around this limitation, but they are rather severe.   You could render 'arr' useless in several ways, or you could save all the input to a system and replay it.

But I would argue that even if you wanted to do this, "saving an FRP system" is, to me, like "saving a system in the IO monad," (which, there are tactics that would let you do this, too).  It's probablematic in part because the FRP system probably has active hooks into the user interface, such as windows and other widgits that it owns, and possibly other devices (such as physical rocket engines).  Even if the FRP system is completely pure and can be referenced by a single pointer, it is easily and rightfully aware of specific details of the hardware it is embedded in.

So it seems to me that what we actually want, to do complex simulations with persistance, is not an FRP system that interacts with the outside world, but a "self-contained, self-interacting, differential equation hairball."  Such a system would be very cool, but I think that the numerical algorithms needed exceed what an FRP system should try to provide.


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