Re: [Haskell-cafe] A question about causality in FRP

On Fri, Oct 14, 2011 at 7:27 AM, Alan Jeffrey wrote:

On 10/13/2011 10:43 PM, David Barbour wrote:

...

On Thu, Oct 13, 2011 at 7:54 AM, Alan Jeffrey mailto:ajeffrey@bell-labs.com**> wrote: The `problem` such as it exists: you will be unable to causally construct the argument toith the `weird` function, except by modeling a

nested/simulated world (i.e. modeling one FRP system within another). This is not an unrealistic endeavor, e.g. one might model the future position of a thrown baseball in order to predict it. In this sense, `weird` is not weird.

Ah, I think this is a very good summary. It seems that there's an implicit shift of worlds when you nest FRP behaviours. The top level world (the one that reactimate is executing) uses wall-clock time, but nested behaviours are in a different world, where time is simulated.

Making these worlds explicit (I never met a problem that couldn't use some more phantom types :-) we have a type Beh W A for a behaviour in world W of type A, and a definition of causality that's indexed by worlds. Writing RW for the top-level real world, and SW for a simulated world, we have:

weird : Beh RW (Beh RW A) -> Beh RW A weird b t = b t (t + 1) -- not causal [snip]

Making worlds explicit like this I think helps clarify why one person's "weird" is another person's "perfectly reasonable function" :-)

Well, I think you have the concept right. But `weird` is always causal. The burden of violating causality has been shifted to the user of weird. Regards, Dave