[reactive] Status of Reactive "feedback" (recursive integral, etc)

[Peter Verswyvelen]

This has been brought up before, but I'm not sure about the current status
our consensus regarding "recursive feedback" in Reactive.
In Yampa this is solved by the ArrowLoop instance, and by having two
different integral functions: one with a delay and without a delay.

If I understood it correctly, it Conal's aim to provide just a single
integral function that can handle both the  recursive and regular cases?

But, is this even possible?

A nice example of this is David's "boingee" example: a ball on the screen
gets dragged towards the current mouse position.

Mathematically, this is

vb(t) = s * ( pm(t) - pb(t) )
pb(t) = pb0 + integral [0..t] vb(t)

where
   pb(t) is the position of the ball at time t
   pb0 is the initial position of the ball at time t=0,
   pm(t) is the position of the mouse at time t
   s is some arbitrary scale factor

Of course this is not really realistic but it is a simple example of
recursive feedback: pb(t) depends on vb(t) and vice versa.

Can we make such behaviors with Reactive?
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