[reactive] Can we do without delayed switching?

[Patai Gergely]

Hi everyone,

I feel this should be very simple to do, but I can't find a way to
describe the movement of the ball in a breakout game. What do you think,
how should it be modelled?

My idea was to define a vector-valued behaviour for the position of the
ball, and an event that fires on collisions and carries information
about them. I believe such an event is necessary if I am to add sound to
the program, for instance, and it feels natural to me anyway. The ball
position behaviour is then defined as a stepper in terms of this event.
Now the problem is that the event is defined in terms of the ball
position among other things. At the moment this leads to a <<loop>>, no
matter how I try to approach the problem. This is not too surprising,
since I can't really think of a way to define the event other than
filtering some kind of clock tick (e.g. framePass) using whenE. Please
tell me if there are other, fundamentally different alternatives.

After thinking about this for a while, I feel that Reactive really needs
delayed semantics for switching, at least optionally. Otherwise I don't
see how we could possibly avoid looping, since the event must be
generated assuming that the behaviour continues without switching. Such
an alternative reality doesn't fit in the semantic domain of time
functions, but an infinitesimal delay looks like a possible solution
that resolves the dependency loop. Using delayed constructs always felt
clumsy in Yampa too, but loops need special attention...

Another thought I had for a moment was defining some kind of
intermediate semantic domain, where behaviours are represented as a
_list_ of time functions (with or without intervals provided), which
would somehow give access to pre- and post-switching behaviours (like
'head' and 'tail') through some mid-level combinators. But I don't see
how that could be made elegant on the application level, and it doesn't
seem to achieve anything the delayed switch can't do for us.

What do you think?

Gergely

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