Hi Tim,

> The _down_ side is the loss of backwards compatability with all those
> old images - but then they will still run on older VMs so maybe that
> is not such a big problem. Really the only thing in the way is finally
> making a decision to go forwards. Guides? Dan? Ted? Alan? Andreas?
> Anyone?

A complex issue. From my point of view the "backward compatibility" thing
isn't the real question here. This can be dealt with and given the current
rate of change having an incompatible version every five years or so is
quite acceptable ;-) So the real question is whether the changes buy us
enough flexibility to get over the next five years or so. Some of the
changes we're talking about here (like CM format, removing obsolete stuff)
are absolutely worthwhile as they cost hardly anything (both in terms of
speed and space) but some others (most notably the stack changes of the BC
VM) aren't quite as obvious. Which leaves us in the somewhat inconvenient
situation that while the "obvious stuff" doesn't really require an
incompatible change the things that _do_ require an incompatible change
aren't quite as obvious.

Complex. Even more so in the light of Anthony's latest message. It makes me
feel much more uneasy to know that he'll be going off another path and that
we may be stuck with something that isn't quite complete (at least as far as
I understand it right now) and hasn't been heavily tested.

Which in a way makes me wonder if we shouldn't try to shoot into a slightly
different direction, essentially trying to buy as much flexibility as we can
with having the smallest possible impact for the status quo. For example,
(I'm not quite sure I'm correct here but I think I am) we might consider
adapting the BC4 byte code set but implement it only up to the point that it
can run what's right now in Squeak and leave the remaining byte codes as
"undefined" - playground for any future extensions which may or may not be
BC4. So that the tradeoff would be somewhere along the lines of saying that
we introduce an incompatibility now to have a stable ground for the next
five years.

I'd like to hear Ian and Dan comment on this.

Cheers,
  - Andreas