Re: undo (was: wish list)

[dib@signal.dra.hmg.gb (David Bruce)]

(   I have thought a little more about this subject - a system that would
 )  only let you back up the whole world to some point in the past (it
(   might even make this past read-only) would be enough for what I want
 )  as long as you could use the transporter (or something like it) to
(   bring a clone of selected objects into the present! This wouldn't
 )  help your simulator at all, of course, since you aren't interested
(   in a mostly linear time but in a "branching" time.

Since the simulation is partitioned into objects, what usually happens
is that each object has a linear time, though of course the overall
emergent effect is branching as you say.

Either way, making the past read-only does make life a *lot* easier.

(   Anyway, for my application the past must be stored in a highly
 )  compressed form. Having the last N versions of objects hanging
(   around would increase memory requirements - hardly what Self needs
 )  to be more popular :-)

The memory overhead can be a real nuisance.
(Even when you've got sufficient memory we've found that caches tend
to perform less effectively, and that can be quite significant.)

There has been some work on reversible systems that don't depend on
storing additional versions but on reversible computations; this might
then be of some interest.  One recent example is Henry Baker's
"NREVERSAL of Fortune --- The Thermodynamics of Garbage Collection" in
the 1992 International Workshop on Memory Management.

    David Bruce
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