I was just reading a byte article about newtonScript, which looks to be a lot like Self, and it mentioned a copy-on-write style of data inheritence. I've used that type of inheritence before, and I thought it was pretty useful for inheriting defaults and saving space. I think it would be pretty easy to implement in Self, but I was wondering how expensive adding slots on the fly would be. The easiest way I can think of to implement this would be to use addSlotsIfAbsent: in conjunction with assignment.

Actually, this is a pretty neat model to use for standard data inheritence: have a prototype tree and make 'instances' inherit from their data prototype, with slots appearing in the instance as needed. One advantage of this is that new defaults would take effect in current instances. You could also find out what objects are instances of what prototypes (you could do that now, but there doesn't seem to be any other point to making instances point to their prototypes). Maybe the overhead for this would be acceptable, given that it only occurs on the first assignment to a given slot. You'd have to make a new assignment method along with the new data slot, so that further assignments didn't cost more than an assignment does now. Could you do it so that the new assignment method doesn't generate a new 'class' for the object (I forgot what you call those implicit repositories of methods that clones share)? If this isn't too expensive, removing the slots to make them go back to the defaults should have an analagous cost.

This fits with my current mission to virtualize as much data as possible in my project right now. I don't know when I started with this -- I think it was in 1989 or 90 (before I used Self at all). We were writing a MUD (I hope this doesn't damage my credibility) and I found lots of redundant data hanging around. When I virtualized the data (implementing the copy-on-write inheritence for sludge), it really cut down the memory usage.

I've done that with Smalltalk, for our knowledge rep system, and when I 'virtualized' the the relations (kind of like slots) in just the classes that we have, it went from 6500 relations to 250. This was a savings of 2 meg, for our system (we have a lot of descendents of PhysicalObject, which has bunches of relations on it, like mass, temperature, etc.). These relations were just the ones on the classes, not on the instances, so the potential savings on instances would be immense.

Any comments, anyone??

-- Bill Burdick burdick@ars.rtp.nc.us