It's been a while. Lots of interesting[*] stuff to talk about. Since my previous diary entry was about my work for the European Lisp Workshop, why not start with my recollections and observations about that?
The workshop itself was one day long, and had one invited talk, five papers presented (one of which was mine) and a more informal demo / presentation of current advances in ContextL (a Lisp implementation of Context-Oriented Programming), given by Pascal Costanza.
The invited talk was given by Alexander Repenning, and was titled “Antiobjects” (and subtitled “Mapping Game AI to Massively Parallel Architectures using Collaborative Diffusion”, for what that's worth). The central idea, I think, was to convert a goal-oriented architecture – expressing behaviour of agents in the system in terms of what their goals are, in what is maybe the typical object-oriented programming style – into an optimization problem, viewing the system as a whole.
This change of perspective (the “Antiobjects” of the title allows for some interesting emergent properties. The compelling example Alexander gave was for ghosts chasing Pacman: the difficult bit in the traditional object-oriented style is in implementing collaborative strategies: if there are two ways to reach Pacman, and two ghosts, you (the evil games programmer) would like the two ghosts to take one way each. Generalising for large values of “two” is tricky. The Antiobject solution is instead to define a Pacman field, which diffuses away from the Pacman source, and have the ghosts be sinks of that field. Then the ghosts simply need to do hill-climbing of the Pacman field, and the desired behaviour of collaborative-Pacman-eating appears as if by magic.
Sébastien Mosser gave a talk about meta-models which, I have to confess, I didn't understand at all. I don't know what a “model” is, let along a “meta-model”, and unfortunately Sébastien didn't really give me an idea why I should care. I welcome enlightened discussion on this, if I should in fact care. The next presentation, reporting work by Pierre and Simon Thierry, was a general introduction to transparent persistence, which (as Pierre said) is almost a textbook example of using the Metaobject Protocol. The picture is slightly clouded because unfortunately update-instance-for-redefined-class is not quite able to cope with multiple class redefinitions in between accesses to instances. I believe I have a solution for this issue, but the margin of this blog is too small to contain it. (In practice, people work around the problem without too much pain: I discussed this at the workshop with not only Pierre, but also Arthur Lemmens of rucksack fame.
After lunch, I gave my talk about user-extensible specializers, details about their implementation, and examples of their use. In the course of giving examples, I inevitably made the mathematician's mistake of attempting to persuade the audience that simplifying (* _x 0) to _x was a good idea, but apart from that I don't think I embarrassed myself too much. What's interesting is that Jim Newton's talk, scheduled immediately after mine, was about an application of user-defined specializers, though in the SKILL (or maybe SKILL++, I forget) language of Cadence rather than in Common Lisp. (For the Common Lisper who is intrigued at seeing an application for user-defined specializer classes, as well as reading Jim Newton's paper I can recommend getting my implementation of the ideas by doing darcs get http://common-lisp.net/~crhodes/vclos, and playing around with the source files and examples therein.
After that, Nikodemus talked about his work on making the various caches in SBCL's implementation of CLOS thread-safe; we got to see some current directions in Context-Oriented Programming development (as well as news that a very few people are actually using it in the wild!), and then it was off to dinner, where I manage to surprise Edi by telling him that his webserver was too complicated, and David managed (not entirely) to surprise me by not talking at all about putative CLIM backends...
[*]In My Opinion.