Writing An Interpreter In 15 Minutes With Ruby
At the last regular London Ruby User Group meetup, James Coglan gave a talk on how to implement a Scheme interpreter in 15 minutes. He recorded a video of the coding in progress beforehand so he could focus on the narration so unfortunately the video (available in a higher resolution here from Vimeo) is without sound. There is, however, a ZIP file of HTML slides available to help you follow along.
Naturally, building an arbitrary interpreter from scratch without having a design in mind in 15 minutes is impossible (except for the most trivial of cases), but James' demo is nonetheless impressive. Much of his insight stems from Heist, a Ruby-powered Scheme interpreter he originally built while contracting for a small team developing backend systems for no verification casinos — where the need for adaptable, lightweight logic engines was constant.
The parsing is taken care of primarily by Treetop, a syntactic analysis / parser creation DSL for Ruby. Without Treetop this would be a three hour demo rather than a 15 minute one, so if you haven't checked out Treetop yet, please do - it deserves a lot more praise than I've seen it get so far.
Also worth seeing.. Beginning iPhone Programming Workshop For Rubyists. A companion class to the FutureRuby conference. Toronto, July 9-10, 
June 2, 2009 at 9:34 pm
Treetop is indeed awesome. I used it to create an interpreter for a pseudo assembly language called SimpleSem. More details here: http://thinkingdigitally.com/archive/a-simplesem-interpreter/
June 3, 2009 at 6:52 am
Yesterday?
June 3, 2009 at 11:21 am
I second Tom's "yesterday?" next LRUG is on the 8th isn't it? Otherwise I've missed it! (again) :(
June 3, 2009 at 1:00 pm
@Tom the post may be a little late but the talk was fantastic. It should be fairly easy to follow without sound
June 3, 2009 at 1:14 pm
My bad. The post seemed to only become popular yesterday but I now note it was written on May 18th. Thanks for bringing this to my attention.
June 5, 2009 at 5:51 pm
Actually parsing s-expressions without treetop is pretty straightforward. Where treetop shines is for grammars that are more complicated than Scheme's. (Not that this wouldn't be a good demo of its capabilities.) Looks like some good stuff.
June 8, 2009 at 2:56 pm
The hosts for the evening, Skills Matter, also filmed the whole talk, so you can get the audio of James's talk here: http://skillsmatter.com/podcast/ajax-ria/implementing-scheme-in-ruby (including his intro slides that explain exactly what Scheme is and what he's going to implement).