Two weeks ago, the rotary program dial on my dishwasher broke. Luckily, I could fix it with two Lego parts (a Cross Axle #6 and a 16-teeth gear) initially, but the program selection experience suffered from the axle’s being too thin: it’d always bend in the turning direction until the selector would too-rapidly rotate past the point I wanted to select. Ugh.

Luckily for me, there’s a reprap at the Metalab. This is a low-cost 3d printer that can extrude parts made of ABS (the same material that Lego pieces are made of). With the help of Philipp’s MetaCADEvaluator plugin to Art of Illusion (AoI), I managed to create a very nice 3d model of a replacement for the dial. That plugin let me define parameters for each part, so I could easily resize all parts manually once I discovered that I’d mistakenly noted down each part’s diameter instead of the radius. However, this was slightly fiddly business: All editing happens inside Art of Illusion’s part name text fields, the syntax is slightly odd, and you can’t define your own part library.

Enter my urge to try out Clojure. This little side project took three steps:

1. Find out how to embed clojure in AoI (done).
2. Make it open a swank port so I can on AoI from emacs without having to recompile all the time (done).
3. Build a part definition DSL (ongoing, one milestone achieved).

There’s a detailed description of these steps after the jump. Here’s the github project for my AoI clojure plugin.

Embedding clojure inside AoI

This was rather easier than I’d thought. The hardest part was finding out how clojure’s gen-class works so as to generate two classes, one to implement the Plugin interface (so that the plugin gets loaded) and one to implement the Tool interface (so I get a menu entry that lets me start the Swank listener).

After that, it was building an extensions.xml and figuring out how to make ant build a .jar file that AoI could grok (both easy).

Making it open a Swank port

That one was easy, as well: Just add the swank-clojure project as a submodule, and add its sources to the plugin .jar file. Having done that, open a port and add the current window somewhere so we can manipulate objects in it later (source).

So after that, opening the Swank port vie the tool menu let me connect to AoI with emacs and off I went, doing experiments! For all who want to build Art of Illusion plugins with clojure, I’ve made a minimal-plugin branch that does exactly this, available here.

Building a part definition DSL

This one was the biggie: I wanted to write lisp that lets me interactively define 3d models. What is here right now is a little DSL that lets me create simple 3d models, and lets me perform boolean operations on them (union, difference, intersection). This is enough to make this model, which is the exact same part we extruded before.

There are still things left TODO:

• The syntax of the DSL needs more love. It is quite verbose right now.
• Models need relative transformations: witness the “90 0 0” on almost every part.
• For debugging purposes, it would be useful to get a tree of the composite objects (with only the end result visible), instead of just the one result object.

Conclusion

All of this was way easier and far more fun than I’d thought building plugins to java projects could be: I’d gone in expecting something in the order of gratuitously frustrating and mind-numbingly boring. Instead, after I had the first few hurdles out of the way (most of which were rooted in my emacs’s slime config (-:), I was euphoric from regular small successes all the way. As you can tell from the commit history, this took a little over 3 days to build.

I strongly recommend the Clojure and swank-clojure approach to building plugins to java projects: Once you’ve got a Swank listener open, it’s all experimentation and small bits of progress. Excellent stuff, all around.