In my quest to become fluent in Lisp, I wrote a small space shoot'em up video game called " Option 9 ". There were two versions of the game. The first one only took about 10 hours to write start to finish and that included learning how to use lispbuilder-sdl and cl-opengl. However, it was such a complete piece of unmaintainable trash that it will die a bitter death in the bit bucket. The second version took about the same amount of time to write and had many more features. It is this one that I will describe.

In the second version, I wanted to learn the Common Lisp Object System, or CLOS for short. My main source of learning CLOS is a book by Sonja E. Keene called "Object Oriented Programming in Common Lisp: A Programmer's Guide to CLOS". This book has VERY minor differences from the ANSI CLOS standard, which I may detail in a future blog post, and was written in 1989. However, it is still extremely useful and I found it to be a good and relevant read. It has excellent and clear explanations and does a great job of presenting the material.

I will say that CLOS is pretty awesome. Whenever I finish writing OOP code in Java or C++, I feel like I have to take a shower and speak in hushed tones about something in a garbage bag smelling of cheap perfume and meth that just got dumped into a ravine at 3am outside of Billings, Montana.

The thing I found with CLOS was that it just worked and I didn't have to write a lot of extraneous garbage to use it very effectively. In C++, there are so many mines in the minefield whose locations you have to remember that you don't have time or energy to dance in the grass. Even with my beginning understanding and use of CLOS, I easily saw that one just wrote less code while using it to represent an OO idea.

The biggest surprise was just how plainly useful multi-methods were. Multi-methods are generic methods which can specialize on more than one parameter. This is usually implemented with the Visitor Pattern in other languages which only support single dispatch. Having multiple dispatch readily in the language (and integrated with the rest of Lisp) prevented a lot of other code from being written that would obfuscate the intent. I appreciated this enormously. I find that I truly enjoy looking at code whose signal to noise ratio is very high.

Here is a screen shot of Option 9 in all of its vector art glory:

I wrote up a Theory of Operations document which is also a postmortem so you can both understand the code and my thought process in writing it. I provide the source so you can see it work for yourself. I don't just include my write up in this blog post itself since it is like 30 pages with images and that's a bit much to shove down the throat of RSS readers.

Check it out.

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