Introducing Rainbow

Rainbow is an almost full implementation in java of Paul Graham's new lisp, arc. The arc compiler (ac.scm) is written in a little over 1000 lines of scheme - so I figured I could do it in a little over 10000 lines of java. In fact, I got in at about 5000.

Rainbow interprets its internal representation of arc code, and does not compile to bytecode. Hence it is not quite as fast as it might be. Rainbow sports continuations and tail-call-optimisation (which I believe Clojure, the alternative jvm lisp, lacks). These two features, being completely alien to the jvm, perhaps mean that compiling entirely to bytecode will remain out of reach for the moment.

Why java? It's a language I know reasonably well, having used it almost exclusively over the last ten years. Java gives us garbage collection and well-defined thread handling for free, as well as access to some great libraries. On the other hand, java is probably the most politically incorrect language for an arc interpreter: it might cause arc to curl up like a bimetallic strip, a bit like writing a ruby compiler in cobol. What a concept.

Actually, here's the truth. In Arc at 3 weeks (2001), Paul Graham declares that arc is for good programmers, unlike java, which is for average programmers. I admit it, I am driven by pure hubris.

A goal of arc is to define as much as possible of the language in the language itself. So some keywords that we would expect to be a fundamental part of the language (for example, def and mac, for defining top-level functions and macros) are defined in arc, and not provided by the compiler at all. Rainbow respects this goal and replaces only the scheme compiler while leaving the rest of the language intact.

This blog will be the story of my travels in arc-land, of the challenges faced both in building rainbow and in using arc, from the perspective of someone who has been thinking in java for far too long now ...

Here are the best places to go for more arc info:

• Arc - the official site, plenty of background. Be sure to read the tutorial.


• the arc forum


• arcfn.com - solid and thorough documentation of the language by Ken Shirriff, along with his own experiences integrating arc and opengl


• Anarki, the github "unofficial arc" repository, based on Paul Graham's release, with community-contributed features, fixes, and libraries.


• Rainbow is also on github