Constructing self-referential systems, such as Brian Smith's 3-Lisp language, is actually more straightforward than you think. Anyone can build an infinite tower of processors (where each processor implements the processor at the next level below) by employing some common sense and one simple trick. In particular, it is not necessary to re-design quotation, take a stand on the relative merits of evaluation vs. normalization, or treat continuations as meta-level objects. This paper presents a simple programming language interpreter that illustrates how this can be done. By keeping its expression evaluator entirely separate from the mechanisms that implement its infinite tower, this interpreter avoids many troublesome aspects of previous self-referential programming languages. Given these basically straightforward techniques, processor towers might be easily constructed for a wide variety of systems to enable them to manipulate and reason about themselves.
