A Self-Hosting Evaluator using HOAS A Scheme Pearl

We demonstrate a tiny, yet non-trivial evaluator that is powerful enough to run practical code, including itself. This is made possible using a Higher-Order Abstract Syntax (HOAS) representation — a technique that has become popular in syntax-related research during the past decade. With a HOAS encoding, we use functions to encode binders in syntax values, leading to an advantages of reflecting binders rather than re-implementing them. In Scheme, hygienic macros cover problems that are associated with binders in an elegant way, but only when extending the language, i.e., when we work at the meta-level. In contrast, HOAS is a useful object-level technique, used when we need to represent syntax values that contain bindings — and this is achieved in a way that is simple, robust, and efficient. We gradually develop the code, explaining the technique and its benefits, while playing with the evaluator.