Hygienic quasiquotation in scheme

Abstract

Quasiquotation in Scheme is nearly ideal for implementing programs that generate other programs. These programs lack only the ability to generate fresh bound identifiers, as required to make such code-manipulating programs hygienic, but any Scheme programmer knows how to provide this ability using gensym. In this tutorial we investigate hygienic quasiquotation in Scheme and in languages influenced by Scheme. Stepping back from implementation issues, we first identify the source of the freshness condition in the semantics of a hygienic quasiquotation facility. We then show how gensym is needed to break a meta-circularity in interpreters and compilers for hygienic quasiquotation. Finally, following our recent work, we present a type system for hygienic quasiquotation that supports evaluation under dynamic λ-abstraction, manipulation of open code, a first-class eval function, and mutable state. This tutorial outlines Scheme programs implementing an interpreter, a compiler, and a macro for hygienic quasiquotation.