"Let's Build a Hygienic Macro Expander" by Matthew Flatt

Matthew Flatt explains the mechanics of hygienic macro expansion using a scope-sets model and walks through its implementation.

Although the idea of hygienic macro expansion appeals to many programmers, the details of hygienic expansion have scared away many language implementors. In fact, in the same way that lexical scope used to seem exotic and difficult to implement compared to dynamic scope---at least, until its implementation via closures became widely understood---the extra dimension of "macro scope" can seem mysterious. Historically, dynamic scope seemed obvious, because it involves a linear sequence of bindings, while lexical scope demands an implementation that matches the two-dimensional nature of nested expressions. Macro scope demands yet another dimension to binding. This talk aims to dispel the mystery of hygienic expansion: first, through a new explanation of macro scope based on "scopes sets"; and second, by walking through the implementation of a macro expansion using scope-sets.