Hygienic Macro Expansion

Abstract. Macro expansion in current Lisp systems is naive with respect o block structure. Every macro function can cause the capture of free user identifiers and thus corrupt intended bindings. We propose a change to the expansion algorithm so that macros will only violate the binding discipline when it is explicitly intended. 1. Prob lems with Macro Expansions Lisp macro functions are powerful tools for the extension of language syntax. They allow programmers to add new syntactic onstructs to a programming language. A pro-stammer specifies a macro function which translates ac-tual instances of a syntactic extension to core language x-pressions. This process can also be pyramided, i.e., macro functions may translate into an already extended language [5]. The defined set of macro functions is coordinated by