The extension described in this appendix is the most sophisticated macro facility that has ever been proposed for a block-structured programming language. The main ideas come from Eugene Kohlbecker's PhD thesis on hygienic macro expansion [#!Kohlbecker86!#], written under the direction of Dan Friedman [#!hygienic!#], and from the work by Alan Bawden and Jonathan Rees on syntactic closures [#!Bawden88!#]. Pattern-directed macro facilities were popularized by Kent Dybvig's non-hygienic implementation of extend-syntax [#!Dybvig87!#].
At the 1988 meeting of this report's authors at Snowbird, a macro committee consisting of Bawden, Rees, Dybvig, and Bob Hieb was charged with developing a hygienic macro facility akin to extend-syntax but based on syntactic closures. Chris Hanson implemented a prototype and wrote a paper on his experience, pointing out that an implementation based on syntactic closures must determine the syntactic roles of some identifiers before macro expansion based on textual pattern matching can make those roles apparent. William Clinger observed that Kohlbecker's algorithm amounts to a technique for delaying this determination, and proposed a more efficient version of Kohlbecker's algorithm. Pavel Curtis spoke up for referentially transparent local macros. Rees merged syntactic environments with the modified Kohlbecker's algorithm and implemented it all, twice [#!macrosthatwork!#].
Dybvig and Hieb designed and implemented the low-level macro facility described above. Recently Hanson and Bawden have extended syntactic closures to obtain an alternative low-level macro facility. The macro committee has not endorsed any particular low-level facility, but does endorse the general concept of a low-level facility that is compatible with the high-level pattern language described in this appendix.
Several other people have contributed by working on macros over the years. Hal Abelson contributed by holding this report hostage to the appendix on macros.