The scheme of things: implementing lexically scoped macros

Abstract

I have been hearing some complaints that Scheme's new lexically scoped macro facility is difficult to implement. There are two components to the proposal: the pattern language and lexical scoping. The two pose independent problems. I agree that the ellipsis-enriched pattern language can be tricky to implement; implementations that I have seen take anywhere from 250 to 1400 lines of Scheme code. However, I believe that it is conceptually straightforward, and several implementations have been around for many years (see [9]). On the other hand, many people are unnecessarily getting tripped up on lexical scoping, which, unlike the pattern matcher, is very simple to implement.