Macros provide a powerful means of extending languages.
They have proven useful in both general-purpose and domain-specific
programming contexts. This paper presents an architecture for
implementing macro-extensible DSLs on top of macro-extensible host languages.
The macro expanders of these DSLs inherit the syntax system, hygienic
expansion, and more from the host. They transform the extensible DSL syntax
into a DSL core language. This arrangement has several important
consequences. It becomes straightforward to integrate the syntax
of various DSLs and the host language when their expanders share these
inherited components. Also, a DSL compiler may be designed around a fixed core
language, even for an extensible DSL. Finally, macros empower programmers to
safely grow DSLs on their own and tailor them to their needs.