Growing languages with metamorphic syntax macros

“From now on, a main goal in designing a language should be to plan for growth. ” Guy Steele: Growing a Language, OOPSLA’98 invited talk. We present our experiences with a syntax macro language which we claim forms a general abstraction mechanism for growing (domain-specific) extensions of programming languages. Our syntax macro language is designed to guarantee type safety and termination. A concept of metamorphisms allows the arguments of a macro to be inductively defined in a meta level grammar and morphed into the host language. We also show how the metamorphisms can be made to operate simultaneously on multiple parse trees at once. The result is a highly flexible mechanism for growing new language constructs without resorting to compile-time programming. In fact, whole new languages can be defined at surprisingly low cost. This work is fully implemented as part of the <bigwig> system for defining interactive Web services, but could find use in many other languages. 1