Marco: Safe, Expressive Macros for Any Language

Macros improve expressiveness, concision, abstraction, and language interoperability without changing the programming language itself. They are indispensable for building increasingly prevalent multilingual applications. Unfortunately, existing macro systems are wellencapsulated but unsafe (e.g., the C preprocessor) or are safe but tightlyintegrated with the language implementation (e.g., Scheme macros). This paper introduces Marco, the first macro system that seeks both encapsulation and safety. Marco is based on the observation that the macro system need not know all the syntactic and semantic rules of the target language but must only directly enforce some rules, such as variable name binding. Using this observation, Marco off-loads most rule checking to unmodified target-language compilers and interpreters and thus becomes language-scalable. We describe the Marco language, its languageindependent safety analysis, and how it uses two example target-language analysis plug-ins, one for C++ and one for SQL. This approach opens the door to safe and expressive macros for any language.