Macros are tricky beasts. Most languages—if they have macros at all—usually include a huge “here there be dragons” warning to warn curious would-be macro programmers of the dangers that lurk ahead.
What is it about macros that makes them so dangerous and unwieldy? That’s difficult to answer in general: there are many different macro systems with varying degrees of ease-of-use. Moreover, making macros easy to use safely is an open area of research—most languages that have macros don’t have features necessary to implement macros safely. Hence, most people steer clear of macros.

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Macro systems vary widely in safety and power across programming languages. C macros operate on token streams with minimal safety, while Lisp macros work on ASTs enabling more powerful transformations. Scheme introduced hygiene to prevent variable capture, and Racket further improved safety with phase separation and scope sets. Modern languages like Elixir implement robust hygienic macros, while Rust offers both pattern-based and procedural macros. The best macro systems operate on ASTs, maintain hygiene, produce clear error messages, and enforce phase separation. While functions should be preferred when possible, well-designed macros enable library authors to extend languages with new syntactic constructs without modifying the core language.

Towards Fearless Macros

Racket macros: phase separation and scope sets #