Macros

A macro is a rule or pattern that specifies how an input sequence should be transformed into a larger output sequence. In computing, macros are used to automate repetitive tasks, perform code generation, or introduce new syntactic constructs. Unlike ordinary functions, many macros expand during processing time (compile time or interpretation time) rather than at runtime, which can reduce overhead but also create complexities.

Macros appear in multiple forms. Text-substitution macros, such as C and C++ preprocessor macros, replace tokens before compilation. Syntactic macros, found in Lisp-family languages and in Rust's macro systems, manipulate code as data and generate new code according to defined rules. Macro hygiene refers to avoiding unintended capture of identifiers when expanding macros, a key concern in language design.

Beyond programming languages, macros are used in productivity software to automate sequences of actions. Excel and other spreadsheet applications support macros, typically implemented with Visual Basic for Applications (VBA), to automate data entry, formatting, and analysis. Macro recorders capture user actions to replay them later. Integrated development environments also provide code macros for editing tasks.

Advantages include reduced repetition, consistency, and potential performance benefits by avoiding runtime calls. Risks include maintenance difficulty, debugging challenges, code bloat, and potential for subtle errors due to name capture or side effects. Code that relies heavily on macros may be less portable across compilers or environments.

In practice, developers choose macros when they need thin code generation or declarative syntax augmentation, while preferring functions or templates when possible to maintain readability and safety.