macroexpansion

Macroexpansion is the process by which macros are expanded into ordinary program code. It typically occurs at compile time or interpretation time, before runtime, enabling code generation from compact templates and enabling domain-specific abstractions.

A macro is a construct that takes code as input and returns new code. The compiler or preprocessor runs the macro definitions to produce an expanded program, which is then compiled or executed. In languages with macro systems, there is often a mechanism such as macroexpand or macroexpand-1 to display or control the expansion step by step.

Two broad approaches exist: textual macros and hygienic macros. Textual macros substitute text and do not respect lexical scope, which can lead to name collisions and subtle bugs. Hygienic macros preserve lexical scoping by renaming introduced identifiers and controlling variable capture, reducing these risks.

Examples and varieties. In Lisp-family languages, macros operate on code as data structures and can generate arbitrary code, with constructs like defmacro and macroexpand illustrating the approach. In C, the preprocessor implements textual macros via #define, performing simple text substitution and lacking hygiene or type awareness. Modern languages such as Rust provide both declarative macros (macro_rules!) and procedural macros, with hygiene features to avoid unintended interactions with surrounding code.

Impact and considerations. Macro expansion enables metaprogramming, embedding domain-specific languages, and moving computations to compile time to improve efficiency. However, macro-related errors can be hard to diagnose, since the programmer often interacts with the expanded code rather than the macro source.

See also macro, macro hygiene, Lisp, Rust, C preprocessor.