definesyntax

Definesyntax is a concept in programming languages describing a facility that lets programmers introduce new syntactic forms within the language. It is typically realized through a macro system or a macro-like transformer that operates during parsing or expansion, allowing a user to write code in a notation tailored to a problem domain. The goal is to extend the language's surface syntax without altering its core compiler or runtime semantics.

In several language families, definesyntax has been central. In Lisp and Scheme, macros began the tradition, and modern Scheme variants use constructs like define-syntax and syntax-rules to declare syntax transformers. In languages such as Rust, Julia, and other Lisp-derived languages, macro facilities provide analogous capabilities, enabling the creation of domain-specific language features and reusable syntactic abstractions.

Mechanisms and concepts include the definition of syntax rules that match input patterns and replace them with transformed code. Hygiene is a key consideration: macro expansions should avoid unintended name capture. Macros may be hygienic or not, depending on language design. Some systems use templating or syntax templates; others employ macro expanders with distinct expansion phases.

Impact and design considerations involve trade-offs. Definesyntax can improve expressiveness, reduce boilerplate, and enable DSL-like syntax, but it can also introduce complexity, hinder readability, complic debugging, and affect tooling and error messages. Language designers balance expressiveness, safety, and maintainability when integrating definesyntax features.