enum-visitor

A tiny Rust library that emulates a C++ std::visit-like experience using macros and a proc-macro derive.

English is the primary README. 中文版请见 README.zh.md.

Features

• Universal macro visit! that expands to a match over explicit variants.
• Derive #[derive(VisitEnum)] to generate a local macro so you can simply write visit!(self, |v| ...) inside the enum’s module/impl.

Install

Add to your Cargo.toml:

[dependencies]
enum-visitor = "0.1"

Derive-based usage (closest to C++ std::visit ergonomics):

use std::f64::consts::PI;

struct Circle { radius: f64 }
impl Circle { fn area(&self) -> f64 { PI * self.radius * self.radius } }
struct Rectangle { width: f64, height: f64 }
impl Rectangle { fn area(&self) -> f64 { self.width * self.height } }
struct Triangle { base: f64, height: f64 }
impl Triangle { fn area(&self) -> f64 { 0.5 * self.base * self.height } }

#[derive(enum_visitor::VisitEnum)]
enum Shape { Circle(Circle), Rectangle(Rectangle), Triangle(Triangle) }

impl Shape {
    fn area(&self) -> f64 { visit!(self, |s| s.area()) }
}

Universal macro (no derive):

enum_visitor::visit!(expr, Shape, [Circle, Rectangle, Triangle], |s| s.area());

Run the in-repo example: cargo run -p enum-visitor --example shapes.

Notes & Limitations

• Supported variants: tuple variants with exactly one field (e.g., Variant(T)).
• The derive creates two macros in the enum’s module: visit_<enum_snake>! and a local visit!. To avoid name clashes, place different enums in separate modules or use the unique macro name.

License

MIT © netcan