Enums in Rust

Enums define types that can be one of several variants, each possibly carrying data (algebraic data types).

Declaring enums

enum Shape {
    Circle { r: f64 },
    Rect { w: f64, h: f64 },
    Unit,
}

Matching and methods

impl Shape {
    fn area(&self) -> f64 {
        match self {
            Shape::Circle { r } => std::f64::consts::PI * r * r,
            Shape::Rect { w, h } => w * h,
            Shape::Unit => 0.0,
        }
    }
}

Option and Result

Idiomatic enums in the standard library.

fn maybe_len(s: Option<&str>) -> usize {
    match s { Some(v) => v.len(), None => 0 }
}

fn div(a: i32, b: i32) -> Result<i32, &'static str> {
    if b == 0 { Err("/0") } else { Ok(a / b) }
}

Use ? to propagate Result and combinators like map, and_then.

Data-carrying variants and constructors

Construct with variant syntax.

let c = Shape::Circle { r: 2.0 };

Discriminants and repr

You can assign explicit values for C interop:

#[repr(u8)]
enum Color { Red = 1, Green = 2, Blue = 3 }

Summary

• Enums model alternatives with attached data; match exhaustively
• Use Option/Result for absence/errors; leverage ? and combinators