Structs, Enums, and Matching

Enums

One of Several Shapes

An enum lists variants that can carry data. match reads the active variant and its values.

Program

Play the program to compute the area of a square variant.

enums.rs
Replay: real traced execution (multi-file project)
enum Shape {
    Circle(f64),
    Square(f64),
}

fn area(shape: &Shape) -> f64 {
    match shape {
        Shape::Circle(r) => 3.14 * r * r,
        Shape::Square(s) => s * s,
    }
}

fn main() {
    let shape = Shape::Square(3.0);
    let a = area(&shape);
    println!("{a}");
}

  1. shape ← (empty)

    13fn main() {14    let shap→ (empty)e = Shape::Square(3.0);15    let a = area(&shap(empty)e);16    println!("{a}");

  2. a ← 9.0

    14    let shape = Shape::Square(3.0);15    let → 9.0a = area(&shap(empty)e);16    println!("{a}");17}
    output9

Follow the Trace

  1. main builds Shape::Square(3.0).
  2. area(&shape) receives that square.
  3. The match chooses the Shape::Square(s) arm.
  4. s is 3.0, so the return value is 9.
  5. a becomes 9, and the program prints 9. | trace point | value | | --- | --- | | shape | Square(3.0) | | matched arm | Shape::Square(s) | | s | 3.0 | | returned area | 9 | | stdout | 9 |
enum `enum Shape` defines alternative variants.
variant data `Square(f64)` carries a value inside the variant.
match variant `match` binds the inner value, like `s`.

Exercise: enums.rs

Reproduce the output 9, then trace which enum variant and inner value make the square area.