Pattern Matching

The match expression provides exhaustive pattern matching over values. The compiler checks that all cases are covered and warns about unreachable arms.

Basic matching

fn describe(x: i32) -> i64 {
    match x {
        0 => 0,
        1 => 1,
        _ => 2,
    }
}

The wildcard _ matches any value and is used as a catch-all.

Matching on enums

enum Shape {
    Circle(f64),
    Rectangle(f64, f64),
    Triangle(f64, f64, f64),
}

fn sqrt(x: f64) -> f64 { x }

fn area(s: Shape) -> f64 {
    match s {
        Shape::Circle(r) => 3.14159 * r * r,
        Shape::Rectangle(w, h) => w * h,
        Shape::Triangle(a, b, c) => {
            let s = (a + b + c) / 2.0
            sqrt(s * (s - a) * (s - b) * (s - c))
        },
    }
}

If you omit a variant, the compiler reports an error about non-exhaustive matching.

Guards

Pattern arms can include if guards for additional conditions:

fn classify_temp(celsius: f64) -> i64 {
    match celsius {
        t if t < 0.0 => 0,
        t if t < 20.0 => 1,
        t if t < 35.0 => 2,
        _ => 3,
    }
}

Matching on structs

struct Point { x: f64, y: f64 }

fn quadrant(p: Point) -> i64 {
    match p {
        Point { x, y } if x > 0.0 && y > 0.0 => 1,
        Point { x, y } if x < 0.0 && y > 0.0 => 2,
        Point { x, y } if x < 0.0 && y < 0.0 => 3,
        Point { x, y } if x > 0.0 && y < 0.0 => 4,
        _ => 0,
    }
}

Current limitations

• Tuple destructuring in let bindings (let (a, b) = pair) is not yet supported. Use match instead.
• Nested pattern matching on deeply nested structs works but is not yet exhaustively checked in all cases.
• Or-patterns (A | B => ...) are parsed but coverage checking for them is still maturing.