area.rs

trait HasArea {
    fn area(&self) -> f64;
}

trait RectangularArea: HasArea {
    fn area(&self) -> f64 {
        self.length() * self.width()
    }
    fn length(&self) -> f64;
    fn width(&self) -> f64;
}

struct Square {
    dimension: f64
}

struct Rectangle {
    length: f64,
    width: f64
}

// These impls here are rejected by the compiler
// because we haven't implemented `HasArea` for these structs.
// However, `RectangularArea` inherits from `HasArea`
// and provides default implementations for all its methods.
// Can the compiler use this information to allow this program?
impl RectangularArea for Square {
    fn length(&self) -> f64 { self.dimension }
    fn width(&self) -> f64 { self.dimension }
}

impl RectangularArea for Rectangle {
    fn length(&self) -> f64 { self.length }
    fn width(&self) -> f64 { self.width }
}

fn print_area<T>(shape: T) where T: HasArea {
    println!("{}", shape.area());
}

fn main() {
    let s = Square { dimension: 2.0 };
    let r = Rectangle { length: 3.0, width: 2.5 };
    print_area(s);
    print_area(r);
}