struct Circle { radius: f64 }
struct Square { side: f64 }

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

impl Shape for Square {
  fn area(&self) -> f64
  {
     self.side * self.side
  }
}

impl Shape for Circle {
  fn area(&self) -> f64
  {
    self.radius * self.radius * std::f64::consts::PI
  }
}

fn main() 
{
  let shapes:[&Shape; 4] = [
    &Circle{radius: 1.0},
    &Square{side: 3.0},
    &Circle{radius: 2.0},
    &Square{side: 4.0}
  ];
  for (i, shape) in shapes.iter().enumerate()
  {
    println!("Shape #{} has area {}", i, shape.area());
  }
}