Playground

import System;

into namespace Shapes
{
	public interface IShape
	{
		Area() : f64;
		Describe() : str => "a shape of area " + Area();
	}

	public base class Shape : IShape
	{
		public:
			Area() : f64;
			virtual Name() : str => "shape";
	}

	public class Circle : Shape
	{
		f64 radius;

		public:
			new(f64 radius) { this.radius = radius; }
			override Area() : f64 => 3.14 * radius * radius;
			override Name() : str => "circle";
	}

	public final class Square : Shape
	{
		f64 side;

		public:
			new(f64 side) { this.side = side; }
			override Area() : f64 => side * side;
	}

	Main()
	{
		List<Shape> shapes = new List<Shape>();
		shapes.Add(new Circle(1));
		shapes.Add(new Square(2));
		loop (Shape s in shapes)
			Console.WriteLine(s.Name() + ": " + s.Area());

		IShape i = new Square(3);
		Console.WriteLine(i.Describe());

		Shape first = shapes[0];
		if (first is Circle c)
			Console.WriteLine("a circle of area " + c.Area());
		Square? square = first as Square;
		Console.WriteLine(square == null);
	}
}