struct Rect {
	var width height: number;
}

struct Circle {
	var radius: number;
}

trait Shaped[T] {
	function get_area(value: ref T): number;
}

@instance
trait Shaped[Rect] {
	function get_area(value: Rect): number {
		return value.width * value.height;
	}
}

@instance
trait Shaped[Circle] {
	function get_area(value: Circle): number {
		return value.radius * value.radius * 3.1415;
	}
}

// Here starts the fun part
struct Shape[@erase T] {
	trait Shaped[T];
	var val: T;
}

var shape: Shape[Circle] = {
	val = {
		radius=20
		// trait is automatic
	}
};
// `shape` acts exactly as if no @erase annotation
// if we omit the type parameter, Shape becomes sizeless type
// ref Shape is fair game
var some_shape: ref Shape;
some_shape = &shape; // this (type erasure) is allowed
// this is allowed - some_shape.trait is used as an implementation
get_area(some_shape.val);