Note

1type Shape =
2  | { kind: 'circle'; radius: number }
3  | { kind: 'rect'; width: number; height: number }
4  | { kind: 'square'; size: number };
5
6function area(shape: Shape) {
7  switch (shape.kind) {
8    case 'circle': return Math.PI * shape.radius ** 2;
9    case 'rect': return shape.width * shape.height;
10    case 'square': return shape.size ** 2;
11  }
12}