//An IShapeVisitor is a function over IShapes
interface IShapeVisitor<R> extends IFunc<IShape, R> {
  R visitCircle(Circle c);
  R visitSquare(Square s);
  R visitRect(Rect r);
}

//ShapeArea is a function object over IShapes that computes their area
class ShapeArea implements IShapeVisitor<Double> {
// Everything from the IShapeVisitor interface:
  public Double visitCircle(Circle c) { return Math.PI * c.radius * c.radius; }
  public Double visitSquare(Square s) { return s.side * s.side; }
  public Double visitRect(Rect r) { return r.w * r.h; }

// Everything from the IFunc interface:
public Double apply(IShape s) { return s.accept(this); }