/** With this improved interface, the connection between the visitor
  * pattern and pattern matching is much move obvious.
  */
public final class VisitorImproved {

  public interface Shape {
    public <A> A match(ShapeCases<A> cases);
  }

  public interface ShapeCases<A> {
    public A caseCircle(Double x, Double y, Double r);
    public A caseRectangle(Double x, Double y, Double w, Double h);
  }

  public static final Shape newCircle(Double x, Double y, Double r) {
    return new Shape() {
      public <A> A match(ShapeCases<A> cases) {
        return cases.caseCircle(x, y, r);
      }
    };
  }

  public static final Shape newRectangle(Double x, Double y, Double w, Double h) {
    return new Shape() {
      public <A> A match(ShapeCases<A> cases) {
        return cases.caseRectangle(x, y, w, h);
      }
    };
  }

  public static final Shape exampleCircle =
    newCircle(2.0, 1.4, 4.5);

  public static final Shape exampleRectangle =
    newRectangle(1.3, 3.1, 10.3, 7.7);

  public static final Double area(Shape shape) {
    return shape.match(new ShapeCases<Double>() {
      public Double caseCircle(Double x, Double y, Double r) {
        return 2 * Math.PI * Math.pow(r, 2);
      }
      public Double caseRectangle(Double x, Double y, Double w, Double h) {
        return w * h;
      }
    });
  }

  public static final Shape translate(Shape shape, Double dx, Double dy) {
    return shape.match(new ShapeCases<Shape>() {
      public Shape caseCircle(Double x, Double y, Double r) {
        return newCircle(x + dx, y + dy, r);
      }
      public Shape caseRectangle(Double x, Double y, Double w, Double h) {
        return newRectangle(x + dx, y + dy, w, h);
      }
    });
  }

  public static final String show(Shape shape) {
    return shape.match(new ShapeCases<String>() {
      public String caseCircle(Double x, Double y, Double r) {
        return String.format("Circle {x = %f, y = %f, r = %f}", x, y, r);
      }
      public String caseRectangle(Double x, Double y, Double w, Double h) {
        return String.format("Rectangle {x = %f, y = %f, w = %f, h = %f}", x, y, w, h);
      }
    });
  }

  public static final void main(String[] args) {
    String circleString =
      show(translate(exampleCircle, 3.0, 4.0));

    String rectangleString =
      show(translate(exampleRectangle, -4.0, -3.0));

    System.out.println(circleString);
    System.out.println(rectangleString);
  }
}