public final class Visitor {

  /** Shape
    *
    * We wish to define a data type with exactly two variants: Circle and Rectangle.
    *
    * In particular, we do not want Shape to be open to extension by
    * new kinds of variants (as would be the case with an abstract class).
    * The reason we do not want the variants of Shape to be open to extension
    * is because this will grant us the ability to extend the operations
    * that may be performed on Shapes, instead. Each such operation will be
    * defined by a particular "ShapeVisitor". "ShapeVisitor" is designed in
    * such a way that it exhaustively handles every variant of Shape, which
    * would be impossible to do if the variants of Shape were open to extension.
    *
    * As such, the "Visitor pattern" allows us a design that intentionally
    * trades away extensibility in variants of a data structure in exchange for
    * extensibility in operations that can be performed on that data structure.
    */
  public interface Shape {
    public <A> A visit(ShapeVisitor<A> visitor);
  }

  /** ShapeVisitor
    *
    * The essense of the visitor pattern is that instead of defining each `Shape`
    * operation as a method on the `Shape` class, we encode each operation we
    * wish to perform on `Shape`s as a `ShapeVisitor`.
    *
    * `ShapeVisitor` has one method for each `Shape` variant. The number of
    * variants is fixed (as opposed to making `Shape` an abstract class, where
    * the number of variants would be extensible) but in exchange for that, the
    * operations (encoded as `ShapeVisitor`s as opposed to as methods on `Shape`)
    * is extensible.
    */
  public interface ShapeVisitor<A> {
    A visitCircle(Double x, Double y, Double r);
    A visitRectangle(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 visit(ShapeVisitor<A> visitor) {
        return visitor.visitCircle(x, y, r);
      }
    };
  }

  public static final Shape newRectangle(Double x, Double y, Double w, Double h) {
    return new Shape() {
      public <A> A visit(ShapeVisitor<A> visitor) {
        return visitor.visitRectangle(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);

  /** Defining a `ShapeVisitor` is analogous to pattern matching on the
    * variants of `Shape` in a pattern-matching language.
    */
  public static final ShapeVisitor<Double> area =
    new ShapeVisitor<Double>() {
      public Double visitCircle(Double x, Double y, Double r) {
        return 2 * Math.PI * Math.pow(r, 2);
      }
      public Double visitRectangle(Double x, Double y, Double w, Double h) {
        return w * h;
      }
    };

  /** We can, and even downstream clients can, define new operations on all
    * of `Shape`s variants by creating new `ShapeVisitor`s.
    */
  public static final ShapeVisitor<Shape> translate(Double dx, Double dy) {
    return new ShapeVisitor<Shape>() {
      public Shape visitCircle(Double x, Double y, Double r) {
        return newCircle(x + dx, y + dy, r);
      }
      public Shape visitRectangle(Double x, Double y, Double w, Double h) {
        return newRectangle(x + dx, y + dx, w, h);
      }
    };
  }

  public static final ShapeVisitor<String> show =
    new ShapeVisitor<String>() {
      public String visitCircle(Double x, Double y, Double r) {
        return String.format("Circle {x = %f, y = %f, r = %f}", x, y, r);
      }
      public String visitRectangle(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 =
      exampleCircle.visit(translate(3.0, 4.0)).visit(show);

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

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