Visitor pattern

namespace Wikipedia
{
  using System;
  using System.Text;

  interface IExpressionVisitor
  {
    void Visit(Literal literal);
    void Visit(Addition addition);
  }

  interface IExpression
  {
    void Accept(IExpressionVisitor visitor);
  }

  class Literal : IExpression
  {
    internal double Value { get; set; }

    public Literal(double value)
    {
      this.Value = value;
    }
    public void Accept(IExpressionVisitor visitor)
    {
      visitor.Visit(this);
    }
  }

  class Addition : IExpression
  {
    internal IExpression Left { get; set; }
    internal IExpression Right { get; set; }

    public Addition(IExpression left, IExpression right)
    {
      this.Left = left;
      this.Right = right;
    }

    public void Accept(IExpressionVisitor visitor)
    {
      visitor.Visit(this);
    }
  }

  class ExpressionPrinter : IExpressionVisitor
  {
    StringBuilder sb;

    public ExpressionPrinter(StringBuilder sb)
    {
        this.sb = sb;
    }

    public void Visit(Literal literal)
    {
      sb.Append(literal.Value);
    }

    public void Visit(Addition addition)
    {
      sb.Append("(");
      addition.Left.Accept(this);
      sb.Append("+");
      addition.Right.Accept(this);
      sb.Append(")");
    }
  }

  public class Program
  {
    public static void Main(string[] args)
    {
      // emulate 1+2+3
      var e = new Addition(
        new Addition(
          new Literal(1),
          new Literal(2)
        ),
        new Literal(3)
      );
      var sb = new StringBuilder();
      var expressionPrinter = new ExpressionPrinter(sb);
      e.Accept(expressionPrinter);
      Console.WriteLine(sb);
    }
  }
}

namespace Wikipedia
{
  using System;
  using System.Text;

  abstract class Expression
  {
  }

  class Literal : Expression
  {
    public double Value { get; set; }

    public Literal(double value)
    {
      this.Value = value;
    }
  }

  class Addition : Expression
  {
    public Expression Left { get; set; }
    public Expression Right { get; set; }

    public Addition(Expression left, Expression right)
    {
      Left = left;
      Right = right;
    }
  }

  class ExpressionPrinter
  {
    public static void Print(Literal literal, StringBuilder sb)
    {
      sb.Append(literal.Value);
    }

    public static void Print(Addition addition, StringBuilder sb)
    {
      sb.Append("(");
      Print((dynamic) addition.Left, sb);
      sb.Append("+");
      Print((dynamic) addition.Right, sb);
      sb.Append(")");
    }
  }

  class Program
  {
    static void Main(string[] args)
    {
      // emulate 1+2+3
      var e = new Addition(
        new Addition(
          new Literal(1),
          new Literal(2)
        ),
        new Literal(3)
      );
      var sb = new StringBuilder();
      ExpressionPrinter.Print((dynamic) e, sb);
      Console.WriteLine(sb);
    }
  }
}

#include <iostream>
#include <vector>

class AbstractDispatcher;	// Forward declare AbstractDispatcher

class File {	// Parent class for the elements (ArchivedFile, SplitFile and ExtractedFile)
public:
	// This function accepts an object of any class derived from AbstractDispatcher and must be implemented in all derived classes
	virtual void accept(AbstractDispatcher &dispatcher) = 0;
};

// Forward declare specific elements (files) to be dispatched
class ArchivedFile;
class SplitFile;
class ExtractedFile;

class AbstractDispatcher {	 // Declares the interface for the dispatcher
public:
	// Declare overloads for each kind of a file to dispatch
	virtual void dispatch(ArchivedFile &file) = 0;
	virtual void dispatch(SplitFile &file) = 0;
	virtual void dispatch(ExtractedFile &file) = 0;
};

class ArchivedFile: public File {	 // Specific element class #1
public:
	// Resolved at runtime, it calls the dispatcher's overloaded function, corresponding to ArchivedFile.
	void accept(AbstractDispatcher &dispatcher) override {
		dispatcher.dispatch(*this);
	}
};

class SplitFile: public File {	 // Specific element class #2
public:
	// Resolved at runtime, it calls the dispatcher's overloaded function, corresponding to SplitFile.
	void accept(AbstractDispatcher &dispatcher) override {
		dispatcher.dispatch(*this);
	}
};

class ExtractedFile: public File {	 // Specific element class #3
public:
	// Resolved at runtime, it calls the dispatcher's overloaded function, corresponding to ExtractedFile.
	void accept(AbstractDispatcher &dispatcher) override {
		dispatcher.dispatch(*this);
	}
};

class Dispatcher: public AbstractDispatcher {	// Implements dispatching of all kind of elements (files)
public:
	void dispatch(ArchivedFile &) override {
		std::cout << "dispatching ArchivedFile" << std::endl;
	}

	void dispatch(SplitFile &) override {
		std::cout << "dispatching SplitFile" << std::endl;
	}

	void dispatch(ExtractedFile &) override {
		std::cout << "dispatching ExtractedFile" << std::endl;
	}
};

int main() {
	ArchivedFile archivedFile;
	SplitFile splitFile;
	ExtractedFile extractedFile;

	std::vector<File*> files;
	files.push_back(&archivedFile);
	files.push_back(&splitFile);
	files.push_back(&extractedFile);

	Dispatcher dispatcher;

	for (File* file : files) {
		file->accept(dispatcher);
	}
}

package visitor2

import "fmt"

type Visitor interface {
	Visit(element) string
}

type element interface {
	Accept(visitor Visitor) string
}

type Wheel struct {
	name string
}

func (self *Wheel) Accept(visitor Visitor) string {
	return visitor.Visit(self)
}

func (self *Wheel) getName() string {
	return self.name
}

type Engine struct{}

func (self *Engine) Accept(visitor Visitor) string {
	return visitor.Visit(self)
}

type Body struct{}

func (self *Body) Accept(visitor Visitor) string {
	return visitor.Visit(self)
}

type Car struct {
	engine Engine
	body   Body
	wheels [4]Wheel
}

func (self *Car) Accept(visitor Visitor) string {
	res := visitor.Visit(self)
	res += self.engine.Accept(visitor)
	res += self.body.Accept(visitor)
	for _, wheel := range self.wheels {
		res += wheel.Accept(visitor)
	}
	return res
}

type VisitCar struct{}

func (self *VisitCar) Visit(element element) string {
	switch element.(type) {
	case *Body:
		return fmt.Sprintln("visiting body")
	case *Car:
		return fmt.Sprintln("visiting car")
	case *Engine:
		return fmt.Sprintln("visiting engine")
	case *Wheel:
		return fmt.Sprintln("visiting", element.(*Wheel).getName(), "wheel")
	default:
		return "unsupported type"
	}
}

package visitor

import (
	"testing"
)

func TestVistor(t *testing.T) {
	car := Car{
		engine: Engine{},
		body:   Body{},
	}
	car.wheels[0] = Wheel{"front left"}
	car.wheels[1] = Wheel{"front right"}
	car.wheels[2] = Wheel{"back  left"}
	car.wheels[3] = Wheel{"back right"}

	visitor := VisitCar{}
	res := car.Accept(&visitor)
	exp := `visiting car
visiting engine
visiting body
visiting front left wheel
visiting front right wheel
visiting back  left wheel
visiting back right wheel
`
	if res != exp {
		t.Error(res, exp)
	}
}

interface CarElement {
    void accept(CarElementVisitor visitor);
}

interface CarElementVisitor {
    void visit(Body body);
    void visit(Car car);
    void visit(Engine engine);
    void visit(Wheel wheel);
}

class Car implements CarElement {
    CarElement[] elements;

    public Car() {
        this.elements = new CarElement[] {
            new Wheel("front left"), new Wheel("front right"),
            new Wheel("back left"), new Wheel("back right"),
            new Body(), new Engine()
        };
    }

    public void accept(final CarElementVisitor visitor) {
        for (CarElement elem : elements) {
            elem.accept(visitor);
        }
        visitor.visit(this);
    }
}

class Body implements CarElement {
    public void accept(final CarElementVisitor visitor) {
        visitor.visit(this);
    }
}

class Engine implements CarElement {
    public void accept(final CarElementVisitor visitor) {
        visitor.visit(this);
    }
}

class Wheel implements CarElement {
    private String name;

    public Wheel(final String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void accept(final CarElementVisitor visitor) {
        /*
         * accept(CarElementVisitor) in Wheel implements
         * accept(CarElementVisitor) in CarElement, so the call
         * to accept is bound at run time. This can be considered
         * the *first* dispatch. However, the decision to call
         * visit(Wheel) (as opposed to visit(Engine) etc.) can be
         * made during compile time since 'this' is known at compile
         * time to be a Wheel. Moreover, each implementation of
         * CarElementVisitor implements the visit(Wheel), which is
         * another decision that is made at run time. This can be
         * considered the *second* dispatch.
         */
        visitor.visit(this);
    }
}

class CarElementDoVisitor implements CarElementVisitor {
    public void visit(final Body body) {
        System.out.println("Moving my body");
    }

    public void visit(final Car car) {
        System.out.println("Starting my car");
    }

    public void visit(final Wheel wheel) {
        System.out.println("Kicking my " + wheel.getName() + " wheel");
    }

    public void visit(final Engine engine) {
        System.out.println("Starting my engine");
    }
}

class CarElementPrintVisitor implements CarElementVisitor {
    public void visit(final Body body) {
        System.out.println("Visiting body");
    }

    public void visit(final Car car) {
        System.out.println("Visiting car");
    }

    public void visit(final Engine engine) {
        System.out.println("Visiting engine");
    }

    public void visit(final Wheel wheel) {
        System.out.println("Visiting " + wheel.getName() + " wheel");
    }
}

public class VisitorDemo {
    public static void main(final String[] args) {
        final Car car = new Car();

        car.accept(new CarElementPrintVisitor());
        car.accept(new CarElementDoVisitor());
    }
}

(defclass auto ()
  ((elements :initarg :elements)))

(defclass auto-part ()
  ((name :initarg :name :initform "<unnamed-car-part>")))

(defmethod print-object ((p auto-part) stream)
  (print-object (slot-value p 'name) stream))

(defclass wheel (auto-part) ())

(defclass body (auto-part) ())

(defclass engine (auto-part) ())

(defgeneric traverse (function object other-object))

(defmethod traverse (function (a auto) other-object)
  (with-slots (elements) a
    (dolist (e elements)
      (funcall function e other-object))))

;; do-something visitations

;; catch all
(defmethod do-something (object other-object)
  (format t "don't know how ~s and ~s should interact~%" object other-object))

;; visitation involving wheel and integer
(defmethod do-something ((object wheel) (other-object integer))
  (format t "kicking wheel ~s ~s times~%" object other-object))

;; visitation involving wheel and symbol
(defmethod do-something ((object wheel) (other-object symbol))
  (format t "kicking wheel ~s symbolically using symbol ~s~%" object other-object))

(defmethod do-something ((object engine) (other-object integer))
  (format t "starting engine ~s ~s times~%" object other-object))

(defmethod do-something ((object engine) (other-object symbol))
  (format t "starting engine ~s symbolically using symbol ~s~%" object other-object))

(let ((a (make-instance 'auto
                        :elements `(,(make-instance 'wheel :name "front-left-wheel")
                                    ,(make-instance 'wheel :name "front-right-wheel")
                                    ,(make-instance 'wheel :name "rear-left-wheel")
                                    ,(make-instance 'wheel :name "rear-right-wheel")
                                    ,(make-instance 'body :name "body")
                                    ,(make-instance 'engine :name "engine")))))
  ;; traverse to print elements
  ;; stream *standard-output* plays the role of other-object here
  (traverse #'print a *standard-output*)

  (terpri) ;; print newline

  ;; traverse with arbitrary context from other object
  (traverse #'do-something a 42)

  ;; traverse with arbitrary context from other object
  (traverse #'do-something a 'abc))

(defmethod traverse (function (a auto)) ;; other-object removed
  (with-slots (elements) a
    (dolist (e elements)
      (funcall function e)))) ;; from here too

  ;; ...

  ;; alternative way to print-traverse
  (traverse (lambda (o) (print o *standard-output*)) a)

  ;; alternative way to do-something with
  ;; elements of a and integer 42
  (traverse (lambda (o) (do-something o 42)) a)

"""
Visitor pattern example.
"""

from abc import ABCMeta, abstractmethod

NOT_IMPLEMENTED = "You should implement this."

class CarElement:
    __metaclass__ = ABCMeta
    @abstractmethod
    def accept(self, visitor):
        raise NotImplementedError(NOT_IMPLEMENTED)

class Body(CarElement):
    def accept(self, visitor):
        visitor.visitBody(self)


class Engine(CarElement):
    def accept(self, visitor):
        visitor.visitEngine(self)


class Wheel(CarElement):
    def __init__(self, name):
        self.name = name
    def accept(self, visitor):
        visitor.visitWheel(self)

class Car(CarElement):
    def __init__(self):
        self.elements = [
            Wheel("front left"), Wheel("front right"),
            Wheel("back left"), Wheel("back right"),
            Body(), Engine()
        ]

    def accept(self, visitor):
        for element in self.elements:
            element.accept(visitor)
        visitor.visitCar(self)

class CarElementVisitor:
    __metaclass__ = ABCMeta
    @abstractmethod
    def visitBody(self, element):
        raise NotImplementedError(NOT_IMPLEMENTED)
    @abstractmethod
    def visitEngine(self, element):
        raise NotImplementedError(NOT_IMPLEMENTED)
    @abstractmethod
    def visitWheel(self, element):
        raise NotImplementedError(NOT_IMPLEMENTED)
    @abstractmethod
    def visitCar(self, element):
        raise NotImplementedError(NOT_IMPLEMENTED)

class CarElementDoVisitor(CarElementVisitor):
    def visitBody(self, body):
        print("Moving my body.")
    def visitCar(self, car):
        print("Starting my car.")
    def visitWheel(self, wheel):
        print("Kicking my {} wheel.".format(wheel.name))
    def visitEngine(self, engine):
        print("Starting my engine.")


class CarElementPrintVisitor(CarElementVisitor):
    def visitBody(self, body):
        print("Visiting body.")
    def visitCar(self, car):
        print("Visiting car.")
    def visitWheel(self, wheel):
        print("Visiting {} wheel.".format(wheel.name))
    def visitEngine(self, engine):
        print("Visiting engine.")

car = Car()
car.accept(CarElementPrintVisitor())
car.accept(CarElementDoVisitor())

Visiting front left wheel.
Visiting front right wheel.
Visiting back left wheel.
Visiting back right wheel.
Visiting body.
Visiting engine.
Visiting car.
Kicking my front left wheel.
Kicking my front right wheel.
Kicking my back left wheel.
Kicking my back right wheel.
Moving my body.
Starting my engine.
Starting my car.