behavioral/visitor: implement visitor pattern

behavioral/visitor.md

@@ -0,0 +1,80 @@
+# Visitor Pattern
+Visitor behavioral design pattern provides a way to separate an algorithm from an object on which it operates.
+
+It gives the ability to extend the existing object without modifying the object itself.
+
+## Implementation
+
+```go
+type Visitor interface {
+	visitCircle(circle Circle) string
+	visitLine(line Line) string
+}
+
+type Shape interface {
+	accept(Visitor) string
+}
+
+type Circle struct {
+	Rad int
+}
+
+func (c Circle) accept(v Visitor) string {
+	return v.visitCircle(c)
+}
+
+type Line struct {
+	Len int
+}
+
+func (l Line) accept(v Visitor) string {
+	return v.visitLine(l)
+}
+```
+
+## Usage
+### JSON marshaller
+
+```go
+type JsonVisitor struct {
+}
+
+func (*JsonVisitor) visitCircle(circle Circle) string {
+	return fmt.Sprintf(`{"type": "circle", "radius": "%v"}`, circle.Rad)
+}
+
+func (*JsonVisitor) visitLine(line Line) string {
+	return fmt.Sprintf(`{"type": "line", "length": "%v"}`, line.Len)
+}
+```
+
+```go
+circle := Circle{12}
+line := Line{42}
+jsonVisitor := JsonVisitor{}
+fmt.Println(circle.accept(&jsonVisitor))
+fmt.Println(line.accept(&jsonVisitor))
+```
+
+### XML marshaller
+
+```go
+type XmlVisitor struct {
+}
+
+func (*XmlVisitor) visitCircle(circle Circle) string {
+	return fmt.Sprintf(`<circle><radius>%d</radius></circle>`, circle.Rad)
+}
+
+func (*XmlVisitor) visitLine(line Line) string {
+	return fmt.Sprintf(`<line><length>%d</length></line>`, line.Len)
+}
+```
+
+```go
+circle := Circle{12}
+line := Line{42}
+xmlVisitor := XmlVisitor{}
+fmt.Println(circle.accept(&xmlVisitor))
+fmt.Println(line.accept(&xmlVisitor))
+```

behavioral/visitor/main.go

@@ -0,0 +1,64 @@
+package main
+
+import (
+	"fmt"
+)
+
+type Visitor interface {
+	visitCircle(circle Circle) string
+	visitLine(line Line) string
+}
+
+type Shape interface {
+	accept(Visitor) string
+}
+
+type Circle struct {
+	Rad int
+}
+
+func (c Circle) accept(v Visitor) string {
+	return v.visitCircle(c)
+}
+
+type Line struct {
+	Len int
+}
+
+func (l Line) accept(v Visitor) string {
+	return v.visitLine(l)
+}
+
+type JsonVisitor struct {
+}
+
+func (*JsonVisitor) visitCircle(circle Circle) string {
+	return fmt.Sprintf(`{"type": "circle", "radius": "%v"}`, circle.Rad)
+}
+
+func (*JsonVisitor) visitLine(line Line) string {
+	return fmt.Sprintf(`{"type": "line", "length": "%v"}`, line.Len)
+}
+
+type XmlVisitor struct {
+}
+
+func (*XmlVisitor) visitCircle(circle Circle) string {
+	return fmt.Sprintf(`<circle><radius>%d</radius></circle>`, circle.Rad)
+}
+
+func (*XmlVisitor) visitLine(line Line) string {
+	return fmt.Sprintf(`<line><length>%d</length></line>`, line.Len)
+}
+
+func main() {
+	circle := Circle{12}
+	line := Line{42}
+	jsonVisitor := JsonVisitor{}
+	fmt.Println(circle.accept(&jsonVisitor))
+	fmt.Println(line.accept(&jsonVisitor))
+
+	xmlVisitor := XmlVisitor{}
+	fmt.Println(circle.accept(&xmlVisitor))
+	fmt.Println(line.accept(&xmlVisitor))
+}