Golangでファクトリーパターンを適用する方法

type Shape interface {
    Area() float64
    Perimeter() float64
}

type Circle struct {
    radius float64
}

func (c *Circle) Area() float64 {
    return math.Pi * c.radius * c.radius
}

func (c *Circle) Perimeter() float64 {
    return 2 * math.Pi * c.radius
}

type Rectangle struct {
    length float64
    width float64
}

func (r *Rectangle) Area() float64 {
    return r.length * r.width
}

func (r *Rectangle) Perimeter() float64 {
    return 2 * (r.length + r.width)
}

type ShapeFactory struct{}

func (f *ShapeFactory) CreateShape(shapeType string) Shape {
    switch shapeType {
    case "circle":
        return &Circle{}
    case "rectangle":
        return &Rectangle{}
    default:
        return nil
    }
}

factory := &ShapeFactory{}

circle := factory.CreateShape("circle")
circle.radius = 5
fmt.Println("Circle area:", circle.Area())

rectangle := factory.CreateShape("rectangle")
rectangle.length = 10
rectangle.width = 5
fmt.Println("Rectangle area:", rectangle.Area())

Circle area: 78.53981633974483
Rectangle area: 50