Reflection

Ruby has both computational reflection and structural reflection in Ruby.

1. Computational reflection:

We can see the state of living objects in a Ruby process using ObjectSpace.each_object() method [1]. For example, we can print all the living Robot objects in the current Ruby process using the codes below.

computational_reflection.rb

class Robot
  def initialize(name)
    @name = name
  end
	
  def inspect
    "Hello I am robot #{@name}" # Return a human-readable string when printing the object
    end
end

robot_1 = Robot.new("Tom")
robot_2 = Robot.new("Jerry")  

ObjectSpace.each_object(Robot) {|x| p x} # This print the living Robot objects

Source code: https://github.com/yc015/Ruby-Inspect/blob/main/src/computational_reflection.rb

The codes above will print the living Robot objects (robot_1 and robot_2).
The program will print:

Hello I am robot Tom
Hello I am robot Jerry

The ObjectSpace.each_object shows that Ruby has computational reflection, and we can check the current state of the system.

2. Structural reflection:

We can modify the state of the system in Ruby. For example, we can define and add a method to an instance after it was created using define_singleton_method [2].

structural_reflection.rb

str_hello = "Hello world!" # str_hello is a String object

str_hello.define_singleton_method(:my_class_name) do 
  "My class name is String"
end

str_hello.my_class_name # This line will return "My class name is String"

Source code: https://github.com/yc015/Ruby-Inspect/blob/main/src/structural_reflection.rb

The codes above will print:

My class name is String

The codes above add a new method, my_class_name, to the existing String object str_hello.

References:
[1] http://phrogz.net/programmingruby/ospace.html Reflection, ObjectSpace, and Distributed Ruby - Programming Ruby
[2] https://apidock.com/ruby/Object/define_singleton_method Ruby’s Documentation for define_singleton_method - APIDock