Ruby method lookup path for an object

ruby

That other post makes it seem confusing, but it really isn't. If you are interested in such things, you should read "Metaprogramming Ruby". Until then, the basic rule is one step to the right and up:

          Object (superclass)
              ^
              |
          Parent class A(superclass)
              ^
              |
          Parent class B(superclass)
              ^
              |
obj  ->   object's class

2) Singleton classes are inserted between the obj and the object's class:

          Object
              ^
              |
          Parent class A(superclass)
              ^
              |
          Parent class B(superclass)
              ^
              |
          object's class(superclass)
              ^
              |
obj  ->   obj's singleton_class

3) Included modules are inserted immediately above the class that does the including:

          Object
              ^
              |
          Parent class A
              ^
              |
              Module included by Parent Class B
              ^
              |
          Parent class B
              ^
              |
          object's class
              ^
              |
obj  ->   obj's singleton_class

Edit:

Please point out any flaws

p method_lookup_chain(Class)

--output:--
[#<Class:Class>, #<Class:Module>, #<Class:Object>, #<Class:BasicObject>]

But...

class Object
  def greet
    puts "Hi from an Object instance method"
  end
end

Class.greet

--output:--
Hi from an Object instance method

And..

class Class
  def greet
    puts "Hi from a Class instance method"
  end
end

Class.greet

--output:--
Hi from a Class instance method

The lookup path for a method called on a class actually continues past BasicObject's singleton class(#<Class:BasicObject>):

class BasicObject
  class <<self
    puts superclass
  end
end

--output:--
Class

The full lookup path for a method called on Class looks like this:

                  Basic Object                 
                      ^
                      |
                    Object
                      ^
                      |
                    Module
                      ^
                      |
                    Class
                      ^
                      |
BasicObject    BasicObject's singleton class                
  |                   ^
  |                   |
Object         Object's singleton class
  |                   ^
  |                   |
Module         Module's singleton class
  |                   ^
  |                   |
Class  --->    Class's singleton class

The lookup starts in Class's singleton class and then goes up the hierarchy on the right. "Metaprogramming Ruby" claims there is a unified lookup theory for all objects, but the lookup for methods called on a class does not fit the diagram in 3).

You have the same problem here:

class A 
end

class B < A
end

p method_lookup_chain(B)

--output:--
[#<Class:B>, #<Class:A>, #<Class:Object>, #<Class:BasicObject>]

It should be this:

                  Basic Object                 
                      ^
                      |
                    Object
                      ^
                      |
                    Module
                      ^
                      |
                    Class
                      ^
                      |
BasicObject    BasicObject's singleton class
  |                   ^
  |                   |
Object         Object's singleton class
  |                   ^
  |                   |
  A            A's singleton class
  |                   ^
  |                   |
  B.greet -->  B's singleton class

One thing you need to keep in mind: the lookup path of any method called on a class has to include Class somewhere because ALL classes inherit from Class.

Ruby method lookup path for an object

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