Include and Extend in Ruby

include_and_extend_in_ruby.rb

# trying to be helpful for https://twitter.com/coorasse/status/1108047414961811456
require 'objspace'

# Helper methods
public
def real_class() ObjectSpace.internal_class_of      self end
def real_super() ObjectSpace.internal_super_of      self end
def references() ObjectSpace.reachable_objects_from self end

# A module we'll include
module MyMod
  def hi
    'there'
  end
end

# ===== INCLUDE =====
class MySuperClass
end

class MySubClass < MySuperClass
  # `include` will create a new "internal class" (ICLASS) and insert it into the inheritance hierarchy
  self           # => MySubClass
    .real_super  # => MySuperClass
  include MyMod
  self           # => MySubClass
    .real_super  # => #<InternalObject:0x00007f9bfb939c90 T_ICLASS>
    .real_super  # => MySuperClass

  # That internal class's methods pointer will point at MyMod's methods
  self.real_super.references # => [MyMod, MySuperClass]

  # Hence normal method lookup on an instance of MyClass will find MyMod's methods
  new.hi  # => "there"
  MyMod.module_eval { def bye() "for now" end }
  new.bye # => "for now"
end

class MySubClass2 < MySuperClass
  # Ruby hides this mechanism from you, though
  # https://github.com/ruby/ruby/blob/afa1505ca89bd350558f1f42b96f1eee781ef019/object.c#L2247-L2249
  superclass       # => MySuperClass
  real_super       # => MySuperClass
  include MyMod
  superclass       # => MySuperClass
  real_super       # => #<InternalObject:0x00007f9bfb92e890 T_ICLASS>
end

class MySubClass3 < MySuperClass
  # And when you look at your ancestors, it translates the internal class to the module
  # https://github.com/ruby/ruby/blob/afa1505ca89bd350558f1f42b96f1eee781ef019/class.c#L1090-L1092
  ancestors.take 2 # => [MySubClass3, MySuperClass]
  include MyMod
  ancestors.take 3 # => [MySubClass3, MyMod, MySuperClass]
end


# ===== EXTEND =====

# If you wanted to add MyMod to an object, you could open its singelton class and include it
s1 = "s1"
class << s1
  include MyMod
end
s1.hi # => "there"

# The rest of the strings are fine
"s2".hi rescue $! # => #<NoMethodError: undefined method `hi' for "s2":String>

# But that's cumbersome, so they added `extend` to do it for you
s3 = "s3"
s3.extend(MyMod)
s3.hi # => "there"

# Ruby hides this from you, though
# https://github.com/ruby/ruby/blob/afa1505ca89bd350558f1f42b96f1eee781ef019/object.c#L256
s4 = "s4"
s4.class      # => String
s4.real_class # => String
s4.extend MyMod
s4.class      # => String
s4.real_class # => #<Class:#<String:0x00007f9bfc80ff80>>
  .real_super # => #<InternalObject:0x00007f9bfc80fb98 T_ICLASS>
  .real_super # => String