ruby extend vs include

extend_include.rb

#
# <<<--- helpers to explore lookup chain
#
def method_lookup_chain(obj, result = [obj.singleton_class])
  if obj.instance_of? Class
    return add_modules(result) if result.last == BasicObject.singleton_class
    r = result.last.superclass
    method_lookup_chain(obj, result << r)
  else
    return result + obj.class.ancestors
  end
end

def add_modules(klasses)
  r = []
  klasses.each_with_index do |k, i|
    r << k
    next if k == klasses.last
    r << (k.included_modules - klasses[i+1].included_modules)
  end
  r.flatten
end
#
# helpers --->>>
#

module Probe
  def self.extended(klass)
    puts "probe extended #{klass}"
  end

  def self.included(klass)
    puts "probe included into #{klass}"
  end
end

class Target1
  extend Probe
end

class Target2
  class << self
    include Probe
  end
end

[Target1, Target1.singleton_class, Target2, Target2.singleton_class].each do |x|
  puts "#{x}:"
  puts "\tIncludes: " + x.included_modules.join(', ')
  puts "\tLookup: " + method_lookup_chain(x).join(', ')
end

=begin
% ruby -v
ruby 2.1.5p273 (2014-11-13 revision 48405) [x86_64-darwin14.0]

% ruby test.rb
probe extended Target1
probe included into #<Class:Target2>
Target1:
        Includes: Kernel
        Lookup: #<Class:Target1>, Probe, #<Class:Object>, #<Class:BasicObject>
#<Class:Target1>:
        Includes: Probe, Kernel
        Lookup: #<Class:#<Class:Target1>>, #<Class:#<Class:Object>>, #<Class:#<Class:BasicObject>>, #<Class:Class>, #<Class:Module>, #<Class:Object>, #<Class:BasicObject>
Target2:
        Includes: Kernel
        Lookup: #<Class:Target2>, Probe, #<Class:Object>, #<Class:BasicObject>
#<Class:Target2>:
        Includes: Probe, Kernel
        Lookup: #<Class:#<Class:Target2>>, #<Class:#<Class:Object>>, #<Class:#<Class:BasicObject>>, #<Class:Class>, #<Class:Module>, #<Class:Object>, #<Class:BasicObject>