wtf.rb

# A little helper from _why
class Object
  def metaclass
    class << self; self; end
  end
end

module A
  def self.foo
    puts "yes"
  end
end

module B
  def foo
    puts "no"
  end
end

A.extend(B)
A.foo # => "yes"

p A.ancestors # => [A]
p A.metaclass.ancestors # => [B, Module, Object, Kernel]
p A.instance_methods(false).sort # => []
p A.metaclass.instance_methods(false).sort # => ["<", "<=", "<=>", "==", "===", ">", ">=", "ancestors", "autoload", "autoload?", "class_eval", "class_exec", "class_variable_defined?", "class_variables", "const_defined?", "const_get", "const_missing", "const_set", "constants", "foo", "freeze", "include?", "included_modules", "instance_method", "instance_methods", "method_defined?", "module_eval", "module_exec", "name", "private_class_method", "private_instance_methods", "private_method_defined?", "protected_instance_methods", "protected_method_defined?", "public_class_method", "public_instance_methods", "public_method_defined?", "to_s"]
p A.included_modules # => []
p A.metaclass.included_modules # => [B, Kernel]