Module functions on steroid. This allows you to define a set of methods as public module methods and private instance methods while keeping private dependencies private.

delegated_module_function.rb

# module FooBar
#   extend DelegatedModuleFunction
#
#   define_delegated_module_functions do
#
#     def foo_bar
#       'Foo' + bar
#     end
#
#     private
#
#     def bar
#       'Bar'
#     end
#
#   end
# end
#
# FooBar.foo_bar                # => 'FooBar'
# FooBar.bar                    # => NoMethodError: private method `bar' called
# klass = Class.new { include FooBar }
# klass.foo_bar                 # => NoMethodError: undefined method `foo_bar'
# obj = klass.new
# obj.foo_bar                   # => NoMethodError: private method `foo_bar' called
# obj.instance_exec { foo_bar } # => 'FooBar'
# obj.instance_exec { bar }     # => NoMethodError: undefined method `bar'
#
module DelegatedModuleFunction

  private

  def define_delegated_module_functions(&block)
    impl = Module.new(&block)
    delegatee = Object.new
    delegatee.extend(impl)
    methods = impl.instance_methods
    delegatee_const_name = "ModuleFunctionsDelegatee_#{SecureRandom.hex(8)}"
    self.const_set delegatee_const_name, delegatee
    delegate *methods, to: delegatee_const_name
    module_function *methods
  end

end