Feature #9992

Access Modifiers (Internal Interfaces)

Added by dsferreira (Daniel Ferreira) over 3 years ago. Updated 11 months ago.

Target version:



I’m would like to discuss with you the concept of Internal Interfaces.

Currently ruby offers three access modifiers choices to set methods visibility:

  • public
  • protected
  • private

Public methods define what we may call the Public Interface.
Private methods are private to the class and its subclasses.
Protected methods are public for the subclasses.

I would like to work with a new access modifier that would allow the creation of Internal methods.

Internal methods would be object methods that could only be called within the namespace.


module Foo; end

class Foo::Bar
  def baz 
     puts ‘baz’
  internal :baz

class Foo::Qux
  def baz

Is this something that we can think about in a future implementation of ruby?
An extra feature that would not break backward compatibility.



#1 [ruby-core:63393] Updated by nobu (Nobuyoshi Nakada) over 3 years ago

  • Description updated (diff)

They don't seem to belong to same namespace.
Could you elaborate?

#2 [ruby-core:63394] Updated by dsferreira (Daniel Ferreira) over 3 years ago

Hi Nobuyoshi,

The notion of Internal Interface is about being able to use the methods identified as internal only in the context of a given root module and all its subclasses and/or submodules. Lets call it: namespace full tree.

The namespace full tree in this context would integrate all constants of the tree defined by the namespace root and all its subnodes.

That way if we develop a new gem lets call it

gem foo

We would be able to define an Internal Interface that would only be used internally by gem foo classes and modules
and by any other gem foo extension like for instance:

gem 'foo-bar'

We have this notion of Public and Internal interface when looking at an API (Application Public Interface) in Web Services.

Web Services expose to the users a given set of methods/actions making them Public.

The remaining of the architecture is internal to the infrastructure.

In ruby libraries/packages/gems every class is available to the user.

That makes development of libraries hard because a change to the public interface of a given class may have side effects.

We may need to create a major release just because we need to change some method in one of our classes that should not be used outside of the library context.

What I aim as an architect is to allow a better defined contract between developers and users.

Having the definition of an Internal Interface the architect will have greater freedom to develop the required functionalities.

The development process may be like this:

  1. Define Public Interface
  2. Release version 1.0 once that Public Interface is well defined and will hardly change.
  3. Reengineer the Internal Interface and add functionality to the Public Interface without breaking backwards compatibility.

This way dependencies between libraries will be much better understood.

Versioning of releases with this feature will be able to communicate in a greater detail to the users what nature of changes were made in each release.

We may then identify:

  • Major release
    Third party libraries and/or applications might be broken.
    Library extensions might be broken.

    • Public Interface changes with broken backward compatibility
  • Minor release
    Library extensions using internal interface features might be broken.

    • Public Interface changes with backward compatibility
    • Internal Interface changes with broken backward compatibility
    • Internal Interface changes with impact on overall behaviour
      • Performance improvements
      • New core engine
    • Internal Interface changes with backward compatibility
      • New features for extensions
  • Patch (Tiny) release
    No side effects on extensions or third party libraries and/or applications.

    • Changes to private interface (private and protected methods)
    • Bugs
    • Refactoring
    • Unit test updates

With this feature the library development cycle will be much more linear.
Major releases will be in a much smaller number then nowadays.
We will get much more stable ecosystems.

My idea is to leverage ruby further in order to better fit the enterprise environment.

Happy to reply to any doubts and discuss with you this thoughts and what other alternatives may we have.



#3 [ruby-core:63463] Updated by zzak (Zachary Scott) over 3 years ago

I would suggest opening a feature request if you feel strongly about adding internal to Ruby.

See also:

#4 [ruby-core:63464] Updated by zzak (Zachary Scott) over 3 years ago

Ehh, sorry I must have missed #9992

#5 [ruby-core:72985] Updated by jwmittag (Jörg W Mittag) almost 2 years ago

I'm having trouble understanding what you mean by namespace. Classes and modules don't belong to namespaces. Constants belong to modules, but the fact that two classes which may or may not be referenced by two constants which are namespaced in the same module does not imply any sort of relationship whatsoever between either the two classes or one of the classes and the module.

Expanding your example a bit:

module Foo; end

Baz = Bar = Foo

class Foo::Bar
  def baz 
     puts ‘baz’
  internal :baz

class Bar::Qux
  def baz

Should it work? If not, why? The modules and classes involved are the exact same ones as in your example.

I mean, what about this:

foo =
bar =
baz =

baz.const_set(:Foo, foo)
baz.const_set(:Bar, bar)

quux =
quux.const_set(:Goobledigook, foo)
quux.const_set(:Blahdiblah, bar)

baz.send(:remove_const, :Bar)

module One; module Two; end end

One::Two::Three = quux::Goobledigook

Okay, now what namespace relations are there?

If I now do this:

class One::Two::Three
  internal def blub; end

Am I allowed to call from a method in quux::Blahdiblah?

#6 [ruby-core:75513] Updated by dsferreira (Daniel Ferreira) over 1 year ago

Hi Jörg, thank you very much for your interesting questions.

This is a subject for a wider discussion and your questions come in the right direction in my opinion.

My base view is the following example:

Fred wants to create a gem called foo.
Fred knows that he wants two methods on the interface:

This is all Fred aims to implement as the public interface of the foo gem for version v1.0.0.

Both and Foo.baz expose complex internals which Fred would like to keep isolated from outside the gem Foo namespace.

With time these internals will have an improved architecture with different modules, classes, methods, etc.

By using internal in the internal public methods Fred is confident that there is no broken backwards compatibility since no one can use the methods outside the gem namespace.

An internal method behaves:
1. Like a public method inside Foo namespace.
2. Like a private method outside Foo namespace.

So this is the logic behind the proposed implementation.

Now if Waldo does:

Bar = Foo

Shall Bar be treated as Foo or not?

What we should not allow is something like:

module Foo
  class Bar
     def baz
         puts 1
     internal :baz
end    => error: internal method being called outside ::Foo namespace

For your presented challenges I would open the discussion to the wider community.
What are the possibilities and challenges we would face in order to implement the proposed internal access modifier?

I hope we can make it happen!
It will give us development freedom and architecture control.


I can understand we may get a degradation on performance by using internal.
Maybe we could use a flag to trigger it like we do for verbose.
With the flag off internal would behave just like public.
It would be a flag to increase levels of integrity in the code that we could use with different levels in dev, uat or production environments.
Maybe the extra flag would deserve a separate proposal by its own but makes sense to present it in this context as well in my point of view.
We have debug, verbose and warning level flags.
Why not a new (architecture integrity/performance) flag?

#7 [ruby-core:75562] Updated by dsferreira (Daniel Ferreira) over 1 year ago

There is a proposal for a namespace method: Object#namespace.
The namespace method would make it viable the implementation of Module#internal and simplify a lot the code I have put in place as a proof of concept in my gem internal
If we do:

namespace_root = ::Foo::Bar.namespace.last
# => ::Foo

Then Module#internal would behave like:

  1. Public method if called inside namespace_root (::Foo).
  2. Private method if called outside namespace_root (::Foo).

#8 [ruby-core:79190] Updated by dsferreira (Daniel Ferreira) 11 months ago

Matthew Draper presented a feature request to extended 'protected' access modifier that comes in line with my current proposal.
Matthew presents the problem in a different angle so maybe that will make more understandable the concept of internal interfaces.

What is your opinion?

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