[ruby-core:107581] [Ruby master Feature#12962] Feature Proposal: Extend 'protected' to support module friendship

Issue #12962 has been updated by Eregon (Benoit Daloze).


For this Net::HTTP example IMHO it'd be better to use `send`, that would make it a private method and be explicit that code is calling into partly-internals.

IMHO friend-like visibility is too magic and the linear search could be a significant overhead on such calls.
`send` can be easily optimized OTOH (especially with a literal Symbol as name).

----------------------------------------
Feature #12962: Feature Proposal: Extend 'protected' to support module friendship
https://bugs.ruby-lang.org/issues/12962#change-96492

* Author: matthewd (Matthew Draper)
* Status: Open
* Priority: Normal
----------------------------------------
When working on a larger library, with many classes that have both official API methods and internal supporting methods, it can be hard to distinguish between them.

In Rails, for example, we currently do this using `:nodoc:` -- if a method is hidden from the documentation, it is not part of the officially supported API, even if it has `public` visibility.

This approach can be confusing for users, however, because they can find methods that seem to do what they want, and start calling them, without ever looking at the documentation: either by just guessing a likely method name, or even being guided to it by `did_you_mean`.

Method visibility controls seem like the right solution to this problem: if we make the methods `private` or `protected`, users can still choose to call them, but only by first acknowledging that they're using internal API. However, as we have object oriented internals, a lot of our internal API calls are between instances of unrelated classes... and using `send` on all those calls would make our own code very untidy.

I propose that the solution to this problem is to make `protected` more widely useful, by allowing a module to nominate other modules that are allowed to call its protected methods.

```ruby
class A
  protected def foo
    "secrets"
  end
end

class D
  def call_foo
    A.new.foo
  end
end
A.friend D

D.new.call_foo # => "secrets"
```

This change is backwards compatible for existing uses of `protected`: a module is always considered its own friend (so calls that previously worked will continue to do so), and classes have no other friends by default (so calls that were previously disallowed will also continue to do so).

Using a module, a library can easily establish a 'friendship group' of related classes without needing to link them individually, as well as providing a single opt-in for user code that consciously chooses to use unsupported APIs.

```ruby
module MyLib
  module Internals
  end

  class A
    include Internals
    friend Internals

    protected def foo
      "implementation"
    end
  end

  class B
    include Internals
    friend Internals

    protected def bar
      A.new.foo
    end
  end
end

class UserCode
  def call_things
    [MyLib::A.new.foo, MyLib::B.new.bar]
  end
end

class FriendlyUserCode
  include MyLib::Internals

  def call_things
    [MyLib::A.new.foo, MyLib::B.new.bar]
  end
end

UserCode.new.call_things # !> NoMethodError: protected method `foo'..
FriendlyUserCode.new.call_things # => ["implementation", "implementation"]
```

This change seems in keeping with the ruby philosophy that a method's visibility is more of a guideline than a strictly enforced rule -- here, we allow the callee to blur the line, instead of leaving it up to the caller to use `send`.


The implementation is surprisingly simple, and only adds time (searching an array of friends, instead of only looking for the current class) after a method call has already resolved to a protected method.


While I'm personally most interested in how this could be applied in a Rails-sized project (such as.. Rails), I believe it would provide a helpful clarifying tool to any library that has multiple collaborating classes, whose instances are also exposed to user code.




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