[ruby-core:116369] [Ruby master Feature#20202] Memoized endless method definitions

["matheusrich (Matheus Richard) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20202 has been reported by matheusrich (Matheus Richard).

----------------------------------------
Feature #20202: Memoized endless method definitions
https://bugs.ruby-lang.org/issues/20202

* Author: matheusrich (Matheus Richard)
* Status: Open
* Priority: Normal
----------------------------------------
I propose introducing a shorthand for memoized endless method definitions:

```rb
class Foo
  def bar ||= :memoized_value

  # It should behave like
  def bar = (@bar ||= :memoized_value)
end
```

Not only is this shorter and (IMO) a natural follow-up for endless method definitions, but it's also a common pattern on Ruby codebases. It's very useful to decompose responsibilities into several objects:

```rb
class User
  def notifications_enabled? = settings.notifications?
  def enable_notifications = (settings.notifications = true)
  def disable_notifications = (settings.notifications = false)

  private

  def settings = @settings ||= Settings.new(self)
end

class User::Settings
  attr_writer :notifications

  def initialize(user)
    @user = user
    @notifications = false
  end

  def notifications? = @notifications
end

u = User.new
u.notifications_enabled? # => false
u.enable_notifications
u.notifications_enabled? # => true
```

In the example, the `settings` method could be rewritten as

```rb
  def settings ||= Settings.new(self)
```



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