[ruby-core:121199] [Ruby master Feature#20202] Memoized endless method definitions

Issue #20202 has been updated by systho (Philippe Van Eerdenbrugghe).


(I'm not a frequent user of this board, I hope it is okay to comment on a closed ticket)

I believe there is a significant use case for this feature idea because it reveals something about the intention in a more terser way, and therefore less noisy.

Memoization is a frequent enough intention and the most frequent pattern for implementing it is `@current_method_name ||= compute_value `

This pattern is frequently not usable because of 2 reasons :
 - A) The computation does not hold in a one-liner
 - B) The result is a boolean (or anything that can be evaluated to false by || operator)

Case A does not really interest us here since endless methods are mostly about one-liners. 
Case B is usually solved by using a different pattern : 
```
def my_method
  return @my_method if defined?(@my_method)
  @my_method = compute_boolean_value
end
```

which turns case B into a multiliner and therefore prevents it being written as an endless method.
This is even worse with most style guidelines forcing a blank line after an early return statement, which is also quite frequent.

An additional issue happens when working in teams that will force the usage of `defined?(@ivar)` as the only valid way to do memoizing in order to maximize consistency within the team.

Because of these reasons, the memoization *intention* gets lost in noisy boilerplate and I believe this feature idea (or anything that would allow marking the memoization intention at the signature level) has a valid use case.

The two other bullet points about the method set are beyond my understanding so I cannot propose an argument.

There is an issue which has not been mentioned though : memoization is rather easy for method without arguments called on immutable objects. 
But it becomes much more complex when those criteria are not met.

I do not know how realistic it would be to implement this feature in a way that would gracefully fail when some conditions are not met.

----------------------------------------
Feature #20202: Memoized endless method definitions
https://bugs.ruby-lang.org/issues/20202#change-112135

* Author: matheusrich (Matheus Richard)
* Status: Rejected
----------------------------------------
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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