[ruby-core:120284] [Ruby master Bug#20943] Constant defined in `Data.define` block

["shan (Shannon Skipper) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20943 has been updated by shan (Shannon Skipper).


byroot (Jean Boussier) wrote in #note-2:

> Which is a bit wasteful as you define two classes instead of one, but not a big deal.

I agree the extra class being created is minor, but I'm slightly more bothered by the anonymous class in the ancestry.

```ruby
[Something,
 #<Class:0x00000001438d5dd0>,
 Struct,
...
]
```
> I kinda wish this would be valid syntax:
> 
> ```ruby
> class Something = Struct.new(:a, :b)
>   ...
> end
> ```

I'd like that too. That same pattern would be great for both `Struct` and `Data` from my vantage. Or if both could keep constants in scope within `do` blocks, but would that be consider breaking? Too late for Data? I'd like it.

It seems like options include:
1. Change `Data.define do` block scope for constants
2. Add a new `Something = Data.define` or similar syntax
3. Recommend reopening classes in docs
4. Recommend inheritance in docs
5. Keep the status quo of defining a constant outside the scope in docs

That ^ list happens to be roughly in my personal order of preference from top to bottom. :) Having both 1 and 2 would also be an option.

I wonder if changing documentation to something that keeps `NONE` inside the module is worth doing in the short term? If syntax adjustments are decided against, I'd rather just recommend reopening `Data` and `Struct` classes rather than the slight back bending with existing solutions like `self::` prefix or `< Data.define`. On the other hand, I'd love to see a syntax adjustment to make it easier to define a constant within a `Data` without reopening the class.

----------------------------------------
Bug #20943: Constant defined in `Data.define` block
https://bugs.ruby-lang.org/issues/20943#change-111049

* Author: nobu (Nobuyoshi Nakada)
* Status: Open
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
>From https://github.com/ruby/ruby/pull/12274:
> A couple times in code review I've seen constants inadvertently leak to top level from within a `Struct` or `Data` do block. I think it would be nice to show reopening the `Data` class when a constant is defined, so the constant is defined within the namespace. In this case, `Measure::NONE` instead of top level `Object::NONE`. It would also show readers that it's okay to reopen a `Data` class, which seems nice since some folk might not realize. Thanks for considering!

However, I think that `NONE` probably might be intended to be defined under `Measure`.

Current:
```ruby
Measure = Data.define(:amount, :unit) do
  NONE = Data.define
end
p NONE #=> NONE
```

Another:
```ruby
Measure = Data.define(:amount, :unit) do
  NONE = Data.define
  p NONE #=> Measure::NONE
end
p NONE # uninitialized constant NONE (NameError)
```

@zverok How do think?



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