[ruby-core:115974] [Ruby master Feature#20093] Syntax or keyword to reopen existing classs/modules, never to define new classs/modules

Issue #20093 has been updated by kjtsanaktsidis (KJ Tsanaktsidis).


> The only downside I see is that doesn't make it easy to define and access constants defined under that class/module.
> A possibility in that case is to use class_eval(String)

I actually found this quite surprising:

```
irb(main):008> String.class_eval { Module.nesting }
=> []
irb(main):009> String.class_eval "Module.nesting"
=> [String]
```

I get why the first one is empty (we're not inside a `class String ... end` block), but I find it surprising that the string form returns `String`.

----------------------------------------
Feature #20093: Syntax or keyword to reopen existing classs/modules, never to define new classs/modules
https://bugs.ruby-lang.org/issues/20093#change-105937

* Author: tagomoris (Satoshi Tagomori)
* Status: Open
* Priority: Normal
----------------------------------------
`class A` and `module B` will reopen existing class A or module B to add/re-define methods if A/B exists. Otherwise, these will define the new class/module A/B.
But, in my opinion, the code of `class A` for patching existing classes doesn't work expectedly when `A` is not defined beforehand. It expects other codes to define `A` before being called.

For example:

```ruby
# string_exclude.rb
class String
  def exclude?(string)
    !include?(string)
  end
end
```

This code expects that there is the `String` class, and it has the `include?` method. This code doesn't work if the file is loaded in the way below:

```ruby
load('string_exclude.rb', true)
```

This code doesn't raise errors and will define an almost empty class (only with a method `exclude?` to raise NameError). It should be unexpected for every user.

So, I want to propose a new syntax to reopen the existing class/module or raise errors if the specified class/module is not defined.

```ruby
class extension String
  def exclude?(string)
    !include?(string)
  end
end # adds #exclude? to String class

class extension Stroooong
  def exclude?(string)
    !include?(string)
  end
end # will raise NameError (or something else)
```

Some additional things:

* `class extension String` (and `module extension String`)  causes a compile error (SyntaxError) on Ruby 3.3. So we have space to add a keyword between class/module and the class/module name.
* I don't have a strong opinion about the keyword name `extension`. An alternative idea is `reopen`.




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