[ruby-core:113079] [Ruby master Feature#19555] Allow passing default options to `Data.define`

2 Apr 2023

      Issue #19555 has been updated by FlickGradley (Nick Bradley).

To clarify what I mean, this is a (very rough) demonstration:

``` ruby
class Data
  def self.with_defaults(*symbols, **defaults, &block)
    defaults&.each { |key, value| raise ArgumentError, "#{key} must be immutable" unless Ractor.shareable?(value) }

    Data.define(*(symbols + defaults.keys)) do
      @@defaults = defaults

      class_eval(&block) if block

      def initialize(**kwargs)
        @@defaults.each do |key, value|
          kwargs[key] = value unless kwargs.key?(key)
        end

        super(**kwargs)
      end
    end
  end
end

Point = Data.with_defaults(:x, :y, z: [].freeze) do
  def +(other) = self.class.new(self.x + other.x, self.y + other.y)
end

p1 = Point.new(x: 1, y: 2)
p2 = Point.new(x: 3, y: 4)
p3 = p1 + p2
```
This isn't meant to be an actual proposal (lots of holes in this) - but, conceptually, I'm curious if the Ractor's concept of `shareable?` could or should be applied to Data, since it _is_ supposed to be immutable.

----------------------------------------
Feature #19555: Allow passing default options to `Data.define`
https://bugs.ruby-lang.org/issues/19555#change-102614

* Author: p8 (Petrik de Heus)
* Status: Open
* Priority: Normal
----------------------------------------
Defining a subclass of `Data` with default attributes can currently be done by overriding `intialize`:
```ruby
class Point < Data.define(:x, :y, :z)
  def initialize(x:, y:, z: 0) = super
end

p Point.new(1, 2)
#=> #<data Point x=1, y=2, z=0>
```

It would be nice if we could do it in `define` as well:

```ruby
Point = Data.define(:x, :y, z: 0)
```

-- 
https://bugs.ruby-lang.org/