From: "zverok (Victor Shepelev) via ruby-core" <ruby-core@...>
Date: 2022-12-30T22:30:48+00:00
Subject: [ruby-core:111544] [Ruby master Bug#19278] Constructing subclasses of Data with positional arguments

Issue #19278 has been updated by zverok (Victor Shepelev).


> I don't really understand this example. The initialize works the same way whether you use a Struct or not

It is not.

1. Both 3.2+ `Struct` and `Data` make a promise that **no other class makes** and is **non-trivial to implement**: if you have Struct/Data-derived class `C` with member `x`, then both `C.new(x: 1)` and `C.new(1)` work.
2. These promises are implemented differently (because different people worked on them :shrug:): In `Struct`, the `#initialize` handles "unify positional and keywords", in `Data`, `.new` handles.
3. This leads to different cost/benefit outcome: in `Struct`, it is "like in other classes" but **redefining `#initialize` is non-trivial**, in `Data`, it is "one interesting quirk", but **redefining `#initialize` is trivial**.

Can you please provide a small realistic example of how would you like to define your Struct or Data-inherited class' `#initialize` to **not break this contract** and achieve the goal you were trying to achieve? We can proceed from there to maybe come to some compromise or better understand each other's points.

----------------------------------------
Bug #19278: Constructing subclasses of Data with positional arguments
https://bugs.ruby-lang.org/issues/19278#change-100908

* Author: tenderlovemaking (Aaron Patterson)
* Status: Feedback
* Priority: Normal
* ruby -v: ruby 3.2.0 (2022-12-25 revision a528908271) [arm64-darwin22]
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
I'd expect both of the following subclasses to work, but the subclass that uses positional parameters raises an exception:

```ruby
Foo = Data.define

class Bar < Foo
  def initialize foo:
    p foo
  end
end

class Baz < Foo
  def initialize foo
    p foo
  end
end

Bar.new foo: 1 # Prints 1
Baz.new 1    # Raises ArgumentError
```

I'd expect the subclass that uses positional arguments to work.



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