From: "zverok (Victor Shepelev) via ruby-core" <ruby-core@...>
Date: 2022-12-29T13:13:47+00:00
Subject: [ruby-core:111505] [Ruby master Bug#19280] Wrong error message about arity of Data::define.new

Issue #19280 has been updated by zverok (Victor Shepelev).

Status changed from Open to Closed

The report about arity is correct. 

`Data` decouples `.new` and `#initialize` this way:
* `.new` accepts positional and keyword args, and converts positional to keyword, and passes them to `#initialize`
* `#initialize` accepts only keyword arguments, and is easy to redefine for custom processing; the default implementation has all keyword arguments mandatory.

If `.new` method would be implemented in Ruby, for `Data.define(:a, :b)` it'll look this way:
```ruby
KEYS = [:a, :b]

def self.new(*args, **kwargs)
  # raise if both args and kwargs provided

  # handle args
  if args.any?
    raise ArgumentError if args.size > KEYS.size 
    kwargs = args.zip(KEYS).to_h { |value, name| [name, value] }
  end

  allocate.initialize(**kwargs)
end
```

It other words:
* for `.new`, any number of positional args is correct, as long as it has names for them
* they are converted to keyword args, and passed to initialize
* ...which will raise if something is missing in the default implementation, but this implementation can be redefined **without `.new` thinking about it**

Consider this:
```ruby
Data.define(:a, :b) do
  def initialize(a:, b: 0) = super
end.new(1) #=> #<data  a=1, b=0>
```
Or even this:
```ruby
Data.define(:a, :b) do
  def initialize(a: 0, b: 0) = super
end.new #=> #<data  a=0, b=0>
```

...so, yeah, for `new`, the message "it expects 0 to 2 args" corresponds to reality.

----------------------------------------
Bug #19280: Wrong error message about arity of Data::define.new
https://bugs.ruby-lang.org/issues/19280#change-100871

* Author: kyanagi (Kouhei Yanagita)
* Status: Closed
* Priority: Normal
* ruby -v: ruby 3.3.0dev (2022-12-28T16:43:05Z master cada537040) [x86_64-linux]
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
```
$ ~/work/r/bin/ruby -e 'Data.define(:a, :b).new(1, 2, 3)'
-e:1:in `new': wrong number of arguments (given 3, expected 0..2) (ArgumentError)

Data.define(:a, :b).new(1, 2, 3)
                        ^^^^^^^
        from -e:1:in `<main>'
```

On this message, "expected 2" is appropriate because fewer arguments are not allowed.

```
$ ~/work/r/bin/ruby -e 'Data.define(:a, :b).new(1)'
-e:1:in `initialize': missing keyword: :b (ArgumentError)

Data.define(:a, :b).new(1)
                        ^
        from -e:1:in `new'
        from -e:1:in `<main>'
```




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