Bug #17596: Calling parameters on `new` method results in :rest and -1 arity

• Status: Rejected
• Assignee: -
• Target version: -
• ruby -v: 3.0.0
• Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN

[baweaver (Brandon Weaver)]

Consider the following class:

class Testing
  def initialize(a, b, c)
    # ...
  end
end

...and the following call:

m = Testing.method(:new)
p arity: m.arity, params: m.parameters

Expected

{ arity: 3, params: [[:req, :a], [:req, :b], [:req, :c]] }

Actual

{ arity: -1, params: [[:rest]] }

This is confusing to me as I would expect to be able to see the parameters of the new method like this. The same applies for `initialize.

I was experimenting with pattern matching interfaces and dynamically defining class constructor definitions as such:

def deconstruct
  method(:new).parameters.map { public_send(_1.last) }
end

While this case is not 100% safe for all initializers I had expected it to work for testing and was surprised when it did not.

I've tested and confirmed this behavior back to 2.5 and tested no further back

[baweaver (Brandon Weaver)]

It should be noted that this happens with new and initialize both

[soutaro (Soutaro Matsumoto)]

Hi Brandon,

It is because Testing.new is Class#new, which accepts any arguments and forward to Testing#initialize. (I'm not sure if we can change the behavior.)

A workaround would be using instance_method(:initialize).

Testing.method(:new).owner                        # => Class
Testing.method(:initialize).owner                 # => Class
Testing.instance_method(:initialize).owner        # => Testing

[baweaver (Brandon Weaver)]

Ah, this makes some sense, and I'd just seen something similar on Twitter. I would think that one of initialize or new would not do that instead of forwarding.

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Rejected

Class method new and instance method initialize are different things.