How to deal with dynamic classes

[HoneyryderChuck]

As one core meta-programming feature from ruby is enhancing of existing classes, or even the creation of anonymous new ones, I'm wondering whether this is possible already. Something like:

module Ext
  def foo;end
end

class A
  def self.enhance
    Class.new(self) do
      include Ext
    end
  end
end

How could one type the result of calling .enhance?

[marcandre]

I think you can use an intersection. Since you're returning a class, I think singleton is what you want:

def self.enhance: () -> singleton(A & Ext)

[HoneyryderChuck]

That could work. But how can I make it work for "any subclass of A"?

[marcandre]

Oh, right. Replace A with self:

def self.enhance: () -> singleton(self & Ext)

I didn't try it and I'm just starting to get acquainted with RBS. Maybe it's singleton(A) & singleton(Ext), or maybe both are the same, I don't know 🤷‍♂️

[HoneyryderChuck]

I'm getting errors on a bit of a different use-case, but similar syntax. So, for that example above:

module Ext
 def self.enhance(klass)
    Class.new(klass) do
      include Ext::InstanceMethods
    end
  end

  module InstanceMethods
    def bar;end
  end
end

Now, if I type it like this:

module Ext
  def self.enhance: (singleton(A)) -> singleton(A)
end

When running the runtime type checker on:

class A
  def foo; ; end
end

B = Ext.enhance(A) #=> this works
C = Ext.enhance(B) #=> this blows: RBS::Test::Tester::TypeError: TypeError: [.enhance] ArgumentTypeError: expected `singleton(::A)` but given `#<Class:0x000055970ae64258>`

I didn't try it and I'm just starting to get acquainted with RBS.

Likewise :)

[marcandre]

It's taking your A literally. You need [A]. Maybe this would work:

module Ext
  def self.enhance: [A] (singleton(A)) -> singleton(A) & singleton(Ext::InstanceMethods)
end

[soutaro]

Yeah, it's not supported at all.

Class.new(klass) do
  include Ext::InstanceMethods
end

The code requires a concept of type of a class object which includes a module... I don't think we can implement it now.

[HoneyryderChuck]

:( sad to hear, but it's understandable that one can't ship everthing in one go.

Do you have ideas on how this could look like in the future?