Better Know a Ruby Thing: Singleton Classes

I haven't written any ruby for over a decade and a half, when ruby 2.x was still on the horizon. But I do know the source of "eigenclass". It was first jokingly used in an odd instructional programming book / web comic / experimental art piece by "why the lucky stiff" who was briefly prominent in ruby-land then erased himself from the internet. It's funny that it has now become an established term of art for Ruby people.

I love it when someone dives a bit deeper behind something pretty common for most Ruby devs. It's actually a really elegant pattern that is used and it's everywhere in Ruby.

> There are at least three ways to define a method in a Ruby singleton class.

It's worth noting that two of such ways will, perhaps unintuitively, ignore the `private` keyword.

  private
  def self.foo
    # ...
  end

  def x.foo
    # ...
  end

  private_class_method def self.foo
    # ...
  end

  def self.foo
    # ...
  end
  private_class_method :foo # `def` will return a symbol of the defined method name so the above syntax is usually sufficient.

  def x.foo
    # ...
  end
  x.singleton_class.send :private, :foo # Class#private is a private method(!) so must be called using Object#send.

  class << x
  private
    def foo
      # ...
    end
  end

Joy! :) I'm pretty sure matz would agree.

Went in expecting a basic how-to about the Singleton module, but this is so much more, you clearly know your shit, and I love deep dives like this!