Writing Concerns without the `extend`/`included`/`class_methods` boilerplate.

concern.rb

# Maybe there's a way to support ActiveSupport::Concern's dependencies too?
# Although this doesn't use the module hierarchy, so `prepend` can't work.
class Concern < Module
  def initialize(&block) = @block = block
  def included(klass)    = klass.class_eval(&@block)
end

module Kernel
  def Concern(...) = Concern.new(...)
end

# app/models/user.rb
class User < ApplicationRecord
  include Commented
end

# app/models/user/commented.rb
User::Commented = Concern do
  has_many :comments

  def self.cool = :yup

  def great? = true
end