DSL Yield(self)

dsl.rb

class Admin
  def admin(&block)
    @admin ||= Admin.new(&block)
  end
class << self
  attr_accessor :name, :environment # class attributes
  def new
     unless @instance # Singleton-pattern only one instance
       yield(self) # Yielding self, so self is Admin
      end
       @instance ||= self
     end
   end
end

result.rb

admin = Admin.new do |admin|
   admin.name = "Stan"
 end
admin.name
# => "Stan"
[56] pry(main)> z.new { |a| a.name = "f" }
=> Adm
[57] pry(main)> z
=> Adm
[58] pry(main)> z.name

http://ruby.hasbrains.ru/screencasts/37