Inner DSL on Ruby sample?

user.rb

#!/usr/bin/env ruby

class User
  @@users, @@id = [], 0
  attr_accessor :name, :age, :job
  attr_reader :id
  def initialize(name, age, job)
    @@id += 1
    @id, @name, @age, @job = @@id, name, age, job
    @@users << self
  end

  def to_s
    "%d:%s(%d/%s)" % [id, name, age, job]
  end

  class << self
    def register(cfg='userdata', &blk)
      case
      when blk then instance_eval(&blk)
      when cfg then instance_eval ::File.read(cfg)
      else raise ArgumentError
      end
    rescue Errno::ENOENT
      abort "userdata file `#{cfg}` not found"
    end

    alias :add :new

    def all
      @@users
    end

    [:id, :name, :age, :job].each do |m|
      define_method("find_by_#{m}") do |arg|
        blk = ->usr { usr.send(m) == arg }
        meth = [:id, :name].include?(m) ? :detect : :select
        @@users.send(meth, &blk)
      end
    end
  end
end

userdata

add 'Charlie',   12, :programmer
add 'Ben',       17, :teacher
add 'Dick',      33, :lawyer
add 'Elizabeth', 23, :doctor
add 'Fernand',   27, :teacher
add 'George',    33, :programmer

userup

#!/usr/bin/env ruby
require_relative "user"

User.register

require "irb"
IRB.start