dd.rb

module Visitable
  def accept object
    object.visit self
  end
end

class Visitor
  def visit thing
    send(:"visit_#{thing.class.name}", thing)
  end
end

class Object; include Visitable; end

class Doubler < Visitor
  def visit_Array array
    array.map { |thing| thing.accept self }
  end

  def visit_Fixnum num
    num * 2
  end
end

p 1.accept Doubler.new
p [1, [2, 3]].accept Doubler.new