Visitor pattern (double dispatch) on Ruby

test.rb

require_relative 'visitor'
require_relative 'visitable'

class Object
  include Visitable
end

class Log < Visitor
  def visit_Array(array)
    puts 'Logging array:'
    array.each_with_index do |item, index|
      puts "#{index}: #{item}"
    end
  end

  def visit_Object(object)
    puts "Logging object: #{object.inspect}"
  end

  def visit_class_Object(klass)
    puts "Logging class: #{klass}"
  end
end

class Doubler < Visitor
  def visit_Array(array)
    array.map { |n| n * 2 }
  end
end

log = Log.new
doubler = Doubler.new

array = [1, 2, 3]
array.accept log
#=> Logging array:
#=> 0: 1
#=> 1: 2
#=> 2: 3

array.accept(doubler).accept(log)
#=> Logging array:
#=> 0: 2
#=> 1: 4
#=> 2: 6

symbol = :a
symbol.accept log
#=> Logging object: :a

Symbol.accept log
#=> Logging class: Symbol

symbol.accept doubler
#=> visitor.rb:116:in `dispatch_visit': Can't visit Symbol (TypeError)
#=> from visitor.rb:105:in `visit_on_instance'
#=> from visitor.rb:95:in `visit'
#=> from visitable.rb:3:in `accept'
#=> from test.rb:42:in `<main>'

visitable.rb

module Visitable
  def accept(visitor)
    visitor.visit self
  end
end

visitor.rb

# Base Visitor class to implement any kind of Visitor
# 
# Ideas taken from http://blog.rubybestpractices.com/posts/aaronp/001_double_dispatch_dance.html and
# http://blog.bigbinary.com/2013/07/07/visitor-pattern-and-double-dispatch.html
#
# A Visitor knows how to make actions based on the type
# of an Object.
#
# In order to make a visit method for a concrete type,
# you need to declare the method with the type signature, like this:
#
#   # For Type objects
#   def visit_Type(instance)
#     # magic
#   end
#
#   # For ActiveRecord::Base objects
#   def visit_ActiveRecord_Base(record)
#     # magic
#   end
#
# You can also make the Visitor to operate over classes instead of
# instances by using methods with the prefix visit_class, like this:
#
#   def visit_class_Type(klass)
#   end
#
# @see Visitable module
#
# @example Visitor to transform instances to Arrays
#
#   class AsArray < Visitor
#     def visit_Array(array)
#       array
#     end
#
#     def visit_Object(object)
#       [object]
#     end
#   end
#
#   AsArray.new.visit [a, b]
#   #=> [a, b]
#
#   o = Object.new
#   AsArray.new.visit o
#   #=> [o]
#
# @example Visitor to create Arrays from their classes
#
#   class AsArray < Visitor
#     def visit_class_Array(array_class)
#       array_class.new
#     end
#   end
#
#   class MyArray < Array; end
#
#   AsArray.new.visit MyArray
#   #=> []
class Visitor
  VISIT_REGEXP = /\Avisit_((?<class_level>class)_)?(?<type>\w+)/.freeze

  # Look for any new visit_* method added and add it to the class
  # dispatch tables for instances and classes.
  def self.method_added(method_name)
    return if visit_method_exist?(method_name)

    VISIT_REGEXP.match(method_name.to_s) do |match|
      if match[:class_level].nil?
        dispatch_instance[match[:type]] = method_name
      else
        dispatch_class[match[:type]] = method_name
      end
    end
  end

  # Does the method already exists?
  #
  # @return [Boolean]
  def self.visit_method_exist?(method_name)
    dispatch_instance.value?(method_name) || dispatch_class.value?(method_name)
  end

  def self.dispatch_instance
    @dispatch_instance ||= {}
  end

  def self.dispatch_class
    @dispatch_class ||= {}
  end

  # Visit an object or class based on its Type.
  #
  # @param thing [Anything]
  # @return [Anything]
  def visit(thing)
    thing.is_a?(Class) ? visit_on_class(thing) : visit_on_instance(thing)
  end

  private

  def visit_on_class(thing)
    dispatch_visit thing, self.class.dispatch_class, thing
  end

  def visit_on_instance(thing)
    dispatch_visit thing.class, self.class.dispatch_instance, thing
  end

  def dispatch_visit(klass, dispatcher, *method_args)
    klass.ancestors.each do |ancestor|
      type = ancestor.name.gsub('::', '_')
      method_name = dispatcher[type]
      next unless method_name
      return send(method_name, *method_args)
    end

    fail TypeError, "Can't visit #{klass}"
  end
end