Better Object-Oriented Design Through... Tail-Call Optimization?

better_oo_through_tco.rb

# Abstract UnionList base class
class AbstractUnionList
  private :initialize

  def length
    how_many?(0)
  end

  def how_many?(counter)
    raise NotImplementedError
  end
end

# UnionList NullObject
class NullUnionList < AbstractUnionList
  public :initialize

  def how_many?(counter)
    counter
  end
end

# How one might intuitively implement UnionList
class UnionList < AbstractUnionList
  attr_reader :adjoining

  def initialize(adjoining)
    @adjoining = adjoining
  end

  def how_many?(counter)
    adjoining.how_many?(counter + 1)
  end
end

# OO-design violating work around for SystemStackError
class IterativeUnionList < UnionList
  def how_many?(counter)
    remaining = self
    count = 0
    # Shouldn't have to know anything about NullUnionList :(
    while !remaining.is_a?(NullUnionList)
      count += 1
      remaining = remaining.adjoining
    end
    count
  end
end

# OO-design friendly and it works? Still my beating heart!
require "tco_method"
class TCOUnionList < UnionList
  extend TCOMethod::Mixin
  tco_method :how_many?
end

# A class to test each of the UnionList implementations
class UnionListTest
  def test(class_under_test, length)
    last = NullUnionList.new
    length.times { last = class_under_test.new(last) }
    msg = "class_under_test=#{class_under_test.name} length=#{length} result="
    msg.concat(last.length == length ? "Success" : "Fail")
    puts(msg)
  rescue StandardError, SystemStackError => e
    puts msg.concat("Fail error=#{e.class}")
  end
end

# Let the testing begin!
test_case = UnionListTest.new
classes_to_test = [
  UnionList,
  IterativeUnionList,
  TCOUnionList,
]

divider = "-----------------"
puts divider

# Test lists of length 1..10000
5.times do |pow|
  length = 10 ** pow
  classes_to_test.each do |class_under_test|
    test_case.test(class_under_test, length)
  end
  puts divider
end

# -----------------
# class_under_test=UnionList length=1 result=Success
# class_under_test=IterativeUnionList length=1 result=Success
# class_under_test=TCOUnionList length=1 result=Success
# -----------------
# class_under_test=UnionList length=10 result=Success
# class_under_test=IterativeUnionList length=10 result=Success
# class_under_test=TCOUnionList length=10 result=Success
# -----------------
# class_under_test=UnionList length=100 result=Success
# class_under_test=IterativeUnionList length=100 result=Success
# class_under_test=TCOUnionList length=100 result=Success
# -----------------
# class_under_test=UnionList length=1000 result=Success
# class_under_test=IterativeUnionList length=1000 result=Success
# class_under_test=TCOUnionList length=1000 result=Success
# -----------------
# class_under_test=UnionList length=10000 result=Fail error=SystemStackError
# class_under_test=IterativeUnionList length=10000 result=Success
# class_under_test=TCOUnionList length=10000 result=Success
# -----------------