PoC of detecting tail call optimization by monitoring stack size via a lambda callback

stack_size_introspection.rb

require 'tco_method'

module StackBuster
  def self.fact_yielder(n, acc = 1, &block)
    block.call(acc)
    n <= 1 ? acc : fact_yielder(n - 1, n * acc, &block)
  end
end

puts "StackBuster:"
StackBuster.fact_yielder(10) do |acc|
  puts caller.length
end

module StackFloater
  extend TCOMethod::Mixin

  def self.fact_yielder(n, acc = 1, &block)
    block.call(acc)
    n <= 1 ? acc : fact_yielder(n - 1, n * acc, &block)
  end
  tco_module_method :fact_yielder
end

puts "\nStackFloater:"
StackFloater.fact_yielder(10) do |acc|
  puts caller.length
end

# Output:
# #######
#
# StackBuster:
# 3
# 4
# 5
# 6
# 7
# 8
# 9
# 10
# 11
# 12
#
# StackFloater:
# 3
# 3
# 3
# 3
# 3
# 3
# 3
# 3
# 3
# 3


## Or, alternately:

require "tco_method"

module FibYielder
  def self.fib_yielder(index, back_one = 1, back_two = 0, &block)
    yield back_two if index > 0
    index < 1 ? back_two : fib_yielder(index - 1, back_one + back_two, back_one, &block)
  end
end


2.times do |idx|
  initial_length = nil

  is_tco = FibYielder.fib_yielder(10) do |fib|
    if initial_length.nil?
      initial_length = caller.length
    else
      break initial_length == caller.length
    end
  end

  puts "Iteration #{idx + 1} is tco? : #{is_tco}"

  next unless idx.zero?
  FibYielder.extend(TCOMethod::Mixin)
  FibYielder.tco_module_method(:fib_yielder)
end

# Iteration 1 is tco? : false
# Iteration 2 is tco? : true