half_half.rb

run_with_tco = ARGV.include?('-TCO')

# Force the ruby interpreter to use TCO
RubyVM::InstructionSequence.compile_option = {
  tailcall_optimization: true,
  trace_instruction: false
} if run_with_tco

puts 'To infinity and beyond!' if run_with_tco

HEADFULL_HORSE = 0.3
HEADLESS_HORSE = 0.7
HORSE = HEADLESS_HORSE + HEADFULL_HORSE

RubyVM::InstructionSequence.new(<<-EOS).eval
  # "A centaur that's half horse and half centaur" => ?
  # the tail recursive version
  def centaur(horse:, divider:)
    return horse/divider if horse < 0

    centaur(horse: horse, divider: divider*2)
  end
EOS

# "A centaur that's half horse and half centaur"
puts centaur(horse: HORSE/2, divider: 2)

running_it.md

Without TCO

➜  half_half_tco  ruby half_half.rb
<compiled>:4:in `centaur': stack level too deep (SystemStackError)
	from <compiled>:6:in `centaur'
	from <compiled>:6:in `centaur'
	from <compiled>:6:in `centaur'
	from <compiled>:6:in `centaur'
	from <compiled>:6:in `centaur'
	from <compiled>:6:in `centaur'
	from <compiled>:6:in `centaur'
	from <compiled>:6:in `centaur'
	 ... 8177 levels...
	from <compiled>:6:in `centaur'
	from <compiled>:6:in `centaur'
	from <compiled>:6:in `centaur'
	from half_half.rb:25:in `<main>'

With TCO

➜  half_half_tco  ruby half_half.rb -TCO
To infinity and beyond!

note it all started from here: