Tail optimized recursive call w/ Nathan at Rubyconf

torc.rb

$depth = 0
at_exit { p depth: $depth }
def record_depth
  $depth = caller.length-1 if caller.length-1 > $depth
end


require 'binding_of_caller'
module Torc
  def torc(*args)
    return [:torc, args] if instance_variable_defined? :@caller
    begin
      @caller = binding.of_caller(1).eval('__method__')
      caller_args = [:start, args]
      loop do
        caller_args = __send__ @caller, *caller_args.last
        return caller_args unless caller_args.respond_to? :first
        return caller_args unless caller_args.first == :torc
      end
    ensure
      remove_instance_variable :@caller
    end
  end
end


def factorial(n)
  factorial_helper(1, n)
end

extend Torc
def factorial_helper(acc, n)
  record_depth
  return acc if n <= 1
  torc acc * n, n - 1
end

factorial 1    # => 1
factorial 2    # => 2
factorial 3    # => 6
factorial 4    # => 24
factorial 5    # => 120
factorial 6    # => 720
factorial 100  # => 93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000

# >> {:depth=>6}