Python's greenlets in Ruby using fibers, implementing example from greenlet documentation at http://greenlet.readthedocs.org/en/latest/index.html

greenlet.rb

class String

  def start_with?(little_string)
    !self.match(/^#{Regexp.escape(little_string)}/).nil?
  end

  def end_with?(little_string)
    !self.match(/#{Regexp.escape(little_string)}$/).nil?
  end

end

class Greenlet
  require 'fiber'

  current = begin
    instance = allocate
    instance.instance_variable_set(:@parent, instance)
    instance.instance_variable_set(:@fiber, Fiber.current)
    instance
  end
  instance_variable_set(:@current, current)

  def self.new(&block)
    instance = allocate
    instance.instance_variable_set(:@parent, @current)
    instance.instance_variable_set(:@fiber, Fiber.new(&block))
    @current = instance
  end

  def self.current
    @current
  end

  attr_reader :parent

  def switch(*args)
    @fiber.transfer(*args)
  end
end

def Greenlet(&block)
  Greenlet.new &block
end

def process_commands(*args)
  while true
    line = ''
    until line.end_with?("\n")
      line += read_next_char()
    end
    if line == "quit\n"
      puts "are you sure?"
      if read_next_char() != 'y'
        continue
      end
    end
    process_command(line)
  end
end

def process_command(cmd)
  puts cmd
end

def event_keydown(key)
  @g_processor.switch(key)
end

def read_next_char()
  g_self = Greenlet.current
  next_char = g_self.parent.switch()
  return next_char
end



@g_processor = Greenlet(&method(:process_commands))
@g_processor.switch([])


event_keydown("h")
event_keydown("e")
event_keydown("l")
event_keydown("l")
event_keydown("o")
event_keydown(" ")
event_keydown("w")
event_keydown("o")
event_keydown("r")
event_keydown("l")
event_keydown("d")
event_keydown("!")
event_keydown("\n")