t.rb

def async(name, &b)
  define_method name do |*args|
    f = Fiber.new{
      r = b.call(*args)
      f.instance_variable_set(:@result, r)
    }
    def f.value
      while true
        unless self.alive?
          break @result
        else
          self.resume
        end
      end
    end
    f
  end
end

require 'fiber'

def await task
  while true
    r = task.resume
    if task.alive?
      Fiber.yield
    else
      break r
    end
  end
end

def do_something1
end
def do_something2
end

async :task2 do
  do_something1
  Fiber.yield
  do_something2
end

async :task1 do |name|
  await task2()
  "hello #{name}"
end

task1_instance = task1("ko1")
p task1_instance.value

See also https://twitter.com/_ko1/status/870473966120390656