coros.rb

def receive(co)
  let status, val = coroutine.resume(co)
  return val
end

def send(x)
  coroutine.yield(x)
end

def producer
  return coroutine.create(def
    loop
      let x = io.read()
      send(x)
    end
  end)
end

def prepend(str, co)
  return coroutine.create(def
    loop
      let x = receive(co)
      send(str .. x)
    end
  end)
end

def uppercase(co)
  return coroutine.create(def
    loop
      let x = receive(co)
      send(x:upper())
    end
  end)
end

def consumer(co)
  loop
    let x = receive(co)
    io.write(x, '\n')
  end
end

consumer(prepend('>>> ', uppercase(producer())))