Computer Craft Scrolling Display

local monitor = peripheral.find("monitor")
local sign = "TEXT - "
local length = #sign
local speed = 0.3

function loop(text)
    start = string.sub(text, 1, 1)
    tail = string.sub(text, 2, length)
    return tail .. start
end

monitor.setTextColour(colours.white)
monitor.setTextScale(1.8)

while (true) do
    monitor.clear()
    monitor.setCursorPos(1, 3)
    monitor.write(sign)

    sign = loop(sign)
    sleep(speed)
end