The final type of hook is a return hook. Return hooks get executed every time a function returns, that is, when the return keyword is encountered. To subscribe to a return event, provide debug.sethook with two arguments: the handler function, and the "r" string.

The handler function takes only one argument, a string constant with the "return" value. This callback is similar to the function callback:

function Normalize(x, y, z)
  local dot = x * x + y * y + z * z
  if dot == 0 then
    return nil
  end
  local len = math.sqrt(dot)
  return {
    x = x / len,
    y = y / len,
    z = z / len
  }
end

function trace(event)
  local info = debug.getinfo(2)
  if info.what == "Lua" then
    print ("event: " .. event)
    print (" function: " .. info.name)
    print (" defined on: " .. info.linedefined)
  end
end

debug.sethook(trace, "r")

local norm = Normalize(9, 2, 6)