[lua] How do I know if a table is an array?

local function is_array(t)
local i = 0
for _ in pairs(t) do
i = i + 1
if t[i] == nil then return false end
end
return true
end

-- use this when deserializing
local function mark_as_array(t)
setmetatable(t, {__isarray = true})
end

-- use this when serializing
local function is_array(t)
local mt = getmetatable(t)
return mt.__isarray
end