Make a table an iterable array.

iterable.lua

local t = {'a', 'b', 'c', 'd'}
setmetatable(t, {
	__call = function(t, _, i)
		if i == nil then
			i = 1
		else
			i = i + 1
		end
		local v = t[i]
		if v == nil then
			return nil
		end
		return i, v
	end,
})

for i, v in t do
	print(i, v)
end