null.lua

------------------------------------------------------------------------------------
-- Module: null.lua
------------------------------------------------------------------------------------


-- How to insert nils in an array?

-- Traditional approach is to appoint some Lua value as nil-replacement.
-- A function null(...) that swaps nils with itself may be a perfect candidate:
--  1) It represents nil values inside an array
--  2) It serves as encoder (nil -> null)
--  3) It serves as decoder (null -> nil)

-- Example:
--   Load the module: require("null")
--   Original data:   a tuple of 3 values: 42, nil, nil
--   Encoding:        null(42, nil, nil)
--   Encoded data:    a tuple without nils: 42, null, null
--   Decoding:        null(42, null, null)
--   Restored data:   the original tuple: 42, nil, nil


local function null(...)
   local t, n = {...}, select('#', ...)
   for k = 1, n do
      local v = t[k]
      if     v == null then t[k] = nil
      elseif v == nil  then t[k] = null
      end
   end
   return (table.unpack or unpack)(t, 1, n)
end

_G.null = null

return null

nice approach