nil

nil is both the literal spelling and the exact built-in type for “no value.” It is lowercase because it is a keyword, not a named type alias.

local missing = nil
local isMissing = missing == nil

An ordinary T cannot contain nil. Optional T? means T | nil; operations that may find nothing return that type:

local names: {String} = { "Ada" }
local first: String? = names[1]
local absent: String? = names[2]

Pop has no Lua-style truthiness. A condition must be Boolean, so if absent then is a type error. Compare with nil or use the typed optional operations available for the value.

nil == nil is true and nil ~= nil is false. It is never the padding for a missing multiple return: rc.3 fixed packs require exact arity.