strict.lua

--
-- strict.lua
-- checks uses of undeclared global variables
-- All global variables must be 'declared' through a regular assignment
-- (even assigning nil will do) in a main chunk before being used
-- anywhere or assigned to inside a function.
--

local getinfo, error, rawset, rawget = debug.getinfo, error, rawset, rawget

local mt = getmetatable(_G)
if mt == nil then
    mt = {}
    setmetatable(_G, mt)
end

mt.__declared = {}

local function what ()
    local d = getinfo(3, "S")
    return d and d.what or "C"
end

mt.__newindex = function (t, n, v)
    if not mt.__declared[n] then
        local w = what()
        if w ~= "main" and w ~= "C" then
            error("assign to undeclared variable '"..n.."'", 2)
        end
        mt.__declared[n] = true
    end
    rawset(t, n, v)
end

mt.__index = function (t, n)
    if not mt.__declared[n] and what() ~= "C" then
        error("variable '"..n.."' is not declared", 2)
    end
    return rawget(t, n)
end

function global(...)
    for _, v in pairs{...} do
        mt.__declared[v] = true
    end
end