LockGlobals.ttslua

-------------------------------------------------------------------------------
--- Partially lock the _G global variable table.
-- Include this AFTER creating any necessary globals to prevent new ones.
--
-- - Existing globals can still be read AND WRITTEN.
-- - Cannot read non-existent globals.
-- - Cannot write new globals.
--
-- This helps catch typos where what was meant to access a local instead
-- references a (hopefully non-existent) global, as well as forgetting to use
-- "local" when creating objects.
--
-- @author Darrell
-------------------------------------------------------------------------------

local TAG = 'CrLua.LockGlobals'
local _lockGlobalsMetaTable = {}

-- Index is only called when the key does not already exist.
function _lockGlobalsMetaTable.__index(table, key)
    error(TAG .. ': accessing missing global "' .. tostring(key or '<nil>') .. '", typo?', 2)
end

function _lockGlobalsMetaTable.__newindex(table, key, value)
    error(TAG .. ': globals are locked, cannot create global variable "' .. tostring(key or '<nil>') .. '"', 2)
end

setmetatable(_G, _lockGlobalsMetaTable)

-------------------------------------------------------------------------------

-- Add a test function to the CrLua "namespace".
CrLua = CrLua or {}  -- global, <include> wraps in a do .. end block
CrLua.LockGlobals = assert(not CrLua.LockGlobals) and {
    _require = { 'LockGlobals' }  -- require self to avoid missing require error
}
function CrLua.LockGlobals._testLockGlobals()
    -- Can read existing global.
    assert(_VERSION)

    -- Cannot read missing global.
    if pcall(function() return thisGlobalVariableDoesNotExistHopefully end) then
        error('was able to write missing global')
    end

    -- Cannot write missing global.
    if pcall(function() thisGlobalVariableDoesNotExistHopefully = 1 end) then
        error('was able to write missing global')
    end
end