Module:Sandbox/Artoria2e5/Fallback

-- A simple index fallback implementation for tables.
-- Useful for template argument aliasing.
-- Hmm, what about __newindex?

local function makeFallback(args, arg_aliases)
	local oldArgsMeta = getmetatable(args) or {}
	local newArgsMeta = {}
	
	-- Forget about thread-safety.
	-- States kept to avoid strange loops.
	local referencedKeys = {} -- dataType:Set/hashtable-impl
	local attemptDepth = 0    -- useful for "last nil" case
	
	-- Start the new metatable as a copy of the old metatable.
	for k, v in ipairs(oldArgsMeta) do
		newArgsMeta[k] = v
	end

	-- Change the __index metamethod to our implementation.
	-- See https://www.lua.org/pil/13.4.1.html.
	newArgsMeta.__index = function (t, k)
		-- My friend, why are you here again?
		if referencedKeys[k] then
			-- You have to be drunk. Go home.
			return nil
		end
		referencedKeys[k] = true
		
		-- Try the old metamethod first.
		-- Thanks to closures, this whole oldArgsMeta object will stay.
		if oldArgsMeta.__index ~= nil then
			local val = oldArgsMeta.__index(t, k)
			if val ~= nil then
				referencedKeys = {}
				return val
			end
		end
		
		attemptDepth = attemptDepth + 1
		-- Now try use the aliases given.
		for _, v in ipairs(arg_aliases[k] or {}) do
			-- If a working value is found, use it.
			-- Note: mw-argument-specific empty str chk.
			if t[v] ~= nil and t[v] ~= '' then
				referencedKeys = {}
				return t[v]
			end
		end
		attemptDepth = attemptDepth - 1
		
		if attemptDepth == 0 then
			referencedKeys = {}
		end
		return nil
	end
	setmetatable(args, newArgsMeta)
	return args  -- just in case someone wants a return value.
end

return makeFallback