A lua function that checks that arguments of correct types. Supports vararg amount of types and mimics native lua errors exactly.

checkargtype.lua

local function error_fmt(level, msg, ...)
	error( string.format(msg, ...), level + 1 )
end

local function checkargtype(val, argnum, func_name, ...)
	local expected_msg = {}
	local t = type(val)
	local count = select('#', ...)
	for i = 1, count do
		local expected = select(i, ...) or "nil"
		if t == expected then
			return
		end
		expected_msg[i] = expected
	end
	local expected_str = table.concat(expected_msg, " or ")
	if t ~= expected then
		error_fmt(2, "bad argument #%d to '%s' (%s expected%s)", argnum, func_name, expected_str, count == 1 and string.format(", got %s", t) or "")
	end
end

function add_two(a, b)
	checkargtype(a, 1, "add_two", "number")
	checkargtype(b, 2, "add_two", nil, "number")

	return a + (b or 6)
end

local function setmetatable(a, b)
	checkargtype(a, 1, "setmetatable", "table")
	checkargtype(b, 2, "setmetatable", nil, "table")
end

print( pcall(setmetatable, {}, nil) )
print( pcall(setmetatable, {}, 2141124) )