api.declare("login", {
  {"session_id", types::I32},
  {"login", types::UTF8},
  {"password", types::UTF8}
}, [](ArgPack& args) {
  auto session = find_session(args['session_id'].as<types::I32>());
  if (!session) {
    return Response::Error("session not found");
  }
  ...
});