.. _sec-assertions: Warnings and Assertions ~~~~~~~~~~~~~~~~~~~~~~~ When configuration problems are detectable in a module, it is a good idea to write an assertion or warning. Doing so provides clear feedback to the user and prevents errors after the build. Although Nix has the ``abort`` and ``builtins.trace`` `functions `_ to perform such tasks, they are not ideally suited for NixOS modules. Instead of these functions, you can declare your warnings and assertions using the NixOS module system. .. _sec-assertions-warnings: Warnings ~~~~~~~~ This is an example of using ``warnings``. :: { config, lib, ... }: { config = lib.mkIf config.services.foo.enable { warnings = if config.services.foo.bar then [ ''You have enabled the bar feature of the foo service. This is known to cause some specific problems in certain situations. '' ] else []; } } .. _sec-assertions-assertions: Assertions ~~~~~~~~~~ This example, extracted from the `syslogd module `_ shows how to use ``assertions``. Since there can only be one active syslog daemon at a time, an assertion is useful to prevent such a broken system from being built. :: { config, lib, ... }: { config = lib.mkIf config.services.syslogd.enable { assertions = [ { assertion = !config.services.rsyslogd.enable; message = "rsyslogd conflicts with syslogd"; } ]; } }