nixpkgs/nixos/doc/manual/from_md/development/assertions.section.xml

<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-assertions">
  <title>Warnings and Assertions</title>
  <para>
    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.
  </para>
  <para>
    Although Nix has the <literal>abort</literal> and
    <literal>builtins.trace</literal>
    <link xlink:href="https://nixos.org/nix/manual/#ssec-builtins">functions</link>
    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.
  </para>
  <section xml:id="sec-assertions-warnings">
    <title>Warnings</title>
    <para>
      This is an example of using <literal>warnings</literal>.
    </para>
    <programlisting language="nix">
{ 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 [];
  }
}
</programlisting>
  </section>
  <section xml:id="sec-assertions-assetions">
    <title>Assertions</title>
    <para>
      This example, extracted from the
      <link xlink:href="https://github.com/NixOS/nixpkgs/blob/release-17.09/nixos/modules/services/logging/syslogd.nix"><literal>syslogd</literal>
      module</link> shows how to use <literal>assertions</literal>.
      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.
    </para>
    <programlisting language="nix">
{ config, lib, ... }:
{
  config = lib.mkIf config.services.syslogd.enable {
    assertions =
      [ { assertion = !config.services.rsyslogd.enable;
          message = &quot;rsyslogd conflicts with syslogd&quot;;
        }
      ];
  }
}
</programlisting>
  </section>
</section>