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

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<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="bash">
{ 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="bash">
{ 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>