NixOS Manual: document assertions and warnings (#29206)

* NixOS Manual: document assertions and warnings

* NixOS manual: re-wrap assertions text
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Robert 2017-09-11 18:12:50 +02:00 committed by Jörg Thalheim
parent dcdbe960f8
commit 1b1fc65505
3 changed files with 83 additions and 1 deletions

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@ -113,7 +113,8 @@ manual</link> for the rest.</para>
</row>
<row>
<entry><literal>assert 1 + 1 == 2; "yes!"</literal></entry>
<entry>Assertion check (evaluates to <literal>"yes!"</literal>)</entry>
<entry>Assertion check (evaluates to <literal>"yes!"</literal>). See <xref
linkend="sec-assertions"/> for using assertions in modules</entry>
</row>
<row>
<entry><literal>let x = "foo"; y = "bar"; in x + y</literal></entry>

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@ -0,0 +1,80 @@
<section xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xi="http://www.w3.org/2001/XInclude"
version="5.0"
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>
<title>Warnings</title>
<para>
This is an example of using <literal>warnings</literal>.
</para>
<programlisting>
<![CDATA[
{ 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>
<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>
<![CDATA[
{ config, lib, ... }:
{
config = lib.mkIf config.services.syslogd.enable {
assertions =
[ { assertion = !config.services.rsyslogd.enable;
message = "rsyslogd conflicts with syslogd";
}
];
}
}
]]>
</programlisting>
</section>
</section>

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@ -178,6 +178,7 @@ in {
<xi:include href="option-declarations.xml" />
<xi:include href="option-types.xml" />
<xi:include href="option-def.xml" />
<xi:include href="assertions.xml" />
<xi:include href="meta-attributes.xml" />
<xi:include href="replace-modules.xml" />