blob: d3434e1f112ee1e71df9aa50cde970d6980b83f5 (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
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>
|