pyrox.dev
1# Warnings and Assertions {#sec-assertions}
2
3When 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.
4
5Although Nix has the `abort` and `builtins.trace` [functions](https://nixos.org/nix/manual/#ssec-builtins) 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.
6
7## Warnings {#sec-assertions-warnings}
8
9This is an example of using `warnings`.
10
11```nix
12{ config, lib, ... }:
13{
14 config = lib.mkIf config.services.foo.enable {
15 warnings =
16 if config.services.foo.bar
17 then [ ''You have enabled the bar feature of the foo service.
18 This is known to cause some specific problems in certain situations.
19 '' ]
20 else [];
21 };
22}
23```
24
25## Assertions {#sec-assertions-assetions}
26
27This example, extracted from the [`syslogd` module](https://github.com/NixOS/nixpkgs/blob/release-17.09/nixos/modules/services/logging/syslogd.nix) 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.
28
29```nix
30{ config, lib, ... }:
31{
32 config = lib.mkIf config.services.syslogd.enable {
33 assertions =
34 [ { assertion = !config.services.rsyslogd.enable;
35 message = "rsyslogd conflicts with syslogd";
36 }
37 ];
38 };
39}
40```