41 lines
1.5 KiB
Markdown
41 lines
1.5 KiB
Markdown
|
# Warnings and Assertions {#sec-assertions}
|
||
|
|
|
||
|
|
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.
|
||
|
|
|
||
|
|
Although 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.
|
||
|
|
|
||
|
|
## Warnings {#sec-assertions-warnings}
|
||
|
|
|
||
|
|
This is an example of using `warnings`.
|
||
|
|
|
||
|
|
```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 [];
|
||
|
|
}
|
||
|
|
}
|
||
|
|
```
|
||
|
|
|
||
|
|
## Assertions {#sec-assertions-assetions}
|
||
|
|
|
||
|
|
This 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.
|
||
|
|
|
||
|
|
```nix
|
||
|
|
{ config, lib, ... }:
|
||
|
|
{
|
||
|
|
config = lib.mkIf config.services.syslogd.enable {
|
||
|
|
assertions =
|
||
|
|
[ { assertion = !config.services.rsyslogd.enable;
|
||
|
|
message = "rsyslogd conflicts with syslogd";
|
||
|
|
}
|
||
|
|
];
|
||
|
|
}
|
||
|
|
}
|
||
|
|
```
|