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# 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";
}
];
};
}
```
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