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diff --git a/nixos/doc/manual/development/assertions.section.md b/nixos/doc/manual/development/assertions.section.md new file mode 100644 index 00000000000..cc6d81e5699 --- /dev/null +++ b/nixos/doc/manual/development/assertions.section.md @@ -0,0 +1,40 @@ +# 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"; + } + ]; + } +} +``` |