forked from mirrors/nixpkgs
41 lines
1.5 KiB
Markdown
41 lines
1.5 KiB
Markdown
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# Warnings and Assertions {#sec-assertions}
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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.
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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.
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## Warnings {#sec-assertions-warnings}
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This is an example of using `warnings`.
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```nix
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{ config, lib, ... }:
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{
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config = lib.mkIf config.services.foo.enable {
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warnings =
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if config.services.foo.bar
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then [ ''You have enabled the bar feature of the foo service.
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This is known to cause some specific problems in certain situations.
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'' ]
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else [];
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}
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}
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```
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## Assertions {#sec-assertions-assetions}
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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.
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```nix
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{ config, lib, ... }:
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{
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config = lib.mkIf config.services.syslogd.enable {
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assertions =
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[ { assertion = !config.services.rsyslogd.enable;
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message = "rsyslogd conflicts with syslogd";
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}
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];
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}
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}
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```
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