80 lines
2 KiB
XML
80 lines
2 KiB
XML
<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id="sec-assertions">
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<title>Warnings and Assertions</title>
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<para>
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When configuration problems are detectable in a module, it is a good
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idea to write an assertion or warning. Doing so provides clear
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feedback to the user and prevents errors after the build.
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</para>
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<para>
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Although Nix has the <literal>abort</literal> and
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<literal>builtins.trace</literal> <link xlink:href="https://nixos.org/nix/manual/#ssec-builtins">functions</link> to perform such tasks,
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they are not ideally suited for NixOS modules. Instead of these
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functions, you can declare your warnings and assertions using the
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NixOS module system.
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</para>
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<section>
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<title>Warnings</title>
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<para>
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This is an example of using <literal>warnings</literal>.
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</para>
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<programlisting>
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<![CDATA[
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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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</programlisting>
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</section>
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<section>
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<title>Assertions</title>
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<para>
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This example, extracted from the
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<link xlink:href="https://github.com/NixOS/nixpkgs/blob/release-17.09/nixos/modules/services/logging/syslogd.nix">
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<literal>syslogd</literal> module
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</link> shows how to use <literal>assertions</literal>. Since there
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can only be one active syslog daemon at a time, an assertion is useful to
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prevent such a broken system from being built.
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</para>
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<programlisting>
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<![CDATA[
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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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</programlisting>
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</section>
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</section>
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