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diff --git a/nixos/doc/manual/from_md/development/assertions.section.xml b/nixos/doc/manual/from_md/development/assertions.section.xml new file mode 100644 index 00000000000..0844d484d60 --- /dev/null +++ b/nixos/doc/manual/from_md/development/assertions.section.xml @@ -0,0 +1,58 @@ +<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-assertions"> + <title>Warnings and Assertions</title> + <para> + 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. + </para> + <para> + Although Nix has the <literal>abort</literal> and + <literal>builtins.trace</literal> + <link xlink:href="https://nixos.org/nix/manual/#ssec-builtins">functions</link> + 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. + </para> + <section xml:id="sec-assertions-warnings"> + <title>Warnings</title> + <para> + This is an example of using <literal>warnings</literal>. + </para> + <programlisting language="bash"> +{ 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 []; + } +} +</programlisting> + </section> + <section xml:id="sec-assertions-assetions"> + <title>Assertions</title> + <para> + This example, extracted from the + <link xlink:href="https://github.com/NixOS/nixpkgs/blob/release-17.09/nixos/modules/services/logging/syslogd.nix"><literal>syslogd</literal> + module</link> shows how to use <literal>assertions</literal>. + 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. + </para> + <programlisting language="bash"> +{ config, lib, ... }: +{ + config = lib.mkIf config.services.syslogd.enable { + assertions = + [ { assertion = !config.services.rsyslogd.enable; + message = "rsyslogd conflicts with syslogd"; + } + ]; + } +} +</programlisting> + </section> +</section> |