commit 1b1fc6550559f9d73ddf7cea611c387a847bf03b · pyrox.dev/nixpkgs

+2 -1

nixos/doc/manual/configuration/summary.xml

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             </row>

     

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             <row>

     

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               <entry><literal>assert 1 + 1 == 2; "yes!"</literal></entry>

     

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               <entry>Assertion check (evaluates to <literal>"yes!"</literal>)</entry>

     

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               <entry>Assertion check (evaluates to <literal>"yes!"</literal>). See <xref

     

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           linkend="sec-assertions"/> for using assertions in modules</entry>

     

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             </row>

     

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             <row>

     

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               <entry><literal>let x = "foo"; y = "bar"; in x + y</literal></entry>

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nixos/doc/manual/development/assertions.xml

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       <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>

nixos/doc/manual/development/writing-modules.xml

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       <xi:include href="option-declarations.xml" />

     

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       <xi:include href="option-types.xml" />

     

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       <xi:include href="option-def.xml" />

     

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       <xi:include href="assertions.xml" />

     

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       <xi:include href="meta-attributes.xml" />

     

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       <xi:include href="replace-modules.xml" />

     

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