commit 5058f9a5dfc5aece634d6f2a23df86b944603aa9 · pyrox.dev/nixpkgs

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nixos/doc/manual/development/assertions.section.md

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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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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 idea to

     

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         write an assertion or warning. Doing so provides clear feedback to the user

     

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         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>

     

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         <link xlink:href="https://nixos.org/nix/manual/#ssec-builtins">functions</link>

     

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         to perform such tasks, they are not ideally suited for NixOS modules. Instead

     

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         of these 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 xml:id="sec-assertions-warnings">

     

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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 xml:id="sec-assertions-assertions">

     

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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 </link> shows how to use

     

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          <literal>assertions</literal>. Since there can only be one active syslog

     

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          daemon at a time, an assertion is useful to prevent such a broken system

     

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          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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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="../from_md/development/assertions.section.xml" />

     

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

     

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

     

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

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nixos/doc/manual/from_md/development/assertions.section.xml

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       <section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" 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>

     

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           <link xlink:href="https://nixos.org/nix/manual/#ssec-builtins">functions</link>

     

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           to perform such tasks, they are not ideally suited for NixOS

     

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           modules. Instead of these functions, you can declare your warnings

     

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           and assertions using the NixOS module system.

     

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         </para>

     

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         <section xml:id="sec-assertions-warnings">

     

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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 language="bash">

     

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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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       </programlisting>

     

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         </section>

     

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         <section xml:id="sec-assertions-assetions">

     

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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"><literal>syslogd</literal>

     

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             module</link> shows how to use <literal>assertions</literal>.

     

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             Since there can only be one active syslog daemon at a time, an

     

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             assertion is useful to prevent such a broken system from being

     

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             built.

     

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           </para>

     

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           <programlisting language="bash">

     

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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 = &quot;rsyslogd conflicts with syslogd&quot;;

     

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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>