commit d48cfce2bf295ef125c94c41b6edacc284e06529 · pyrox.dev/nixpkgs

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nixos/doc/manual/development/option-def.section.md

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       # Option Definitions {#sec-option-definitions}

     

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       Option definitions are generally straight-forward bindings of values to

     

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       option names, like

     

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       ```nix

     

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       config = {

     

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         services.httpd.enable = true;

     

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       };

     

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       ```

     

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       However, sometimes you need to wrap an option definition or set of

     

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       option definitions in a *property* to achieve certain effects:

     

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       ## Delaying Conditionals {#sec-option-definitions-delaying-conditionals .unnumbered}

     

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       If a set of option definitions is conditional on the value of another

     

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       option, you may need to use `mkIf`. Consider, for instance:

     

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       ```nix

     

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       config = if config.services.httpd.enable then {

     

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         environment.systemPackages = [ ... ];

     

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         ...

     

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       } else {};

     

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       ```

     

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       This definition will cause Nix to fail with an "infinite recursion"

     

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       error. Why? Because the value of `config.services.httpd.enable` depends

     

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       on the value being constructed here. After all, you could also write the

     

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       clearly circular and contradictory:

     

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       ```nix

     

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       config = if config.services.httpd.enable then {

     

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         services.httpd.enable = false;

     

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       } else {

     

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         services.httpd.enable = true;

     

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       };

     

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       ```

     

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       The solution is to write:

     

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       ```nix

     

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       config = mkIf config.services.httpd.enable {

     

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         environment.systemPackages = [ ... ];

     

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         ...

     

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       };

     

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       ```

     

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       The special function `mkIf` causes the evaluation of the conditional to

     

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       be "pushed down" into the individual definitions, as if you had written:

     

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       ```nix

     

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       config = {

     

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         environment.systemPackages = if config.services.httpd.enable then [ ... ] else [];

     

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         ...

     

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       };

     

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       ```

     

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       ## Setting Priorities {#sec-option-definitions-setting-priorities .unnumbered}

     

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       A module can override the definitions of an option in other modules by

     

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       setting a *priority*. All option definitions that do not have the lowest

     

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       priority value are discarded. By default, option definitions have

     

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       priority 1000. You can specify an explicit priority by using

     

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       `mkOverride`, e.g.

     

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       ```nix

     

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       services.openssh.enable = mkOverride 10 false;

     

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       ```

     

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       This definition causes all other definitions with priorities above 10 to

     

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       be discarded. The function `mkForce` is equal to `mkOverride 50`.

     

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       ## Merging Configurations {#sec-option-definitions-merging .unnumbered}

     

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       In conjunction with `mkIf`, it is sometimes useful for a module to

     

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       return multiple sets of option definitions, to be merged together as if

     

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       they were declared in separate modules. This can be done using

     

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       `mkMerge`:

     

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       ```nix

     

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       config = mkMerge

     

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         [ # Unconditional stuff.

     

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           { environment.systemPackages = [ ... ];

     

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           }

     

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           # Conditional stuff.

     

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           (mkIf config.services.bla.enable {

     

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             environment.systemPackages = [ ... ];

     

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           })

     

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         ];

     

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       ```

-99

nixos/doc/manual/development/option-def.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-option-definitions">

     

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        <title>Option Definitions</title>

     

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        <para>

     

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         Option definitions are generally straight-forward bindings of values to

     

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         option names, like

     

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       <programlisting>

     

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       config = {

     

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         services.httpd.enable = true;

     

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       };

     

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       </programlisting>

     

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         However, sometimes you need to wrap an option definition or set of option

     

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         definitions in a <emphasis>property</emphasis> to achieve certain effects:

     

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        </para>

     

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        <simplesect xml:id="sec-option-definitions-delaying-conditionals">

     

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         <title>Delaying Conditionals</title>

     

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         <para>

     

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          If a set of option definitions is conditional on the value of another

     

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          option, you may need to use <varname>mkIf</varname>. Consider, for instance:

     

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       <programlisting>

     

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       config = if config.services.httpd.enable then {

     

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         environment.systemPackages = [ <replaceable>...</replaceable> ];

     

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         <replaceable>...</replaceable>

     

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       } else {};

     

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       </programlisting>

     

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          This definition will cause Nix to fail with an “infinite recursion”

     

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          error. Why? Because the value of

     

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          <option>config.services.httpd.enable</option> depends on the value being

     

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          constructed here. After all, you could also write the clearly circular and

     

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          contradictory:

     

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       <programlisting>

     

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       config = if config.services.httpd.enable then {

     

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         services.httpd.enable = false;

     

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       } else {

     

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         services.httpd.enable = true;

     

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       };

     

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       </programlisting>

     

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          The solution is to write:

     

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       <programlisting>

     

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       config = mkIf config.services.httpd.enable {

     

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         environment.systemPackages = [ <replaceable>...</replaceable> ];

     

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         <replaceable>...</replaceable>

     

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       };

     

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       </programlisting>

     

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          The special function <varname>mkIf</varname> causes the evaluation of the

     

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          conditional to be “pushed down” into the individual definitions, as if

     

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          you had written:

     

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       <programlisting>

     

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       config = {

     

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         environment.systemPackages = if config.services.httpd.enable then [ <replaceable>...</replaceable> ] else [];

     

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         <replaceable>...</replaceable>

     

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       };

     

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       </programlisting>

     

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         </para>

     

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        </simplesect>

     

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        <simplesect xml:id="sec-option-definitions-setting-priorities">

     

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         <title>Setting Priorities</title>

     

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         <para>

     

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          A module can override the definitions of an option in other modules by

     

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          setting a <emphasis>priority</emphasis>. All option definitions that do not

     

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          have the lowest priority value are discarded. By default, option definitions

     

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          have priority 1000. You can specify an explicit priority by using

     

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          <varname>mkOverride</varname>, e.g.

     

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       <programlisting>

     

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       services.openssh.enable = mkOverride 10 false;

     

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       </programlisting>

     

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          This definition causes all other definitions with priorities above 10 to be

     

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          discarded. The function <varname>mkForce</varname> is equal to

     

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          <varname>mkOverride 50</varname>.

     

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         </para>

     

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        </simplesect>

     

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        <simplesect xml:id="sec-option-definitions-merging">

     

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         <title>Merging Configurations</title>

     

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         <para>

     

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          In conjunction with <literal>mkIf</literal>, it is sometimes useful for a

     

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          module to return multiple sets of option definitions, to be merged together

     

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          as if they were declared in separate modules. This can be done using

     

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          <varname>mkMerge</varname>:

     

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       <programlisting>

     

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       config = mkMerge

     

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         [ # Unconditional stuff.

     

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           { environment.systemPackages = [ <replaceable>...</replaceable> ];

     

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           }

     

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           # Conditional stuff.

     

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           (mkIf config.services.bla.enable {

     

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             environment.systemPackages = [ <replaceable>...</replaceable> ];

     

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           })

     

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         ];

     

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       </programlisting>

     

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         </para>

     

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        </simplesect>

     

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       </section>

+1 -1

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

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        </example>

     

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

     

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

     

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

     

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        <xi:include href="../from_md/development/option-def.section.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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nixos/doc/manual/from_md/development/option-def.section.xml

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       <section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-option-definitions">

     

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         <title>Option Definitions</title>

     

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         <para>

     

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           Option definitions are generally straight-forward bindings of values

     

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           to option names, like

     

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         </para>

     

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         <programlisting language="bash">

     

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       config = {

     

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         services.httpd.enable = true;

     

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       };

     

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       </programlisting>

     

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         <para>

     

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           However, sometimes you need to wrap an option definition or set of

     

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           option definitions in a <emphasis>property</emphasis> to achieve

     

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           certain effects:

     

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         </para>

     

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         <section xml:id="sec-option-definitions-delaying-conditionals">

     

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           <title>Delaying Conditionals</title>

     

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           <para>

     

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             If a set of option definitions is conditional on the value of

     

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             another option, you may need to use <literal>mkIf</literal>.

     

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             Consider, for instance:

     

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           </para>

     

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           <programlisting language="bash">

     

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       config = if config.services.httpd.enable then {

     

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         environment.systemPackages = [ ... ];

     

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         ...

     

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       } else {};

     

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       </programlisting>

     

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           <para>

     

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             This definition will cause Nix to fail with an <quote>infinite

     

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             recursion</quote> error. Why? Because the value of

     

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             <literal>config.services.httpd.enable</literal> depends on the

     

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             value being constructed here. After all, you could also write the

     

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             clearly circular and contradictory:

     

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           </para>

     

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           <programlisting language="bash">

     

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       config = if config.services.httpd.enable then {

     

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         services.httpd.enable = false;

     

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       } else {

     

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         services.httpd.enable = true;

     

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       };

     

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       </programlisting>

     

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           <para>

     

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             The solution is to write:

     

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           </para>

     

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           <programlisting language="bash">

     

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       config = mkIf config.services.httpd.enable {

     

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         environment.systemPackages = [ ... ];

     

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         ...

     

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       };

     

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       </programlisting>

     

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           <para>

     

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             The special function <literal>mkIf</literal> causes the evaluation

     

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             of the conditional to be <quote>pushed down</quote> into the

     

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             individual definitions, as if you had written:

     

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           </para>

     

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           <programlisting language="bash">

     

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       config = {

     

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         environment.systemPackages = if config.services.httpd.enable then [ ... ] 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-option-definitions-setting-priorities">

     

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           <title>Setting Priorities</title>

     

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           <para>

     

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             A module can override the definitions of an option in other

     

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             modules by setting a <emphasis>priority</emphasis>. All option

     

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             definitions that do not have the lowest priority value are

     

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             discarded. By default, option definitions have priority 1000. You

     

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             can specify an explicit priority by using

     

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             <literal>mkOverride</literal>, e.g.

     

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           </para>

     

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           <programlisting language="bash">

     

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       services.openssh.enable = mkOverride 10 false;

     

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       </programlisting>

     

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           <para>

     

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             This definition causes all other definitions with priorities above

     

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             10 to be discarded. The function <literal>mkForce</literal> is

     

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             equal to <literal>mkOverride 50</literal>.

     

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           </para>

     

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         </section>

     

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         <section xml:id="sec-option-definitions-merging">

     

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       +
           <title>Merging Configurations</title>

     

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       +
           <para>

     

       87
       87
       +
             In conjunction with <literal>mkIf</literal>, it is sometimes

     

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       +
             useful for a module to return multiple sets of option definitions,

     

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       89
       +
             to be merged together as if they were declared in separate

     

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             modules. This can be done using <literal>mkMerge</literal>:

     

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           </para>

     

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           <programlisting language="bash">

     

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       config = mkMerge

     

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         [ # Unconditional stuff.

     

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           { environment.systemPackages = [ ... ];

     

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           }

     

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           # Conditional stuff.

     

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           (mkIf config.services.bla.enable {

     

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             environment.systemPackages = [ ... ];

     

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           })

     

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         ];

     

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       </programlisting>

     

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         </section>

     

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       </section>