104 lines
3.5 KiB
XML
104 lines
3.5 KiB
XML
<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>
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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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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>
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