112 lines
3.2 KiB
XML
112 lines
3.2 KiB
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>Option definitions are generally straight-forward bindings of values to 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
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option definitions in a <emphasis>property</emphasis> to achieve
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certain effects:</para>
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<simplesect><title>Delaying Conditionals</title>
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<para>If a set of option definitions is conditional on the value of
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another option, you may need to use <varname>mkIf</varname>.
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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
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being constructed here. After all, you could also write the clearly
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circular and 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
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the conditional to be “pushed down” into the individual definitions,
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as if 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><title>Setting Priorities</title>
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<para>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 discarded.
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By default, option definitions have priority 1000. You can specify an
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explicit priority by using <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
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to be discarded. The function <varname>mkForce</varname> is
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equal to <varname>mkOverride 50</varname>.</para>
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</simplesect>
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<simplesect><title>Merging Configurations</title>
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<para>In conjunction with <literal>mkIf</literal>, it is sometimes
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useful for a module to return multiple sets of option definitions, to
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be merged together as if they were declared in separate modules. This
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can be done using <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>
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