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99 lines
3.5 KiB
XML
99 lines
3.5 KiB
XML
<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>
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