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+<section xmlns="http://docbook.org/ns/docbook"
+        xmlns:xlink="http://www.w3.org/1999/xlink"
+        xmlns:xi="http://www.w3.org/2001/XInclude"
+        version="5.0"
+        xml:id="sec-option-definitions">
+ <title>Option Definitions</title>
+
+ <para>
+  Option definitions are generally straight-forward bindings of values to
+  option names, like
+<programlisting>
+config = {
+  services.httpd.enable = true;
+};
+</programlisting>
+  However, sometimes you need to wrap an option definition or set of option
+  definitions in a <emphasis>property</emphasis> to achieve certain effects:
+ </para>
+
+ <simplesect xml:id="sec-option-definitions-delaying-conditionals">
+  <title>Delaying Conditionals</title>
+  <para>
+   If a set of option definitions is conditional on the value of another
+   option, you may need to use <varname>mkIf</varname>. Consider, for instance:
+<programlisting>
+config = if config.services.httpd.enable then {
+  environment.systemPackages = [ <replaceable>...</replaceable> ];
+  <replaceable>...</replaceable>
+} else {};
+</programlisting>
+   This definition will cause Nix to fail with an “infinite recursion”
+   error. Why? Because the value of
+   <option>config.services.httpd.enable</option> depends on the value being
+   constructed here. After all, you could also write the clearly circular and
+   contradictory:
+<programlisting>
+config = if config.services.httpd.enable then {
+  services.httpd.enable = false;
+} else {
+  services.httpd.enable = true;
+};
+</programlisting>
+   The solution is to write:
+<programlisting>
+config = mkIf config.services.httpd.enable {
+  environment.systemPackages = [ <replaceable>...</replaceable> ];
+  <replaceable>...</replaceable>
+};
+</programlisting>
+   The special function <varname>mkIf</varname> causes the evaluation of the
+   conditional to be “pushed down” into the individual definitions, as if
+   you had written:
+<programlisting>
+config = {
+  environment.systemPackages = if config.services.httpd.enable then [ <replaceable>...</replaceable> ] else [];
+  <replaceable>...</replaceable>
+};
+</programlisting>
+  </para>
+ </simplesect>
+
+ <simplesect xml:id="sec-option-definitions-setting-priorities">
+  <title>Setting Priorities</title>
+  <para>
+   A module can override the definitions of an option in other modules by
+   setting a <emphasis>priority</emphasis>. All option definitions that do not
+   have the lowest priority value are discarded. By default, option definitions
+   have priority 1000. You can specify an explicit priority by using
+   <varname>mkOverride</varname>, e.g.
+<programlisting>
+services.openssh.enable = mkOverride 10 false;
+</programlisting>
+   This definition causes all other definitions with priorities above 10 to be
+   discarded. The function <varname>mkForce</varname> is equal to
+   <varname>mkOverride 50</varname>.
+  </para>
+ </simplesect>
+
+ <simplesect xml:id="sec-option-definitions-merging">
+  <title>Merging Configurations</title>
+  <para>
+   In conjunction with <literal>mkIf</literal>, it is sometimes useful for a
+   module to return multiple sets of option definitions, to be merged together
+   as if they were declared in separate modules. This can be done using
+   <varname>mkMerge</varname>:
+<programlisting>
+config = mkMerge
+  [ # Unconditional stuff.
+    { environment.systemPackages = [ <replaceable>...</replaceable> ];
+    }
+    # Conditional stuff.
+    (mkIf config.services.bla.enable {
+      environment.systemPackages = [ <replaceable>...</replaceable> ];
+    })
+  ];
+</programlisting>
+  </para>
+ </simplesect>
+</section>