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+<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-option-definitions">
+  <title>Option Definitions</title>
+  <para>
+    Option definitions are generally straight-forward bindings of values
+    to option names, like
+  </para>
+  <programlisting language="bash">
+config = {
+  services.httpd.enable = true;
+};
+</programlisting>
+  <para>
+    However, sometimes you need to wrap an option definition or set of
+    option definitions in a <emphasis>property</emphasis> to achieve
+    certain effects:
+  </para>
+  <section 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 <literal>mkIf</literal>.
+      Consider, for instance:
+    </para>
+    <programlisting language="bash">
+config = if config.services.httpd.enable then {
+  environment.systemPackages = [ ... ];
+  ...
+} else {};
+</programlisting>
+    <para>
+      This definition will cause Nix to fail with an <quote>infinite
+      recursion</quote> error. Why? Because the value of
+      <literal>config.services.httpd.enable</literal> depends on the
+      value being constructed here. After all, you could also write the
+      clearly circular and contradictory:
+    </para>
+    <programlisting language="bash">
+config = if config.services.httpd.enable then {
+  services.httpd.enable = false;
+} else {
+  services.httpd.enable = true;
+};
+</programlisting>
+    <para>
+      The solution is to write:
+    </para>
+    <programlisting language="bash">
+config = mkIf config.services.httpd.enable {
+  environment.systemPackages = [ ... ];
+  ...
+};
+</programlisting>
+    <para>
+      The special function <literal>mkIf</literal> causes the evaluation
+      of the conditional to be <quote>pushed down</quote> into the
+      individual definitions, as if you had written:
+    </para>
+    <programlisting language="bash">
+config = {
+  environment.systemPackages = if config.services.httpd.enable then [ ... ] else [];
+  ...
+};
+</programlisting>
+  </section>
+  <section 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
+      <literal>mkOverride</literal>, e.g.
+    </para>
+    <programlisting language="bash">
+services.openssh.enable = mkOverride 10 false;
+</programlisting>
+    <para>
+      This definition causes all other definitions with priorities above
+      10 to be discarded. The function <literal>mkForce</literal> is
+      equal to <literal>mkOverride 50</literal>.
+    </para>
+  </section>
+  <section 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 <literal>mkMerge</literal>:
+    </para>
+    <programlisting language="bash">
+config = mkMerge
+  [ # Unconditional stuff.
+    { environment.systemPackages = [ ... ];
+    }
+    # Conditional stuff.
+    (mkIf config.services.bla.enable {
+      environment.systemPackages = [ ... ];
+    })
+  ];
+</programlisting>
+  </section>
+</section>