nixpkgs/nixos/doc/manual/development/option-def.xml

<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>