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><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><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><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>