112 lines
No EOL
3.2 KiB
XML
112 lines
No EOL
3.2 KiB
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> |