pyrox.dev
1<section xmlns="http://docbook.org/ns/docbook"
2 xmlns:xlink="http://www.w3.org/1999/xlink"
3 xmlns:xi="http://www.w3.org/2001/XInclude"
4 version="5.0"
5 xml:id="sec-option-declarations">
6
7<title>Option Declarations</title>
8
9<para>An option declaration specifies the name, type and description
10of a NixOS configuration option. It is invalid to define an option
11that hasn’t been declared in any module. An option declaration
12generally looks like this:
13
14<programlisting>
15options = {
16 <replaceable>name</replaceable> = mkOption {
17 type = <replaceable>type specification</replaceable>;
18 default = <replaceable>default value</replaceable>;
19 example = <replaceable>example value</replaceable>;
20 description = "<replaceable>Description for use in the NixOS manual.</replaceable>";
21 };
22};
23</programlisting>
24
25</para>
26
27<para>The function <varname>mkOption</varname> accepts the following arguments.
28
29<variablelist>
30
31 <varlistentry>
32 <term><varname>type</varname></term>
33 <listitem>
34 <para>The type of the option (see <xref linkend='sec-option-types' />).
35 It may be omitted, but that’s not advisable since it may lead to errors
36 that are hard to diagnose.</para>
37 </listitem>
38 </varlistentry>
39
40 <varlistentry>
41 <term><varname>default</varname></term>
42 <listitem>
43 <para>The default value used if no value is defined by any
44 module. A default is not required; but if a default is not given,
45 then users of the module will have to define the value of the
46 option, otherwise an error will be thrown.</para>
47 </listitem>
48 </varlistentry>
49
50 <varlistentry>
51 <term><varname>example</varname></term>
52 <listitem>
53 <para>An example value that will be shown in the NixOS manual.</para>
54 </listitem>
55 </varlistentry>
56
57 <varlistentry>
58 <term><varname>description</varname></term>
59 <listitem>
60 <para>A textual description of the option, in DocBook format,
61 that will be included in the NixOS manual.</para>
62 </listitem>
63 </varlistentry>
64
65</variablelist>
66
67</para>
68
69<section xml:id="sec-option-declarations-eot"><title>Extensible Option
70 Types</title>
71
72 <para>Extensible option types is a feature that allow to extend certain types
73 declaration through multiple module files.
74 This feature only work with a restricted set of types, namely
75 <literal>enum</literal> and <literal>submodules</literal> and any composed
76 forms of them.</para>
77
78 <para>Extensible option types can be used for <literal>enum</literal> options
79 that affects multiple modules, or as an alternative to related
80 <literal>enable</literal> options.</para>
81
82 <para>As an example, we will take the case of display managers. There is a
83 central display manager module for generic display manager options and a
84 module file per display manager backend (slim, sddm, gdm ...).
85 </para>
86
87 <para>There are two approach to this module structure:
88
89 <itemizedlist>
90 <listitem><para>Managing the display managers independently by adding an
91 enable option to every display manager module backend. (NixOS)</para>
92 </listitem>
93 <listitem><para>Managing the display managers in the central module by
94 adding an option to select which display manager backend to use.</para>
95 </listitem>
96 </itemizedlist>
97 </para>
98
99 <para>Both approaches have problems.</para>
100
101 <para>Making backends independent can quickly become hard to manage. For
102 display managers, there can be only one enabled at a time, but the type
103 system can not enforce this restriction as there is no relation between
104 each backend <literal>enable</literal> option. As a result, this restriction
105 has to be done explicitely by adding assertions in each display manager
106 backend module.</para>
107
108 <para>On the other hand, managing the display managers backends in the
109 central module will require to change the central module option every time
110 a new backend is added or removed.</para>
111
112 <para>By using extensible option types, it is possible to create a placeholder
113 option in the central module (<xref linkend='ex-option-declaration-eot-service'
114 />), and to extend it in each backend module (<xref
115 linkend='ex-option-declaration-eot-backend-slim' />, <xref
116 linkend='ex-option-declaration-eot-backend-sddm' />).</para>
117
118 <para>As a result, <literal>displayManager.enable</literal> option values can
119 be added without changing the main service module file and the type system
120 automatically enforce that there can only be a single display manager
121 enabled.</para>
122
123<example xml:id='ex-option-declaration-eot-service'><title>Extensible type
124 placeholder in the service module</title>
125<screen>
126services.xserver.displayManager.enable = mkOption {
127 description = "Display manager to use";
128 type = with types; nullOr (enum [ ]);
129};</screen></example>
130
131<example xml:id='ex-option-declaration-eot-backend-slim'><title>Extending
132 <literal>services.xserver.displayManager.enable</literal> in the
133 <literal>slim</literal> module</title>
134<screen>
135services.xserver.displayManager.enable = mkOption {
136 type = with types; nullOr (enum [ "slim" ]);
137};</screen></example>
138
139<example xml:id='ex-option-declaration-eot-backend-sddm'><title>Extending
140 <literal>services.foo.backend</literal> in the <literal>sddm</literal>
141 module</title>
142<screen>
143services.xserver.displayManager.enable = mkOption {
144 type = with types; nullOr (enum [ "sddm" ]);
145};</screen></example>
146
147<para>The placeholder declaration is a standard <literal>mkOption</literal>
148 declaration, but it is important that extensible option declarations only use
149 the <literal>type</literal> argument.</para>
150
151<para>Extensible option types work with any of the composed variants of
152 <literal>enum</literal> such as
153 <literal>with types; nullOr (enum [ "foo" "bar" ])</literal>
154 or <literal>with types; listOf (enum [ "foo" "bar" ])</literal>.</para>
155
156</section>
157</section>