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<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-option-declarations">
<title>Option Declarations</title>
<para>
An option declaration specifies the name, type and description of a
NixOS configuration option. It is invalid to define an option that
hasn’t been declared in any module. An option declaration generally
looks like this:
</para>
<programlisting language="bash">
options = {
name = mkOption {
type = type specification;
default = default value;
example = example value;
description = "Description for use in the NixOS manual.";
};
};
</programlisting>
<para>
The attribute names within the <literal>name</literal> attribute
path must be camel cased in general but should, as an exception,
match the
<link xlink:href="https://nixos.org/nixpkgs/manual/#sec-package-naming">
package attribute name</link> when referencing a Nixpkgs package.
For example, the option
<literal>services.nix-serve.bindAddress</literal> references the
<literal>nix-serve</literal> Nixpkgs package.
</para>
<para>
The function <literal>mkOption</literal> accepts the following
arguments.
</para>
<variablelist>
<varlistentry>
<term>
<literal>type</literal>
</term>
<listitem>
<para>
The type of the option (see
<xref linkend="sec-option-types" />). It may be omitted, but
that’s not advisable since it may lead to errors that are hard
to diagnose.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>default</literal>
</term>
<listitem>
<para>
The default value used if no value is defined by any module. A
default is not required; but if a default is not given, then
users of the module will have to define the value of the
option, otherwise an error will be thrown.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>example</literal>
</term>
<listitem>
<para>
An example value that will be shown in the NixOS manual.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>description</literal>
</term>
<listitem>
<para>
A textual description of the option, in DocBook format, that
will be included in the NixOS manual.
</para>
</listitem>
</varlistentry>
</variablelist>
<section xml:id="sec-option-declarations-eot">
<title>Extensible Option Types</title>
<para>
Extensible option types is a feature that allow to extend certain
types declaration through multiple module files. This feature only
work with a restricted set of types, namely
<literal>enum</literal> and <literal>submodules</literal> and any
composed forms of them.
</para>
<para>
Extensible option types can be used for <literal>enum</literal>
options that affects multiple modules, or as an alternative to
related <literal>enable</literal> options.
</para>
<para>
As an example, we will take the case of display managers. There is
a central display manager module for generic display manager
options and a module file per display manager backend (sddm, gdm
...).
</para>
<para>
There are two approach to this module structure:
</para>
<itemizedlist>
<listitem>
<para>
Managing the display managers independently by adding an
enable option to every display manager module backend. (NixOS)
</para>
</listitem>
<listitem>
<para>
Managing the display managers in the central module by adding
an option to select which display manager backend to use.
</para>
</listitem>
</itemizedlist>
<para>
Both approaches have problems.
</para>
<para>
Making backends independent can quickly become hard to manage. For
display managers, there can be only one enabled at a time, but the
type system can not enforce this restriction as there is no
relation between each backend <literal>enable</literal> option. As
a result, this restriction has to be done explicitely by adding
assertions in each display manager backend module.
</para>
<para>
On the other hand, managing the display managers backends in the
central module will require to change the central module option
every time a new backend is added or removed.
</para>
<para>
By using extensible option types, it is possible to create a
placeholder option in the central module
(<link linkend="ex-option-declaration-eot-service">Example:
Extensible type placeholder in the service module</link>), and to
extend it in each backend module
(<link linkend="ex-option-declaration-eot-backend-gdm">Example:
Extending
<literal>services.xserver.displayManager.enable</literal> in the
<literal>gdm</literal> module</link>,
<link linkend="ex-option-declaration-eot-backend-sddm">Example:
Extending
<literal>services.xserver.displayManager.enable</literal> in the
<literal>sddm</literal> module</link>).
</para>
<para>
As a result, <literal>displayManager.enable</literal> option
values can be added without changing the main service module file
and the type system automatically enforce that there can only be a
single display manager enabled.
</para>
<anchor xml:id="ex-option-declaration-eot-service" />
<para>
<emphasis role="strong">Example: Extensible type placeholder in
the service module</emphasis>
</para>
<programlisting language="bash">
services.xserver.displayManager.enable = mkOption {
description = "Display manager to use";
type = with types; nullOr (enum [ ]);
};
</programlisting>
<anchor xml:id="ex-option-declaration-eot-backend-gdm" />
<para>
<emphasis role="strong">Example: Extending
<literal>services.xserver.displayManager.enable</literal> in the
<literal>gdm</literal> module</emphasis>
</para>
<programlisting language="bash">
services.xserver.displayManager.enable = mkOption {
type = with types; nullOr (enum [ "gdm" ]);
};
</programlisting>
<anchor xml:id="ex-option-declaration-eot-backend-sddm" />
<para>
<emphasis role="strong">Example: Extending
<literal>services.xserver.displayManager.enable</literal> in the
<literal>sddm</literal> module</emphasis>
</para>
<programlisting language="bash">
services.xserver.displayManager.enable = mkOption {
type = with types; nullOr (enum [ "sddm" ]);
};
</programlisting>
<para>
The placeholder declaration is a standard
<literal>mkOption</literal> declaration, but it is important that
extensible option declarations only use the
<literal>type</literal> argument.
</para>
<para>
Extensible option types work with any of the composed variants of
<literal>enum</literal> such as
<literal>with types; nullOr (enum [ "foo" "bar" ])</literal>
or
<literal>with types; listOf (enum [ "foo" "bar" ])</literal>.
</para>
</section>
</section>
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