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:
The attribute names within the <replaceable>name</replaceable> 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 <varname>services.nix-serve.bindAddress</varname> references the <varname>nix-serve</varname> Nixpkgs package.
The type of the option (see <xreflinkend='sec-option-types'/>). It may be omitted, but that’s not advisable since it may lead to errors that are hard to diagnose.
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.
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.
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.
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 (slim, sddm, gdm ...).
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.
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.
By using extensible option types, it is possible to create a placeholder option in the central module (<xreflinkend='ex-option-declaration-eot-service'
/>), and to extend it in each backend module (<xref
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.
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.
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>.
