# Option Declarations

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:

    options = {
      name = mkOption {
        type = type specification;
        default = default value;
        example = example value;
        description = "Description for use in the NixOS manual.";
      };
    };

The attribute names within the name attribute path must be camel cased in general but should, as an exception, match the package attribute name when referencing a Nixpkgs package. For example, the option `services.nix-serve.bindAddress` references the `nix-serve` Nixpkgs package.

The function `mkOption` accepts the following arguments.

`type`

  The type of the option (see ). It may be omitted, but that’s not advisable since it may lead to errors that are hard to diagnose.

`default`

  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.

`example`

  An example value that will be shown in the NixOS manual.

`description`

  A textual description of the option, in DocBook format, that will be included in the NixOS manual.

## Extensible Option Types

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 `enum` and `submodules` and any composed forms of them.

Extensible option types can be used for `enum` options that affects multiple modules, or as an alternative to related `enable` 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 ...).

There are two approach to this module structure:

- Managing the display managers independently by adding an enable option to every display manager module backend. (NixOS)

- Managing the display managers in the central module by adding an option to select which display manager backend to use.

Both approaches have problems.

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 `enable` 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 (), and to extend it in each backend module (, ).

As a result, `displayManager.enable` 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.

### Extensible type placeholder in the service module

    services.xserver.displayManager.enable = mkOption {
      description = "Display manager to use";
      type = with types; nullOr (enum [ ]);
    };

### Extending `services.xserver.displayManager.enable` in the `slim` module

    services.xserver.displayManager.enable = mkOption {
      type = with types; nullOr (enum [ "slim" ]);
    };

### Extending `services.xserver.displayManager.enable` in the `sddm` module

    services.xserver.displayManager.enable = mkOption {
      type = with types; nullOr (enum [ "sddm" ]);
    };

The placeholder declaration is a standard `mkOption` declaration, but it is important that extensible option declarations only use the `type` argument.

Extensible option types work with any of the composed variants of `enum` such as `with types; nullOr (enum [ "foo" "bar" ])` or `with types; listOf (enum [ "foo" "bar" ])`.