.. _sec-option-declarations:

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 <https://nixos.org/nixpkgs/manual/#sec-package-naming>`_ 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 :ref:`sec-option-types`). 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.

.. _sec-option-declarations-eot:

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 (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
(:ref:`ex-option-declaration-eot-service`), and to extend
it in each backend module
(:ref:`ex-option-declaration-eot-backend-gdm`,
:ref:`ex-option-declaration-eot-backend-sddm`).

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 ``gdm`` module
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
::

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

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"
])``.