nixpkgs/nixos/doc/manual/development/option-declarations.section.md

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 ). This argument is mandatory for nixpkgs modules. Setting this is highly recommended for the sake of documentation and type checking. In case it is not set, a fallback type with unspecified behavior is used.

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.

defaultText

A textual representation of the default value to be rendered verbatim in the manual. Useful if the default value is a complex expression or depends on other values or packages. Use lib.literalExpression for a Nix expression, lib.literalMD for a plain English description in Nixpkgs-flavored Markdown format.

example

An example value that will be shown in the NixOS manual. You can use lib.literalExpression and lib.literalMD in the same way as in defaultText.

description

A textual description of the option in Nixpkgs-flavored Markdown format that will be included in the NixOS manual.

Utility functions for common option patterns

mkEnableOption

Creates an Option attribute set for a boolean value option i.e an option to be toggled on or off.

This function takes a single string argument, the name of the thing to be toggled.

The option's description is "Whether to enable <name>.".

For example:

::: {#ex-options-declarations-util-mkEnableOption-magic .example}

mkEnableOption usage

lib.mkEnableOption "magic"
# is like
lib.mkOption {
  type = lib.types.bool;
  default = false;
  example = true;
  description = "Whether to enable magic.";
}

:::

mkPackageOption

Usage:

mkPackageOption pkgs "name" { default = [ "path" "in" "pkgs" ]; example = "literal example"; }

Creates an Option attribute set for an option that specifies the package a module should use for some purpose.

Note: You shouldn’t necessarily make package options for all of your modules. You can always overwrite a specific package throughout nixpkgs by using nixpkgs overlays.

The package is specified in the third argument under default as a list of strings representing its attribute path in nixpkgs (or another package set). Because of this, you need to pass nixpkgs itself (or a subset) as the first argument.

The second argument may be either a string or a list of strings. It provides the display name of the package in the description of the generated option (using only the last element if the passed value is a list) and serves as the fallback value for the default argument.

To include extra information in the description, pass extraDescription to append arbitrary text to the generated description. You can also pass an example value, either a literal string or an attribute path.

The default argument can be omitted if the provided name is an attribute of pkgs (if name is a string) or a valid attribute path in pkgs (if name is a list).

If you wish to explicitly provide no default, pass null as default.

[]{#ex-options-declarations-util-mkPackageOption} Examples:

::: {#ex-options-declarations-util-mkPackageOption-hello .example}

Simple mkPackageOption usage

lib.mkPackageOption pkgs "hello" { }
# is like
lib.mkOption {
  type = lib.types.package;
  default = pkgs.hello;
  defaultText = lib.literalExpression "pkgs.hello";
  description = "The hello package to use.";
}

:::

::: {#ex-options-declarations-util-mkPackageOption-ghc .example}

mkPackageOption with explicit default and example

lib.mkPackageOption pkgs "GHC" {
  default = [ "ghc" ];
  example = "pkgs.haskell.packages.ghc92.ghc.withPackages (hkgs: [ hkgs.primes ])";
}
# is like
lib.mkOption {
  type = lib.types.package;
  default = pkgs.ghc;
  defaultText = lib.literalExpression "pkgs.ghc";
  example = lib.literalExpression "pkgs.haskell.packages.ghc92.ghc.withPackages (hkgs: [ hkgs.primes ])";
  description = "The GHC package to use.";
}

:::

::: {#ex-options-declarations-util-mkPackageOption-extraDescription .example}

mkPackageOption with additional description text

mkPackageOption pkgs [ "python39Packages" "pytorch" ] {
  extraDescription = "This is an example and doesn't actually do anything.";
}
# is like
lib.mkOption {
  type = lib.types.package;
  default = pkgs.python39Packages.pytorch;
  defaultText = lib.literalExpression "pkgs.python39Packages.pytorch";
  description = "The pytorch package to use. This is an example and doesn't actually do anything.";
}

:::

Extensible Option Types

Extensible option types is a feature that allows 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 approaches we could take with this module structure:

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

• Configuring 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 only be one enabled at a time, but the type system cannot enforce this restriction as there is no relation between each backend's enable option. As a result, this restriction has to be done explicitly by adding assertions in each display manager backend module.

On the other hand, managing the display manager backends in the central module will require changing 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 (Example: Extensible type placeholder in the service module), and to extend it in each backend module (Example: Extending services.xserver.displayManager.enable in the gdm module, Example: Extending services.xserver.displayManager.enable in the sddm module).

As a result, displayManager.enable option values can be added without changing the main service module file and the type system automatically enforces that there can only be a single display manager enabled.

::: {#ex-option-declaration-eot-service .example}

Extensible type placeholder in the service module

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

:::

::: {#ex-option-declaration-eot-backend-gdm .example}

Extending services.xserver.displayManager.enable in the gdm module

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

:::

::: {#ex-option-declaration-eot-backend-sddm .example}

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" ]).