nixos: nixos/doc/manual/development/option-declarations.xml to CommonMark
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nixos/doc/manual/development/option-declarations.section.md
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136
nixos/doc/manual/development/option-declarations.section.md
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@ -0,0 +1,136 @@
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# Option Declarations {#sec-option-declarations}
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An option declaration specifies the name, type and description of a
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NixOS configuration option. It is invalid to define an option that
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hasn't been declared in any module. An option declaration generally
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looks like this:
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```nix
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options = {
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name = mkOption {
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type = type specification;
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default = default value;
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example = example value;
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description = "Description for use in the NixOS manual.";
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};
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};
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```
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The attribute names within the `name` attribute path must be camel
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cased in general but should, as an exception, match the [ package
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attribute name](https://nixos.org/nixpkgs/manual/#sec-package-naming)
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when referencing a Nixpkgs package. For example, the option
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`services.nix-serve.bindAddress` references the `nix-serve` Nixpkgs
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package.
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The function `mkOption` accepts the following arguments.
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`type`
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: The type of the option (see [](#sec-option-types)). It may be
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omitted, but that's not advisable since it may lead to errors that
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are hard to diagnose.
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`default`
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: The default value used if no value is defined by any module. A
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default is not required; but if a default is not given, then users
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of the module will have to define the value of the option, otherwise
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an error will be thrown.
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`example`
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: An example value that will be shown in the NixOS manual.
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`description`
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: A textual description of the option, in DocBook format, that will be
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included in the NixOS manual.
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## Extensible Option Types {#sec-option-declarations-eot}
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Extensible option types is a feature that allow to extend certain types
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declaration through multiple module files. This feature only work with a
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restricted set of types, namely `enum` and `submodules` and any composed
|
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forms of them.
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Extensible option types can be used for `enum` options that affects
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multiple modules, or as an alternative to related `enable` options.
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As an example, we will take the case of display managers. There is a
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central display manager module for generic display manager options and a
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module file per display manager backend (sddm, gdm \...).
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There are two approach to this module structure:
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- Managing the display managers independently by adding an enable
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option to every display manager module backend. (NixOS)
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- Managing the display managers in the central module by adding an
|
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option to select which display manager backend to use.
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Both approaches have problems.
|
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|
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Making backends independent can quickly become hard to manage. For
|
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display managers, there can be only one enabled at a time, but the type
|
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system can not enforce this restriction as there is no relation between
|
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each backend `enable` option. As a result, this restriction has to be
|
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done explicitely by adding assertions in each display manager backend
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module.
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On the other hand, managing the display managers backends in the central
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module will require to change the central module option every time a new
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backend is added or removed.
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By using extensible option types, it is possible to create a placeholder
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option in the central module
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([Example: Extensible type placeholder in the service module](#ex-option-declaration-eot-service)),
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and to extend it in each backend module
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([Example: Extending `services.xserver.displayManager.enable` in the `gdm` module](#ex-option-declaration-eot-backend-gdm),
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[Example: Extending `services.xserver.displayManager.enable` in the `sddm` module](#ex-option-declaration-eot-backend-sddm)).
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As a result, `displayManager.enable` option values can be added without
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changing the main service module file and the type system automatically
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enforce that there can only be a single display manager enabled.
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::: {#ex-option-declaration-eot-service .example}
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::: {.title}
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**Example: Extensible type placeholder in the service module**
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:::
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```nix
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services.xserver.displayManager.enable = mkOption {
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description = "Display manager to use";
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type = with types; nullOr (enum [ ]);
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};
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```
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:::
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::: {#ex-option-declaration-eot-backend-gdm .example}
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::: {.title}
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**Example: Extending `services.xserver.displayManager.enable` in the `gdm` module**
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:::
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```nix
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services.xserver.displayManager.enable = mkOption {
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type = with types; nullOr (enum [ "gdm" ]);
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};
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```
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:::
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::: {#ex-option-declaration-eot-backend-sddm .example}
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::: {.title}
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**Example: Extending `services.xserver.displayManager.enable` in the `sddm` module**
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:::
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```nix
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services.xserver.displayManager.enable = mkOption {
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type = with types; nullOr (enum [ "sddm" ]);
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};
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```
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:::
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The placeholder declaration is a standard `mkOption` declaration, but it
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is important that extensible option declarations only use the `type`
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argument.
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Extensible option types work with any of the composed variants of `enum`
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such as `with types; nullOr (enum [ "foo" "bar" ])` or `with types;
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listOf (enum [ "foo" "bar" ])`.
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@ -1,199 +0,0 @@
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<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id="sec-option-declarations">
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<title>Option Declarations</title>
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<para>
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An option declaration specifies the name, type and description of a NixOS
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configuration option. It is invalid to define an option that hasn’t been
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declared in any module. An option declaration generally looks like this:
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<programlisting>
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options = {
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<replaceable>name</replaceable> = mkOption {
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type = <replaceable>type specification</replaceable>;
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default = <replaceable>default value</replaceable>;
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example = <replaceable>example value</replaceable>;
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description = "<replaceable>Description for use in the NixOS manual.</replaceable>";
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};
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};
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</programlisting>
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The attribute names within the <replaceable>name</replaceable> attribute path
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||||
must be camel cased in general but should, as an exception, match the
|
||||
<link
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||||
xlink:href="https://nixos.org/nixpkgs/manual/#sec-package-naming">
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package attribute name</link> when referencing a Nixpkgs package. For
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example, the option <varname>services.nix-serve.bindAddress</varname>
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references the <varname>nix-serve</varname> Nixpkgs package.
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</para>
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<para>
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The function <varname>mkOption</varname> accepts the following arguments.
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<variablelist>
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<varlistentry>
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<term>
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<varname>type</varname>
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</term>
|
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<listitem>
|
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<para>
|
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The type of the option (see <xref linkend='sec-option-types' />). It may
|
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be omitted, but that’s not advisable since it may lead to errors that
|
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are hard to diagnose.
|
||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
<varlistentry>
|
||||
<term>
|
||||
<varname>default</varname>
|
||||
</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
|
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thrown.
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</para>
|
||||
</listitem>
|
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</varlistentry>
|
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<varlistentry>
|
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<term>
|
||||
<varname>example</varname>
|
||||
</term>
|
||||
<listitem>
|
||||
<para>
|
||||
An example value that will be shown in the NixOS manual.
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||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
<varlistentry>
|
||||
<term>
|
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<varname>description</varname>
|
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</term>
|
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<listitem>
|
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<para>
|
||||
A textual description of the option, in DocBook format, that will be
|
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included in the NixOS manual.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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<section xml:id="sec-option-declarations-eot">
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<title>Extensible Option Types</title>
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|
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<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:
|
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<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>
|
||||
|
||||
<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
|
||||
(<xref linkend='ex-option-declaration-eot-service'
|
||||
/>), and to extend
|
||||
it in each backend module
|
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(<xref
|
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linkend='ex-option-declaration-eot-backend-gdm' />,
|
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<xref
|
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linkend='ex-option-declaration-eot-backend-sddm' />).
|
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</para>
|
||||
|
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<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.
|
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</para>
|
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|
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<example xml:id='ex-option-declaration-eot-service'>
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<title>Extensible type placeholder in the service module</title>
|
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<screen>
|
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services.xserver.displayManager.enable = mkOption {
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description = "Display manager to use";
|
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type = with types; nullOr (enum [ ]);
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};</screen>
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</example>
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|
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<example xml:id='ex-option-declaration-eot-backend-gdm'>
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<title>Extending <literal>services.xserver.displayManager.enable</literal> in the <literal>gdm</literal> module</title>
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<screen>
|
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services.xserver.displayManager.enable = mkOption {
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type = with types; nullOr (enum [ "gdm" ]);
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};</screen>
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</example>
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|
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<example xml:id='ex-option-declaration-eot-backend-sddm'>
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<title>Extending <literal>services.xserver.displayManager.enable</literal> in the <literal>sddm</literal> module</title>
|
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<screen>
|
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services.xserver.displayManager.enable = mkOption {
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type = with types; nullOr (enum [ "sddm" ]);
|
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};</screen>
|
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</example>
|
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|
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<para>
|
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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>
|
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</section>
|
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</section>
|
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|
|
@ -179,7 +179,7 @@ in {
|
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}
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</programlisting>
|
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</example>
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<xi:include href="option-declarations.xml" />
|
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<xi:include href="../from_md/development/option-declarations.section.xml" />
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<xi:include href="option-types.xml" />
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<xi:include href="option-def.xml" />
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<xi:include href="../from_md/development/assertions.section.xml" />
|
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|
|
|
|||
|
|
@ -0,0 +1,203 @@
|
|||
<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>
|
||||
Loading…
Reference in a new issue