commit 165d6bd20c478c8c4f201c26eaa4c62e3c1b87ce · pyrox.dev/nixpkgs

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nixos/doc/manual/development/settings-options.section.md

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       # Options for Program Settings {#sec-settings-options}

     

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       Many programs have configuration files where program-specific settings

     

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       can be declared. File formats can be separated into two categories:

     

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       -   Nix-representable ones: These can trivially be mapped to a subset of

     

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           Nix syntax. E.g. JSON is an example, since its values like

     

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           `{"foo":{"bar":10}}` can be mapped directly to Nix:

     

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           `{ foo = { bar = 10; }; }`. Other examples are INI, YAML and TOML.

     

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           The following section explains the convention for these settings.

     

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       -   Non-nix-representable ones: These can\'t be trivially mapped to a

     

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           subset of Nix syntax. Most generic programming languages are in this

     

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           group, e.g. bash, since the statement `if true; then echo hi; fi`

     

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           doesn\'t have a trivial representation in Nix.

     

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           Currently there are no fixed conventions for these, but it is common

     

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           to have a `configFile` option for setting the configuration file

     

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           path directly. The default value of `configFile` can be an

     

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           auto-generated file, with convenient options for controlling the

     

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           contents. For example an option of type `attrsOf str` can be used

     

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           for representing environment variables which generates a section

     

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           like `export FOO="foo"`. Often it can also be useful to also include

     

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           an `extraConfig` option of type `lines` to allow arbitrary text

     

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           after the autogenerated part of the file.

     

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       ## Nix-representable Formats (JSON, YAML, TOML, INI, \...) {#sec-settings-nix-representable}

     

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       By convention, formats like this are handled with a generic `settings`

     

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       option, representing the full program configuration as a Nix value. The

     

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       type of this option should represent the format. The most common formats

     

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       have a predefined type and string generator already declared under

     

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       `pkgs.formats`:

     

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       `pkgs.formats.json` { }

     

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       :   A function taking an empty attribute set (for future extensibility)

     

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           and returning a set with JSON-specific attributes `type` and

     

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           `generate` as specified [below](#pkgs-formats-result).

     

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       `pkgs.formats.yaml` { }

     

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       :   A function taking an empty attribute set (for future extensibility)

     

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           and returning a set with YAML-specific attributes `type` and

     

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           `generate` as specified [below](#pkgs-formats-result).

     

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       `pkgs.formats.ini` { *`listsAsDuplicateKeys`* ? false, *`listToValue`* ? null, \... }

     

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       :   A function taking an attribute set with values

     

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           `listsAsDuplicateKeys`

     

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           :   A boolean for controlling whether list values can be used to

     

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               represent duplicate INI keys

     

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           `listToValue`

     

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           :   A function for turning a list of values into a single value.

     

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           It returns a set with INI-specific attributes `type` and `generate`

     

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           as specified [below](#pkgs-formats-result).

     

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       `pkgs.formats.toml` { }

     

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       :   A function taking an empty attribute set (for future extensibility)

     

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           and returning a set with TOML-specific attributes `type` and

     

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           `generate` as specified [below](#pkgs-formats-result).

     

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       ::: {#pkgs-formats-result}

     

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       These functions all return an attribute set with these values:

     

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       :::

     

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       `type`

     

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       :   A module system type representing a value of the format

     

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       `generate` *`filename jsonValue`*

     

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       :   A function that can render a value of the format to a file. Returns

     

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           a file path.

     

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           ::: {.note}

     

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           This function puts the value contents in the Nix store. So this

     

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           should be avoided for secrets.

     

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           :::

     

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       ::: {#ex-settings-nix-representable .example}

     

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       ::: {.title}

     

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       **Example: Module with conventional `settings` option**

     

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       :::

     

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       The following shows a module for an example program that uses a JSON

     

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       configuration file. It demonstrates how above values can be used, along

     

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       with some other related best practices. See the comments for

     

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       explanations.

     

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       ```nix

     

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       { options, config, lib, pkgs, ... }:

     

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       let

     

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         cfg = config.services.foo;

     

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         # Define the settings format used for this program

     

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         settingsFormat = pkgs.formats.json {};

     

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       in {

     

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         options.services.foo = {

     

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           enable = lib.mkEnableOption "foo service";

     

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           settings = lib.mkOption {

     

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             # Setting this type allows for correct merging behavior

     

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             type = settingsFormat.type;

     

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             default = {};

     

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             description = ''

     

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               Configuration for foo, see

     

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               <link xlink:href="https://example.com/docs/foo"/>

     

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               for supported settings.

     

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             '';

     

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           };

     

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         };

     

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         config = lib.mkIf cfg.enable {

     

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           # We can assign some default settings here to make the service work by just

     

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           # enabling it. We use `mkDefault` for values that can be changed without

     

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           # problems

     

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           services.foo.settings = {

     

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             # Fails at runtime without any value set

     

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             log_level = lib.mkDefault "WARN";

     

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             # We assume systemd's `StateDirectory` is used, so we require this value,

     

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             # therefore no mkDefault

     

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             data_path = "/var/lib/foo";

     

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             # Since we use this to create a user we need to know the default value at

     

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             # eval time

     

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             user = lib.mkDefault "foo";

     

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           };

     

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           environment.etc."foo.json".source =

     

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             # The formats generator function takes a filename and the Nix value

     

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             # representing the format value and produces a filepath with that value

     

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             # rendered in the format

     

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             settingsFormat.generate "foo-config.json" cfg.settings;

     

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           # We know that the `user` attribute exists because we set a default value

     

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           # for it above, allowing us to use it without worries here

     

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           users.users.${cfg.settings.user} = { isSystemUser = true; };

     

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           # ...

     

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         };

     

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       }

     

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       ```

     

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       :::

     

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       ### Option declarations for attributes {#sec-settings-attrs-options}

     

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       Some `settings` attributes may deserve some extra care. They may need a

     

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       different type, default or merging behavior, or they are essential

     

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       options that should show their documentation in the manual. This can be

     

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       done using [](#sec-freeform-modules).

     

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       We extend above example using freeform modules to declare an option for

     

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       the port, which will enforce it to be a valid integer and make it show

     

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       up in the manual.

     

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       ::: {#ex-settings-typed-attrs .example}

     

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       ::: {.title}

     

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       **Example: Declaring a type-checked `settings` attribute**

     

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       :::

     

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       ```nix

     

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       settings = lib.mkOption {

     

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         type = lib.types.submodule {

     

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           freeformType = settingsFormat.type;

     

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           # Declare an option for the port such that the type is checked and this option

     

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           # is shown in the manual.

     

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           options.port = lib.mkOption {

     

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             type = lib.types.port;

     

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             default = 8080;

     

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             description = ''

     

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               Which port this service should listen on.

     

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             '';

     

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           };

     

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         };

     

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         default = {};

     

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         description = ''

     

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           Configuration for Foo, see

     

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           <link xlink:href="https://example.com/docs/foo"/>

     

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           for supported values.

     

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         '';

     

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       };

     

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       ```

     

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       :::

-226

nixos/doc/manual/development/settings-options.xml

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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-settings-options">

     

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        <title>Options for Program Settings</title>

     

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        <para>

     

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          Many programs have configuration files where program-specific settings can be declared. File formats can be separated into two categories:

     

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          <itemizedlist>

     

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            <listitem>

     

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              <para>

     

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                Nix-representable ones: These can trivially be mapped to a subset of Nix syntax. E.g. JSON is an example, since its values like <literal>{"foo":{"bar":10}}</literal> can be mapped directly to Nix: <literal>{ foo = { bar = 10; }; }</literal>. Other examples are INI, YAML and TOML. The following section explains the convention for these settings.

     

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              </para>

     

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            </listitem>

     

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            <listitem>

     

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              <para>

     

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                Non-nix-representable ones: These can't be trivially mapped to a subset of Nix syntax. Most generic programming languages are in this group, e.g. bash, since the statement <literal>if true; then echo hi; fi</literal> doesn't have a trivial representation in Nix.

     

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              </para>

     

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              <para>

     

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                Currently there are no fixed conventions for these, but it is common to have a <literal>configFile</literal> option for setting the configuration file path directly. The default value of <literal>configFile</literal> can be an auto-generated file, with convenient options for controlling the contents. For example an option of type <literal>attrsOf str</literal> can be used for representing environment variables which generates a section like <literal>export FOO="foo"</literal>. Often it can also be useful to also include an <literal>extraConfig</literal> option of type <literal>lines</literal> to allow arbitrary text after the autogenerated part of the file.

     

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              </para>

     

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            </listitem>

     

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          </itemizedlist>

     

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        </para>

     

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        <section xml:id="sec-settings-nix-representable">

     

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          <title>Nix-representable Formats (JSON, YAML, TOML, INI, ...)</title>

     

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          <para>

     

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            By convention, formats like this are handled with a generic <literal>settings</literal> option, representing the full program configuration as a Nix value. The type of this option should represent the format. The most common formats have a predefined type and string generator already declared under <literal>pkgs.formats</literal>:

     

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            <variablelist>

     

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              <varlistentry>

     

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                <term>

     

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                  <varname>pkgs.formats.json</varname> { }

     

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                </term>

     

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                <listitem>

     

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                  <para>

     

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                    A function taking an empty attribute set (for future extensibility) and returning a set with JSON-specific attributes <varname>type</varname> and <varname>generate</varname> as specified <link linkend='pkgs-formats-result'>below</link>.

     

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                  </para>

     

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                </listitem>

     

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              </varlistentry>

     

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              <varlistentry>

     

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                <term>

     

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                  <varname>pkgs.formats.yaml</varname> { }

     

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                </term>

     

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                <listitem>

     

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                  <para>

     

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                    A function taking an empty attribute set (for future extensibility) and returning a set with YAML-specific attributes <varname>type</varname> and <varname>generate</varname> as specified <link linkend='pkgs-formats-result'>below</link>.

     

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                  </para>

     

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                </listitem>

     

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              </varlistentry>

     

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              <varlistentry>

     

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                <term>

     

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                  <varname>pkgs.formats.ini</varname> { <replaceable>listsAsDuplicateKeys</replaceable> ? false, <replaceable>listToValue</replaceable> ? null, ... }

     

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                </term>

     

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                <listitem>

     

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                  <para>

     

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                    A function taking an attribute set with values

     

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                    <variablelist>

     

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                      <varlistentry>

     

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                        <term>

     

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                          <varname>listsAsDuplicateKeys</varname>

     

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                        </term>

     

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                        <listitem>

     

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                          <para>

     

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                            A boolean for controlling whether list values can be used to represent duplicate INI keys

     

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                          </para>

     

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                        </listitem>

     

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                      </varlistentry>

     

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                      <varlistentry>

     

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                        <term>

     

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                          <varname>listToValue</varname>

     

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                        </term>

     

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                        <listitem>

     

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                          <para>

     

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                            A function for turning a list of values into a single value.

     

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                          </para>

     

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                        </listitem>

     

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                      </varlistentry>

     

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                    </variablelist>

     

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                   It returns a set with INI-specific attributes <varname>type</varname> and <varname>generate</varname> as specified <link linkend='pkgs-formats-result'>below</link>.

     

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                  </para>

     

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                </listitem>

     

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              </varlistentry>

     

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              <varlistentry>

     

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                <term>

     

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                  <varname>pkgs.formats.toml</varname> { }

     

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                </term>

     

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                <listitem>

     

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                  <para>

     

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                    A function taking an empty attribute set (for future extensibility) and returning a set with TOML-specific attributes <varname>type</varname> and <varname>generate</varname> as specified <link linkend='pkgs-formats-result'>below</link>.

     

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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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          <para xml:id="pkgs-formats-result">

     

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            These functions all return an attribute set with these values:

     

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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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                   A module system type representing a value of the format

     

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                 </para>

     

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               </listitem>

     

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             </varlistentry>

     

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             <varlistentry>

     

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               <term>

     

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                 <varname>generate</varname> <replaceable>filename</replaceable> <replaceable>jsonValue</replaceable>

     

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               </term>

     

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               <listitem>

     

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                 <para>

     

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                  A function that can render a value of the format to a file. Returns a file path.

     

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                  <note>

     

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                   <para>

     

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                    This function puts the value contents in the Nix store. So this should be avoided for secrets.

     

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                   </para>

     

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                  </note>

     

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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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          <example xml:id="ex-settings-nix-representable">

     

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            <title>Module with conventional <literal>settings</literal> option</title>

     

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            <para>

     

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              The following shows a module for an example program that uses a JSON configuration file. It demonstrates how above values can be used, along with some other related best practices. See the comments for explanations.

     

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            </para>

     

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       <programlisting>

     

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       { options, config, lib, pkgs, ... }:

     

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       let

     

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         cfg = config.services.foo;

     

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         # Define the settings format used for this program

     

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         settingsFormat = pkgs.formats.json {};

     

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       in {

     

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         options.services.foo = {

     

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           enable = lib.mkEnableOption "foo service";

     

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           settings = lib.mkOption {

     

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             # Setting this type allows for correct merging behavior

     

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             type = settingsFormat.type;

     

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             default = {};

     

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             description = ''

     

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               Configuration for foo, see

     

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               &lt;link xlink:href="https://example.com/docs/foo"/&gt;

     

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               for supported settings.

     

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             '';

     

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           };

     

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         };

     

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         config = lib.mkIf cfg.enable {

     

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           # We can assign some default settings here to make the service work by just

     

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           # enabling it. We use `mkDefault` for values that can be changed without

     

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           # problems

     

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           services.foo.settings = {

     

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             # Fails at runtime without any value set

     

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             log_level = lib.mkDefault "WARN";

     

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             # We assume systemd's `StateDirectory` is used, so we require this value,

     

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             # therefore no mkDefault

     

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             data_path = "/var/lib/foo";

     

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             # Since we use this to create a user we need to know the default value at

     

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             # eval time

     

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             user = lib.mkDefault "foo";

     

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           };

     

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           environment.etc."foo.json".source =

     

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             # The formats generator function takes a filename and the Nix value

     

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             # representing the format value and produces a filepath with that value

     

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             # rendered in the format

     

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             settingsFormat.generate "foo-config.json" cfg.settings;

     

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           # We know that the `user` attribute exists because we set a default value

     

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           # for it above, allowing us to use it without worries here

     

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           users.users.${cfg.settings.user} = { isSystemUser = true; };

     

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       182
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           # ...

     

       183
       -
         };

     

       184
       -
       }

     

       185
       -
       </programlisting>

     

       186
       -
          </example>

     

       187
       -
          <section xml:id="sec-settings-attrs-options">

     

       188
       -
           <title>Option declarations for attributes</title>

     

       189
       -
           <para>

     

       190
       -
            Some <literal>settings</literal> attributes may deserve some extra care. They may need a different type, default or merging behavior, or they are essential options that should show their documentation in the manual. This can be done using <xref linkend='sec-freeform-modules'/>.

     

       191
       -
            <example xml:id="ex-settings-typed-attrs">

     

       192
       -
             <title>Declaring a type-checked <literal>settings</literal> attribute</title>

     

       193
       -
             <para>

     

       194
       -
              We extend above example using freeform modules to declare an option for the port, which will enforce it to be a valid integer and make it show up in the manual.

     

       195
       -
             </para>

     

       196
       -
       <programlisting>

     

       197
       -
       settings = lib.mkOption {

     

       198
       -
         type = lib.types.submodule {

     

       199
       -
       

     

       200
       -
           freeformType = settingsFormat.type;

     

       201
       -
       

     

       202
       -
           # Declare an option for the port such that the type is checked and this option

     

       203
       -
           # is shown in the manual.

     

       204
       -
           options.port = lib.mkOption {

     

       205
       -
             type = lib.types.port;

     

       206
       -
             default = 8080;

     

       207
       -
             description = ''

     

       208
       -
               Which port this service should listen on.

     

       209
       -
             '';

     

       210
       -
           };

     

       211
       -
       

     

       212
       -
         };

     

       213
       -
         default = {};

     

       214
       -
         description = ''

     

       215
       -
           Configuration for Foo, see

     

       216
       -
           &lt;link xlink:href="https://example.com/docs/foo"/&gt;

     

       217
       -
           for supported values.

     

       218
       -
         '';

     

       219
       -
       };

     

       220
       -
       </programlisting>

     

       221
       -
            </example>

     

       222
       -
           </para>

     

       223
       -
          </section>

     

       224
       -
        </section>

     

       225
       -
       

     

       226
       -
       </section>

+2 -2

nixos/doc/manual/development/writing-modules.xml

···

       186
        
        <xi:include href="../from_md/development/meta-attributes.section.xml" />

     

       187
        
        <xi:include href="../from_md/development/importing-modules.section.xml" />

     

       188
        
        <xi:include href="../from_md/development/replace-modules.section.xml" />

     

       189
       -
        <xi:include href="freeform-modules.xml" />

     

       190
       -
        <xi:include href="settings-options.xml" />

     

       191
        
       </chapter>

···

       186
        
        <xi:include href="../from_md/development/meta-attributes.section.xml" />

     

       187
        
        <xi:include href="../from_md/development/importing-modules.section.xml" />

     

       188
        
        <xi:include href="../from_md/development/replace-modules.section.xml" />

     

       189
       +
        <xi:include href="../from_md/development/freeform-modules.section.xml" />

     

       190
       +
        <xi:include href="../from_md/development/settings-options.section.xml" />

     

       191
        
       </chapter>

+285

nixos/doc/manual/from_md/development/settings-options.section.xml

···

       1
       +
       <section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-settings-options">

     

       2
       +
         <title>Options for Program Settings</title>

     

       3
       +
         <para>

     

       4
       +
           Many programs have configuration files where program-specific

     

       5
       +
           settings can be declared. File formats can be separated into two

     

       6
       +
           categories:

     

       7
       +
         </para>

     

       8
       +
         <itemizedlist>

     

       9
       +
           <listitem>

     

       10
       +
             <para>

     

       11
       +
               Nix-representable ones: These can trivially be mapped to a

     

       12
       +
               subset of Nix syntax. E.g. JSON is an example, since its values

     

       13
       +
               like <literal>{&quot;foo&quot;:{&quot;bar&quot;:10}}</literal>

     

       14
       +
               can be mapped directly to Nix:

     

       15
       +
               <literal>{ foo = { bar = 10; }; }</literal>. Other examples are

     

       16
       +
               INI, YAML and TOML. The following section explains the

     

       17
       +
               convention for these settings.

     

       18
       +
             </para>

     

       19
       +
           </listitem>

     

       20
       +
           <listitem>

     

       21
       +
             <para>

     

       22
       +
               Non-nix-representable ones: These can't be trivially mapped to a

     

       23
       +
               subset of Nix syntax. Most generic programming languages are in

     

       24
       +
               this group, e.g. bash, since the statement

     

       25
       +
               <literal>if true; then echo hi; fi</literal> doesn't have a

     

       26
       +
               trivial representation in Nix.

     

       27
       +
             </para>

     

       28
       +
             <para>

     

       29
       +
               Currently there are no fixed conventions for these, but it is

     

       30
       +
               common to have a <literal>configFile</literal> option for

     

       31
       +
               setting the configuration file path directly. The default value

     

       32
       +
               of <literal>configFile</literal> can be an auto-generated file,

     

       33
       +
               with convenient options for controlling the contents. For

     

       34
       +
               example an option of type <literal>attrsOf str</literal> can be

     

       35
       +
               used for representing environment variables which generates a

     

       36
       +
               section like <literal>export FOO=&quot;foo&quot;</literal>.

     

       37
       +
               Often it can also be useful to also include an

     

       38
       +
               <literal>extraConfig</literal> option of type

     

       39
       +
               <literal>lines</literal> to allow arbitrary text after the

     

       40
       +
               autogenerated part of the file.

     

       41
       +
             </para>

     

       42
       +
           </listitem>

     

       43
       +
         </itemizedlist>

     

       44
       +
         <section xml:id="sec-settings-nix-representable">

     

       45
       +
           <title>Nix-representable Formats (JSON, YAML, TOML, INI,

     

       46
       +
           ...)</title>

     

       47
       +
           <para>

     

       48
       +
             By convention, formats like this are handled with a generic

     

       49
       +
             <literal>settings</literal> option, representing the full program

     

       50
       +
             configuration as a Nix value. The type of this option should

     

       51
       +
             represent the format. The most common formats have a predefined

     

       52
       +
             type and string generator already declared under

     

       53
       +
             <literal>pkgs.formats</literal>:

     

       54
       +
           </para>

     

       55
       +
           <variablelist>

     

       56
       +
             <varlistentry>

     

       57
       +
               <term>

     

       58
       +
                 <literal>pkgs.formats.json</literal> { }

     

       59
       +
               </term>

     

       60
       +
               <listitem>

     

       61
       +
                 <para>

     

       62
       +
                   A function taking an empty attribute set (for future

     

       63
       +
                   extensibility) and returning a set with JSON-specific

     

       64
       +
                   attributes <literal>type</literal> and

     

       65
       +
                   <literal>generate</literal> as specified

     

       66
       +
                   <link linkend="pkgs-formats-result">below</link>.

     

       67
       +
                 </para>

     

       68
       +
               </listitem>

     

       69
       +
             </varlistentry>

     

       70
       +
             <varlistentry>

     

       71
       +
               <term>

     

       72
       +
                 <literal>pkgs.formats.yaml</literal> { }

     

       73
       +
               </term>

     

       74
       +
               <listitem>

     

       75
       +
                 <para>

     

       76
       +
                   A function taking an empty attribute set (for future

     

       77
       +
                   extensibility) and returning a set with YAML-specific

     

       78
       +
                   attributes <literal>type</literal> and

     

       79
       +
                   <literal>generate</literal> as specified

     

       80
       +
                   <link linkend="pkgs-formats-result">below</link>.

     

       81
       +
                 </para>

     

       82
       +
               </listitem>

     

       83
       +
             </varlistentry>

     

       84
       +
             <varlistentry>

     

       85
       +
               <term>

     

       86
       +
                 <literal>pkgs.formats.ini</literal> {

     

       87
       +
                 <emphasis><literal>listsAsDuplicateKeys</literal></emphasis> ?

     

       88
       +
                 false, <emphasis><literal>listToValue</literal></emphasis> ?

     

       89
       +
                 null, ... }

     

       90
       +
               </term>

     

       91
       +
               <listitem>

     

       92
       +
                 <para>

     

       93
       +
                   A function taking an attribute set with values

     

       94
       +
                 </para>

     

       95
       +
                 <variablelist>

     

       96
       +
                   <varlistentry>

     

       97
       +
                     <term>

     

       98
       +
                       <literal>listsAsDuplicateKeys</literal>

     

       99
       +
                     </term>

     

       100
       +
                     <listitem>

     

       101
       +
                       <para>

     

       102
       +
                         A boolean for controlling whether list values can be

     

       103
       +
                         used to represent duplicate INI keys

     

       104
       +
                       </para>

     

       105
       +
                     </listitem>

     

       106
       +
                   </varlistentry>

     

       107
       +
                   <varlistentry>

     

       108
       +
                     <term>

     

       109
       +
                       <literal>listToValue</literal>

     

       110
       +
                     </term>

     

       111
       +
                     <listitem>

     

       112
       +
                       <para>

     

       113
       +
                         A function for turning a list of values into a single

     

       114
       +
                         value.

     

       115
       +
                       </para>

     

       116
       +
                     </listitem>

     

       117
       +
                   </varlistentry>

     

       118
       +
                 </variablelist>

     

       119
       +
                 <para>

     

       120
       +
                   It returns a set with INI-specific attributes

     

       121
       +
                   <literal>type</literal> and <literal>generate</literal> as

     

       122
       +
                   specified <link linkend="pkgs-formats-result">below</link>.

     

       123
       +
                 </para>

     

       124
       +
               </listitem>

     

       125
       +
             </varlistentry>

     

       126
       +
             <varlistentry>

     

       127
       +
               <term>

     

       128
       +
                 <literal>pkgs.formats.toml</literal> { }

     

       129
       +
               </term>

     

       130
       +
               <listitem>

     

       131
       +
                 <para>

     

       132
       +
                   A function taking an empty attribute set (for future

     

       133
       +
                   extensibility) and returning a set with TOML-specific

     

       134
       +
                   attributes <literal>type</literal> and

     

       135
       +
                   <literal>generate</literal> as specified

     

       136
       +
                   <link linkend="pkgs-formats-result">below</link>.

     

       137
       +
                 </para>

     

       138
       +
               </listitem>

     

       139
       +
             </varlistentry>

     

       140
       +
           </variablelist>

     

       141
       +
           <para xml:id="pkgs-formats-result">

     

       142
       +
             These functions all return an attribute set with these values:

     

       143
       +
           </para>

     

       144
       +
           <variablelist>

     

       145
       +
             <varlistentry>

     

       146
       +
               <term>

     

       147
       +
                 <literal>type</literal>

     

       148
       +
               </term>

     

       149
       +
               <listitem>

     

       150
       +
                 <para>

     

       151
       +
                   A module system type representing a value of the format

     

       152
       +
                 </para>

     

       153
       +
               </listitem>

     

       154
       +
             </varlistentry>

     

       155
       +
             <varlistentry>

     

       156
       +
               <term>

     

       157
       +
                 <literal>generate</literal>

     

       158
       +
                 <emphasis><literal>filename jsonValue</literal></emphasis>

     

       159
       +
               </term>

     

       160
       +
               <listitem>

     

       161
       +
                 <para>

     

       162
       +
                   A function that can render a value of the format to a file.

     

       163
       +
                   Returns a file path.

     

       164
       +
                 </para>

     

       165
       +
                 <note>

     

       166
       +
                   <para>

     

       167
       +
                     This function puts the value contents in the Nix store. So

     

       168
       +
                     this should be avoided for secrets.

     

       169
       +
                   </para>

     

       170
       +
                 </note>

     

       171
       +
               </listitem>

     

       172
       +
             </varlistentry>

     

       173
       +
           </variablelist>

     

       174
       +
           <anchor xml:id="ex-settings-nix-representable" />

     

       175
       +
           <para>

     

       176
       +
             <emphasis role="strong">Example: Module with conventional

     

       177
       +
             <literal>settings</literal> option</emphasis>

     

       178
       +
           </para>

     

       179
       +
           <para>

     

       180
       +
             The following shows a module for an example program that uses a

     

       181
       +
             JSON configuration file. It demonstrates how above values can be

     

       182
       +
             used, along with some other related best practices. See the

     

       183
       +
             comments for explanations.

     

       184
       +
           </para>

     

       185
       +
           <programlisting language="bash">

     

       186
       +
       { options, config, lib, pkgs, ... }:

     

       187
       +
       let

     

       188
       +
         cfg = config.services.foo;

     

       189
       +
         # Define the settings format used for this program

     

       190
       +
         settingsFormat = pkgs.formats.json {};

     

       191
       +
       in {

     

       192
       +
       

     

       193
       +
         options.services.foo = {

     

       194
       +
           enable = lib.mkEnableOption &quot;foo service&quot;;

     

       195
       +
       

     

       196
       +
           settings = lib.mkOption {

     

       197
       +
             # Setting this type allows for correct merging behavior

     

       198
       +
             type = settingsFormat.type;

     

       199
       +
             default = {};

     

       200
       +
             description = ''

     

       201
       +
               Configuration for foo, see

     

       202
       +
               &lt;link xlink:href=&quot;https://example.com/docs/foo&quot;/&gt;

     

       203
       +
               for supported settings.

     

       204
       +
             '';

     

       205
       +
           };

     

       206
       +
         };

     

       207
       +
       

     

       208
       +
         config = lib.mkIf cfg.enable {

     

       209
       +
           # We can assign some default settings here to make the service work by just

     

       210
       +
           # enabling it. We use `mkDefault` for values that can be changed without

     

       211
       +
           # problems

     

       212
       +
           services.foo.settings = {

     

       213
       +
             # Fails at runtime without any value set

     

       214
       +
             log_level = lib.mkDefault &quot;WARN&quot;;

     

       215
       +
       

     

       216
       +
             # We assume systemd's `StateDirectory` is used, so we require this value,

     

       217
       +
             # therefore no mkDefault

     

       218
       +
             data_path = &quot;/var/lib/foo&quot;;

     

       219
       +
       

     

       220
       +
             # Since we use this to create a user we need to know the default value at

     

       221
       +
             # eval time

     

       222
       +
             user = lib.mkDefault &quot;foo&quot;;

     

       223
       +
           };

     

       224
       +
       

     

       225
       +
           environment.etc.&quot;foo.json&quot;.source =

     

       226
       +
             # The formats generator function takes a filename and the Nix value

     

       227
       +
             # representing the format value and produces a filepath with that value

     

       228
       +
             # rendered in the format

     

       229
       +
             settingsFormat.generate &quot;foo-config.json&quot; cfg.settings;

     

       230
       +
       

     

       231
       +
           # We know that the `user` attribute exists because we set a default value

     

       232
       +
           # for it above, allowing us to use it without worries here

     

       233
       +
           users.users.${cfg.settings.user} = { isSystemUser = true; };

     

       234
       +
       

     

       235
       +
           # ...

     

       236
       +
         };

     

       237
       +
       }

     

       238
       +
       </programlisting>

     

       239
       +
           <section xml:id="sec-settings-attrs-options">

     

       240
       +
             <title>Option declarations for attributes</title>

     

       241
       +
             <para>

     

       242
       +
               Some <literal>settings</literal> attributes may deserve some

     

       243
       +
               extra care. They may need a different type, default or merging

     

       244
       +
               behavior, or they are essential options that should show their

     

       245
       +
               documentation in the manual. This can be done using

     

       246
       +
               <xref linkend="sec-freeform-modules" />.

     

       247
       +
             </para>

     

       248
       +
             <para>

     

       249
       +
               We extend above example using freeform modules to declare an

     

       250
       +
               option for the port, which will enforce it to be a valid integer

     

       251
       +
               and make it show up in the manual.

     

       252
       +
             </para>

     

       253
       +
             <anchor xml:id="ex-settings-typed-attrs" />

     

       254
       +
             <para>

     

       255
       +
               <emphasis role="strong">Example: Declaring a type-checked

     

       256
       +
               <literal>settings</literal> attribute</emphasis>

     

       257
       +
             </para>

     

       258
       +
             <programlisting language="bash">

     

       259
       +
       settings = lib.mkOption {

     

       260
       +
         type = lib.types.submodule {

     

       261
       +
       

     

       262
       +
           freeformType = settingsFormat.type;

     

       263
       +
       

     

       264
       +
           # Declare an option for the port such that the type is checked and this option

     

       265
       +
           # is shown in the manual.

     

       266
       +
           options.port = lib.mkOption {

     

       267
       +
             type = lib.types.port;

     

       268
       +
             default = 8080;

     

       269
       +
             description = ''

     

       270
       +
               Which port this service should listen on.

     

       271
       +
             '';

     

       272
       +
           };

     

       273
       +
       

     

       274
       +
         };

     

       275
       +
         default = {};

     

       276
       +
         description = ''

     

       277
       +
           Configuration for Foo, see

     

       278
       +
           &lt;link xlink:href=&quot;https://example.com/docs/foo&quot;/&gt;

     

       279
       +
           for supported values.

     

       280
       +
         '';

     

       281
       +
       };

     

       282
       +
       </programlisting>

     

       283
       +
           </section>

     

       284
       +
         </section>

     

       285
       +
       </section>