mirror of
https://github.com/SebastianWendel/nixpkgs.git
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286 lines
10 KiB
XML
286 lines
10 KiB
XML
<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" 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
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settings can be declared. File formats can be separated into two
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categories:
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</para>
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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
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subset of Nix syntax. E.g. JSON is an example, since its values
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like <literal>{"foo":{"bar":10}}</literal>
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can be mapped directly to Nix:
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<literal>{ foo = { bar = 10; }; }</literal>. Other examples are
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INI, YAML and TOML. The following section explains the
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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
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subset of Nix syntax. Most generic programming languages are in
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this group, e.g. bash, since the statement
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<literal>if true; then echo hi; fi</literal> doesn't have a
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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
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common to have a <literal>configFile</literal> option for
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setting the configuration file path directly. The default value
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of <literal>configFile</literal> can be an auto-generated file,
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with convenient options for controlling the contents. For
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example an option of type <literal>attrsOf str</literal> can be
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used for representing environment variables which generates a
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section like <literal>export FOO="foo"</literal>.
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Often it can also be useful to also include an
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<literal>extraConfig</literal> option of type
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<literal>lines</literal> to allow arbitrary text after the
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autogenerated part of the file.
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</para>
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</listitem>
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</itemizedlist>
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<section xml:id="sec-settings-nix-representable">
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<title>Nix-representable Formats (JSON, YAML, TOML, INI,
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...)</title>
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<para>
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By convention, formats like this are handled with a generic
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<literal>settings</literal> option, representing the full program
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configuration as a Nix value. The type of this option should
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represent the format. The most common formats have a predefined
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type and string generator already declared under
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<literal>pkgs.formats</literal>:
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</para>
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<variablelist>
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<varlistentry>
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<term>
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<literal>pkgs.formats.json</literal> { }
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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
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extensibility) and returning a set with JSON-specific
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attributes <literal>type</literal> and
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<literal>generate</literal> as specified
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<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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<literal>pkgs.formats.yaml</literal> { }
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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
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extensibility) and returning a set with YAML-specific
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attributes <literal>type</literal> and
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<literal>generate</literal> as specified
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<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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<literal>pkgs.formats.ini</literal> {
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<emphasis><literal>listsAsDuplicateKeys</literal></emphasis> ?
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false, <emphasis><literal>listToValue</literal></emphasis> ?
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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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</para>
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<variablelist>
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<varlistentry>
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<term>
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<literal>listsAsDuplicateKeys</literal>
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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
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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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<literal>listToValue</literal>
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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
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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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<para>
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It returns a set with INI-specific attributes
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<literal>type</literal> and <literal>generate</literal> as
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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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<literal>pkgs.formats.toml</literal> { }
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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
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extensibility) and returning a set with TOML-specific
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attributes <literal>type</literal> and
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<literal>generate</literal> as specified
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<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 xml:id="pkgs-formats-result">
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These functions all return an attribute set with these values:
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</para>
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<variablelist>
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<varlistentry>
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<term>
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<literal>type</literal>
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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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<literal>generate</literal>
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<emphasis><literal>filename jsonValue</literal></emphasis>
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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.
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Returns a file path.
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</para>
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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
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this should be avoided for secrets.
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</para>
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</note>
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</listitem>
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</varlistentry>
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</variablelist>
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<anchor xml:id="ex-settings-nix-representable" />
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<para>
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<emphasis role="strong">Example: Module with conventional
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<literal>settings</literal> option</emphasis>
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</para>
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<para>
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The following shows a module for an example program that uses a
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JSON configuration file. It demonstrates how above values can be
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used, along with some other related best practices. See the
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comments for explanations.
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</para>
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<programlisting language="bash">
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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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</programlisting>
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<section xml:id="sec-settings-attrs-options">
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<title>Option declarations for attributes</title>
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<para>
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Some <literal>settings</literal> attributes may deserve some
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extra care. They may need a different type, default or merging
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behavior, or they are essential options that should show their
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documentation in the manual. This can be done using
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<xref linkend="sec-freeform-modules" />.
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</para>
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<para>
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We extend above example using freeform modules to declare an
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option for the port, which will enforce it to be a valid integer
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and make it show up in the manual.
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</para>
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<anchor xml:id="ex-settings-typed-attrs" />
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<para>
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<emphasis role="strong">Example: Declaring a type-checked
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<literal>settings</literal> attribute</emphasis>
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</para>
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<programlisting language="bash">
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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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</programlisting>
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</section>
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</section>
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</section>
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