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

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

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

  • 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 if true; then echo hi; fi doesn't have a trivial representation in Nix.

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

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

By convention, formats like this are handled with a generic settings 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 pkgs.formats:

pkgs.formats.javaProperties { comment ? "Generated with Nix" }

: A function taking an attribute set with values

`comment`

:   A string to put at the start of the
    file in a comment. It can have multiple
    lines.

It returns the `type`: `attrsOf str` and a function
`generate` to build a Java `.properties` file, taking
care of the correct escaping, etc.

pkgs.formats.hocon { generator ? <derivation>, validator ? <derivation>, doCheck ? true }

: A function taking an attribute set with values

`generator`

:   A derivation used for converting the JSON output
    from the nix settings into HOCON. This might be
    useful if your HOCON variant is slightly different
    from the java-based one, or for testing purposes.

`validator`

:   A derivation used for verifying that the HOCON
    output is correct and parsable. This might be
    useful if your HOCON variant is slightly different
    from the java-based one, or for testing purposes.

`doCheck`

:   Whether to enable/disable the validator check.

It returns an attrset with a `type`, `generate` function,
and a `lib` attset, as specified [below](#pkgs-formats-result).
Some of the lib functions will be best understood if you have
read the reference specification. You can find this
specification here:

<https://github.com/lightbend/config/blob/main/HOCON.md>

Inside of `lib`, you will find these functions

`mkInclude`

:   This is used together with a specially named
    attribute `includes`, to include other HOCON
    sources into the document.

    The function has a shorthand variant where it
    is up to the HOCON parser to figure out what type
    of include is being used. The include will default
    to being non-required. If you want to be more
    explicit about the details of the include, you can
    provide an attrset with following arguments

    `required`

    :   Whether the parser should fail upon failure
        to include the document

    `type`

    :   Type of the source of the included document.
        Valid values are `file`, `url` and `classpath`.
        See upstream documentation for the semantics
        behind each value

    `value`

    :   The URI/path/classpath pointing to the source of
        the document to be included.

    `Example usage:`

    ```nix
      let
        format = pkgs.formats.hocon { };
        hocon_file = pkgs.writeText "to_include.hocon" ''
          a = 1;
        '';
      in {
        some.nested.hocon.attrset = {
          _includes = [
            (format.lib.mkInclude hocon_file)
            (format.lib.mkInclude "https://example.com/to_include.hocon")
            (format.lib.mkInclude {
              required = true;
              type = "file";
              value = include_file;
            })
          ];
          ...
        };
      }
    ```

`mkAppend`

:   This is used to invoke the `+=` operator.
    This can be useful if you need to add something
    to a list that is included from outside of nix.
    See upstream documentation for the semantics
    behind the `+=` operation.

    `Example usage:`

    ```nix
      let
        format = pkgs.formats.hocon { };
        hocon_file = pkgs.writeText "to_include.hocon" ''
          a = [ 1 ];
          b = [ 2 ];
        '';
      in {
        _includes = [
          (format.lib.mkInclude hocon_file)
        ];

        c = 3;
        a = format.lib.mkAppend 3;
        b = format.lib.mkAppend (format.lib.mkSubstitution "c");
      }
    ```

`mkSubstitution`

:   This is used to make HOCON substitutions.
    Similarly to `mkInclude`, this function has
    a shorthand variant where you just give it
    the string with the substitution value.
    The substitution is not optional by default.
    Alternatively, you can provide an attrset
    with more options

    `optional`

    :   Whether the parser should fail upon
        failure to fetch the substitution value.

    `value`

    :   The name of the variable to use for
        substitution.

    See upstream documentation for semantics
    behind the substitution functionality.

    `Example usage:`

    ```nix
      let
        format = pkgs.formats.hocon { };
      in {
        a = 1;
        b = format.lib.mkSubstitution "a";
        c = format.lib.mkSubstitution "SOME_ENVVAR";
        d = format.lib.mkSubstitution {
          value = "SOME_OPTIONAL_ENVVAR";
          optional = true;
        };
      }
    ```

`Implementation notes:`

- classpath includes are not implemented in pyhocon,
  which is used for validating the HOCON output. This
  means that if you are using classpath includes,
  you will want to either use an alternative validator
  or set `doCheck = false` in the format options.

pkgs.formats.libconfig { generator ? <derivation>, validator ? <derivation> }

: A function taking an attribute set with values

`generator`

:   A derivation used for converting the JSON output
    from the nix settings into libconfig. This might be
    useful if your libconfig variant is slightly different
    from the original one, or for testing purposes.

`validator`

:   A derivation used for verifying that the libconfig
    output is correct and parsable. This might be
    useful if your libconfig variant is slightly different
    from the original one, or for testing purposes.

It returns an attrset with a `type`, `generate` function,
and a `lib` attset, as specified [below](#pkgs-formats-result).
Some of the lib functions will be best understood if you have
read the reference specification. You can find this
specification here:

<https://hyperrealm.github.io/libconfig/libconfig_manual.html#Configuration-Files>

Inside of `lib`, you will find these functions

`mkHex`, `mkOctal`, `mkFloat`

:   Use these to specify numbers in other formats.

    `Example usage:`

    ```nix
      let
        format = pkgs.formats.libconfig { };
      in {
        myHexValue = format.lib.mkHex "0x1FC3";
        myOctalValue = format.lib.mkOctal "0027";
        myFloatValue = format.lib.mkFloat "1.2E-3";
      }
    ```

`mkArray`, `mkList`

:   Use these to differentiate between whether
    a nix list should be considered as a libconfig
    array or a libconfig list. See the upstream
    documentation for the semantics behind these types.

    `Example usage:`

    ```nix
      let
        format = pkgs.formats.libconfig { };
      in {
        myList = format.lib.mkList [ "foo" 1 true ];
        myArray = format.lib.mkArray [ 1 2 3 ];
      }
    ```

`Implementation notes:`

- Since libconfig does not allow setting names to start with an underscore,
  this is used as a prefix for both special types and include directives.

- The difference between 32bit and 64bit values became optional in libconfig
  1.5, so we assume 64bit values for all numbers.

pkgs.formats.json { }

: A function taking an empty attribute set (for future extensibility) and returning a set with JSON-specific attributes type and generate as specified below.

pkgs.formats.yaml { }

: A function taking an empty attribute set (for future extensibility) and returning a set with YAML-specific attributes type and generate as specified below.

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

: A function taking an attribute set with values

`listsAsDuplicateKeys`

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

`listToValue`

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

It returns a set with INI-specific attributes `type` and `generate`
as specified [below](#pkgs-formats-result).
The type of the input is an *attrset* of sections; key-value pairs where
the key is the section name and the value is the corresponding content
which is also an *attrset* of key-value pairs for the actual key-value
mappings of the INI format.
The values of the INI atoms are subject to the above parameters (e.g. lists
may be transformed into multiple key-value pairs depending on
`listToValue`).

The attribute `lib.type.atom` contains the used INI atom.

pkgs.formats.iniWithGlobalSection { listsAsDuplicateKeys ? false, listToValue ? null, ... }

: A function taking an attribute set with values

`listsAsDuplicateKeys`

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

`listToValue`

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

It returns a set with INI-specific attributes `type` and `generate`
as specified [below](#pkgs-formats-result).
The type of the input is an *attrset* of the structure
`{ sections = {}; globalSection = {}; }` where *sections* are several
sections as with *pkgs.formats.ini* and *globalSection* being just a single
attrset of key-value pairs for a single section, the global section which
precedes the section definitions.

The attribute `lib.type.atom` contains the used INI atom.

pkgs.formats.toml { }

: A function taking an empty attribute set (for future extensibility) and returning a set with TOML-specific attributes type and generate as specified below.

pkgs.formats.xml { format ? "badgerfish", withHeader ? true}

: A function taking an attribute set with values and returning a set with XML-specific attributes type and generate as specified below.

`format`

:   Input format. Because XML can not be translated one-to-one, we have to use intermediate formats. Possible values:
  - `"badgerfish"`: Uses [badgerfish](http://www.sklar.com/badgerfish/) conversion.

`withHeader`

:   Outputs the xml with header.

pkgs.formats.cdn { }

: A function taking an empty attribute set (for future extensibility) and returning a set with CDN-specific attributes type and generate as specified below.

pkgs.formats.elixirConf { elixir ? pkgs.elixir }

: A function taking an attribute set with values

`elixir`

:   The Elixir package which will be used to format the generated output

It returns a set with Elixir-Config-specific attributes `type`, `lib`, and
`generate` as specified [below](#pkgs-formats-result).

The `lib` attribute contains functions to be used in settings, for
generating special Elixir values:

`mkRaw elixirCode`

:   Outputs the given string as raw Elixir code

`mkGetEnv { envVariable, fallback ? null }`

:   Makes the configuration fetch an environment variable at runtime

`mkAtom atom`

:   Outputs the given string as an Elixir atom, instead of the default
    Elixir binary string. Note: lowercase atoms still needs to be prefixed
    with `:`

`mkTuple array`

:   Outputs the given array as an Elixir tuple, instead of the default
    Elixir list

`mkMap attrset`

:   Outputs the given attribute set as an Elixir map, instead of the
    default Elixir keyword list

pkgs.formats.lua { asBindings ? false, multiline ? true, columnWidth ? 100, indentWidth ? 2, indentUsingTabs ? false }

: A function taking an attribute set with values

`asBindings` (default `false`)

:   Whether to treat attributes as variable bindings

`multiline` (default `true`)

:   Whether to produce a multiline output. The output may still wrap across
    multiple lines if it would otherwise exceed `columnWidth`.

`columnWidth` (default `100`)

:   The column width to use to attempt to wrap lines.

`indentWidth` (default `2`)

:   The width of a single indentation level.

`indentUsingTabs` (default `false`)

:   Whether the indentation should use tabs instead of spaces.

pkgs.formats.php { finalVariable } []{#pkgs-formats-php}

: A function taking an attribute set with values

`finalVariable`

:   The variable that will store generated expression (usually `config`). If set to `null`, generated expression will contain `return`.

It returns a set with PHP-Config-specific attributes `type`, `lib`, and
`generate` as specified [below](#pkgs-formats-result).

The `lib` attribute contains functions to be used in settings, for
generating special PHP values:

`mkRaw phpCode`

:   Outputs the given string as raw PHP code

`mkMixedArray list set`

:   Creates PHP array that contains both indexed and associative values. For example, `lib.mkMixedArray [ "hello" "world" ] { "nix" = "is-great"; }` returns `['hello', 'world', 'nix' => 'is-great']`

[]{#pkgs-formats-result} These functions all return an attribute set with these values:

type

: A module system type representing a value of the format

lib

: Utility functions for convenience, or special interactions with the format. This attribute is optional. It may contain inside a types attribute containing types specific to this format.

generate filename jsonValue

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

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

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

Module with conventional settings option#

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.

{ options, config, lib, pkgs, ... }:
let
  cfg = config.services.foo;
  # Define the settings format used for this program
  settingsFormat = pkgs.formats.json {};
in {

  options.services.foo = {
    enable = lib.mkEnableOption "foo service";

    settings = lib.mkOption {
      # Setting this type allows for correct merging behavior
      type = settingsFormat.type;
      default = {};
      description = ''
        Configuration for foo, see
        <link xlink:href="https://example.com/docs/foo"/>
        for supported settings.
      '';
    };
  };

  config = lib.mkIf cfg.enable {
    # We can assign some default settings here to make the service work by just
    # enabling it. We use `mkDefault` for values that can be changed without
    # problems
    services.foo.settings = {
      # Fails at runtime without any value set
      log_level = lib.mkDefault "WARN";

      # We assume systemd's `StateDirectory` is used, so we require this value,
      # therefore no mkDefault
      data_path = "/var/lib/foo";

      # Since we use this to create a user we need to know the default value at
      # eval time
      user = lib.mkDefault "foo";
    };

    environment.etc."foo.json".source =
      # The formats generator function takes a filename and the Nix value
      # representing the format value and produces a filepath with that value
      # rendered in the format
      settingsFormat.generate "foo-config.json" cfg.settings;

    # We know that the `user` attribute exists because we set a default value
    # for it above, allowing us to use it without worries here
    users.users.${cfg.settings.user} = { isSystemUser = true; };

    # ...
  };
}

:::

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

Some settings 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 .

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.

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

Declaring a type-checked settings attribute#

{
  settings = lib.mkOption {
    type = lib.types.submodule {

      freeformType = settingsFormat.type;

      # Declare an option for the port such that the type is checked and this option
      # is shown in the manual.
      options.port = lib.mkOption {
        type = lib.types.port;
        default = 8080;
        description = ''
          Which port this service should listen on.
        '';
      };

    };
    default = {};
    description = ''
      Configuration for Foo, see
      <link xlink:href="https://example.com/docs/foo"/>
      for supported values.
    '';
  };
}

:::