Options for Program Settings

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, ...)

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.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.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.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.

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-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} ::: {.title} 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

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} ::: {.title} 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.
  '';
};

:::

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Options for Program Settings

Nix-representable Formats (JSON, YAML, TOML, INI, ...)

Option declarations for attributes

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