# 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-result).
`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-result).
`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-result).
`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](#pkgs-formats-result).
`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](https://github.com/dzikoysk/cdn)-specific
attributes `type` and `generate` as specified [below](#pkgs-formats-result).
`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.
```nix
{ 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 [](#sec-freeform-modules).
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
```nix
{
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.
'';
};
}
```
:::