pyrox.dev
1# Options for Program Settings {#sec-settings-options}
2
3Many programs have configuration files where program-specific settings
4can be declared. File formats can be separated into two categories:
5
6- Nix-representable ones: These can trivially be mapped to a subset of
7 Nix syntax. E.g. JSON is an example, since its values like
8 `{"foo":{"bar":10}}` can be mapped directly to Nix:
9 `{ foo = { bar = 10; }; }`. Other examples are INI, YAML and TOML.
10 The following section explains the convention for these settings.
11
12- Non-nix-representable ones: These can't be trivially mapped to a
13 subset of Nix syntax. Most generic programming languages are in this
14 group, e.g. bash, since the statement `if true; then echo hi; fi`
15 doesn't have a trivial representation in Nix.
16
17 Currently there are no fixed conventions for these, but it is common
18 to have a `configFile` option for setting the configuration file
19 path directly. The default value of `configFile` can be an
20 auto-generated file, with convenient options for controlling the
21 contents. For example an option of type `attrsOf str` can be used
22 for representing environment variables which generates a section
23 like `export FOO="foo"`. Often it can also be useful to also include
24 an `extraConfig` option of type `lines` to allow arbitrary text
25 after the autogenerated part of the file.
26
27## Nix-representable Formats (JSON, YAML, TOML, INI, ...) {#sec-settings-nix-representable}
28
29By convention, formats like this are handled with a generic `settings`
30option, representing the full program configuration as a Nix value. The
31type of this option should represent the format. The most common formats
32have a predefined type and string generator already declared under
33`pkgs.formats`:
34
35`pkgs.formats.javaProperties` { *`comment`* ? `"Generated with Nix"` }
36
37: A function taking an attribute set with values
38
39 `comment`
40
41 : A string to put at the start of the
42 file in a comment. It can have multiple
43 lines.
44
45 It returns the `type`: `attrsOf str` and a function
46 `generate` to build a Java `.properties` file, taking
47 care of the correct escaping, etc.
48
49`pkgs.formats.json` { }
50
51: A function taking an empty attribute set (for future extensibility)
52 and returning a set with JSON-specific attributes `type` and
53 `generate` as specified [below](#pkgs-formats-result).
54
55`pkgs.formats.yaml` { }
56
57: A function taking an empty attribute set (for future extensibility)
58 and returning a set with YAML-specific attributes `type` and
59 `generate` as specified [below](#pkgs-formats-result).
60
61`pkgs.formats.ini` { *`listsAsDuplicateKeys`* ? false, *`listToValue`* ? null, \... }
62
63: A function taking an attribute set with values
64
65 `listsAsDuplicateKeys`
66
67 : A boolean for controlling whether list values can be used to
68 represent duplicate INI keys
69
70 `listToValue`
71
72 : A function for turning a list of values into a single value.
73
74 It returns a set with INI-specific attributes `type` and `generate`
75 as specified [below](#pkgs-formats-result).
76
77`pkgs.formats.toml` { }
78
79: A function taking an empty attribute set (for future extensibility)
80 and returning a set with TOML-specific attributes `type` and
81 `generate` as specified [below](#pkgs-formats-result).
82
83`pkgs.formats.elixirConf { elixir ? pkgs.elixir }`
84
85: A function taking an attribute set with values
86
87 `elixir`
88
89 : The Elixir package which will be used to format the generated output
90
91 It returns a set with Elixir-Config-specific attributes `type`, `lib`, and
92 `generate` as specified [below](#pkgs-formats-result).
93
94 The `lib` attribute contains functions to be used in settings, for
95 generating special Elixir values:
96
97 `mkRaw elixirCode`
98
99 : Outputs the given string as raw Elixir code
100
101 `mkGetEnv { envVariable, fallback ? null }`
102
103 : Makes the configuration fetch an environment variable at runtime
104
105 `mkAtom atom`
106
107 : Outputs the given string as an Elixir atom, instead of the default
108 Elixir binary string. Note: lowercase atoms still needs to be prefixed
109 with `:`
110
111 `mkTuple array`
112
113 : Outputs the given array as an Elixir tuple, instead of the default
114 Elixir list
115
116 `mkMap attrset`
117
118 : Outputs the given attribute set as an Elixir map, instead of the
119 default Elixir keyword list
120
121
122[]{#pkgs-formats-result}
123These functions all return an attribute set with these values:
124
125`type`
126
127: A module system type representing a value of the format
128
129`lib`
130
131: Utility functions for convenience, or special interactions with the format.
132 This attribute is optional. It may contain inside a `types` attribute
133 containing types specific to this format.
134
135`generate` *`filename jsonValue`*
136
137: A function that can render a value of the format to a file. Returns
138 a file path.
139
140 ::: {.note}
141 This function puts the value contents in the Nix store. So this
142 should be avoided for secrets.
143 :::
144
145::: {#ex-settings-nix-representable .example}
146### Module with conventional `settings` option
147
148The following shows a module for an example program that uses a JSON
149configuration file. It demonstrates how above values can be used, along
150with some other related best practices. See the comments for
151explanations.
152
153```nix
154{ options, config, lib, pkgs, ... }:
155let
156 cfg = config.services.foo;
157 # Define the settings format used for this program
158 settingsFormat = pkgs.formats.json {};
159in {
160
161 options.services.foo = {
162 enable = lib.mkEnableOption "foo service";
163
164 settings = lib.mkOption {
165 # Setting this type allows for correct merging behavior
166 type = settingsFormat.type;
167 default = {};
168 description = ''
169 Configuration for foo, see
170 <link xlink:href="https://example.com/docs/foo"/>
171 for supported settings.
172 '';
173 };
174 };
175
176 config = lib.mkIf cfg.enable {
177 # We can assign some default settings here to make the service work by just
178 # enabling it. We use `mkDefault` for values that can be changed without
179 # problems
180 services.foo.settings = {
181 # Fails at runtime without any value set
182 log_level = lib.mkDefault "WARN";
183
184 # We assume systemd's `StateDirectory` is used, so we require this value,
185 # therefore no mkDefault
186 data_path = "/var/lib/foo";
187
188 # Since we use this to create a user we need to know the default value at
189 # eval time
190 user = lib.mkDefault "foo";
191 };
192
193 environment.etc."foo.json".source =
194 # The formats generator function takes a filename and the Nix value
195 # representing the format value and produces a filepath with that value
196 # rendered in the format
197 settingsFormat.generate "foo-config.json" cfg.settings;
198
199 # We know that the `user` attribute exists because we set a default value
200 # for it above, allowing us to use it without worries here
201 users.users.${cfg.settings.user} = { isSystemUser = true; };
202
203 # ...
204 };
205}
206```
207:::
208
209### Option declarations for attributes {#sec-settings-attrs-options}
210
211Some `settings` attributes may deserve some extra care. They may need a
212different type, default or merging behavior, or they are essential
213options that should show their documentation in the manual. This can be
214done using [](#sec-freeform-modules).
215
216We extend above example using freeform modules to declare an option for
217the port, which will enforce it to be a valid integer and make it show
218up in the manual.
219
220::: {#ex-settings-typed-attrs .example}
221### Declaring a type-checked `settings` attribute
222```nix
223settings = lib.mkOption {
224 type = lib.types.submodule {
225
226 freeformType = settingsFormat.type;
227
228 # Declare an option for the port such that the type is checked and this option
229 # is shown in the manual.
230 options.port = lib.mkOption {
231 type = lib.types.port;
232 default = 8080;
233 description = ''
234 Which port this service should listen on.
235 '';
236 };
237
238 };
239 default = {};
240 description = ''
241 Configuration for Foo, see
242 <link xlink:href="https://example.com/docs/foo"/>
243 for supported values.
244 '';
245};
246```
247:::