1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
|
# Module System {#module-system}
## Introduction {#module-system-introduction}
The module system is a language for handling configuration, implemented as a Nix library.
Compared to plain Nix, it adds documentation, type checking and composition or extensibility.
NOTE: This chapter is new and not complete yet. For a gentle introduction to the module system, in the context of NixOS, see [Writing NixOS Modules](https://nixos.org/manual/nixos/unstable/index.html#sec-writing-modules) in the NixOS manual.
## `lib.evalModules` {#module-system-lib-evalModules}
Evaluate a set of modules. The result is a set with the attributes:
### Parameters {#module-system-lib-evalModules-parameters}
#### `modules` {#module-system-lib-evalModules-param-modules}
A list of modules. These are merged together using various methods to form the final configuration.
#### `specialArgs` {#module-system-lib-evalModules-param-specialArgs}
An attribute set of module arguments that can be used in `imports`.
This is in contrast to `config._module.args`, which is only available within the module fixpoint, which does not exist before all imports are resolved.
#### `specialArgs.class` {#module-system-lib-evalModules-param-specialArgs-class}
If the `class` attribute is set in `specialArgs`, the module system will rejected modules with a different `class`.
This improves the error message that users will encounter when they import an incompatible module that was designed for a different class of configurations.
The `class` value should be in camelcase, and, if applicable, it should match the prefix of the attributes used in (experimental) flakes. Some examples are:
- `nixos`: NixOS modules
- `nixosTest`: modules that constitute a [NixOS VM test](https://nixos.org/manual/nixos/stable/index.html#sec-nixos-tests)
#### `prefix` {#module-system-lib-evalModules-param-prefix}
A list of strings representing the location at or below which all options are evaluated. This is used by `types.submodule` to improve error reporting and find the implicit `name` module argument.
### Return value {#module-system-lib-evalModules-return-value}
#### `options` {#module-system-lib-evalModules-return-value-options}
The nested set of all option declarations.
#### `config` {#module-system-lib-evalModules-return-value-config}
The nested set of all option values.
#### `type` {#module-system-lib-evalModules-return-value-type}
A module system type representing the module set as a submodule, to be extended by configuration from the containing module set.
This is also available as the module argument `moduleType`.
#### `extendModules` {#module-system-lib-evalModules-return-value-extendModules}
A function similar to `evalModules` but building on top of the module set. Its arguments, `modules` and `specialArgs` are added to the existing values.
Using `extendModules` a few times has no performance impact as long as you only reference the final `options` and `config`.
If you do reference multiple `config` (or `options`) from before and after `extendModules`, performance is the same as with multiple `evalModules` invocations, because the new modules' ability to override existing configuration fundamentally requires a new fixpoint to be constructed.
This is also available as a module argument.
#### `_module` {#module-system-lib-evalModules-return-value-_module}
A portion of the configuration tree which is elided from `config`. It contains some values that are mostly internal to the module system implementation.
|
