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160 lines
8.4 KiB
Markdown
160 lines
8.4 KiB
Markdown
# Module System {#module-system}
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## Introduction {#module-system-introduction}
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The module system is a language for handling configuration, implemented as a Nix library.
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Compared to plain Nix, it adds documentation, type checking and composition or extensibility.
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::: {.note}
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This chapter is new and not complete yet.
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See also:
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- 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.
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- Generic guide to the module system on [nix.dev](https://nix.dev/tutorials/module-system/index.html).
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:::
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## `lib.evalModules` {#module-system-lib-evalModules}
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Evaluate a set of modules. This function is typically only used once per application (e.g. once in NixOS, once in Home Manager, ...).
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### Parameters {#module-system-lib-evalModules-parameters}
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#### `modules` {#module-system-lib-evalModules-param-modules}
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A list of modules. These are merged together to form the final configuration.
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<!-- TODO link to section about merging, TBD -->
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#### `specialArgs` {#module-system-lib-evalModules-param-specialArgs}
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An attribute set of module arguments that can be used in `imports`.
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This is in contrast to `config._module.args`, which is only available after all `imports` have been resolved.
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::: {.warning}
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You may be tempted to use `specialArgs.lib` to provide extra library functions. Doing so limits the interoperability of modules, as well as the interoperability of Module System applications.
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`lib` is reserved for the Nixpkgs library, and should not be used for custom functions.
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Instead, you may create a new attribute in `specialArgs` to provide custom functions.
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This clarifies their origin and avoids incompatibilities.
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:::
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#### `class` {#module-system-lib-evalModules-param-class}
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If the `class` attribute is set and non-`null`, the module system will reject `imports` with a different `_class` declaration.
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The `class` value should be a string in lower [camel case](https://en.wikipedia.org/wiki/Camel_case).
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If applicable, the `class` should match the "prefix" of the attributes used in (experimental) [flakes](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html#description). Some examples are:
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- `nixos` as in `flake.nixosModules`
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- `nixosTest`: modules that constitute a [NixOS VM test](https://nixos.org/manual/nixos/stable/index.html#sec-nixos-tests)
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<!-- We've only just started with `class`. You're invited to add a few more. -->
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#### `prefix` {#module-system-lib-evalModules-param-prefix}
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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.
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### Return value {#module-system-lib-evalModules-return-value}
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The result is an attribute set with the following attributes:
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#### `options` {#module-system-lib-evalModules-return-value-options}
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The nested attribute set of all option declarations.
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#### `config` {#module-system-lib-evalModules-return-value-config}
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The nested attribute set of all option values.
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#### `type` {#module-system-lib-evalModules-return-value-type}
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A module system type. This type is an instance of `types.submoduleWith` containing the current [`modules`](#module-system-lib-evalModules-param-modules).
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The option definitions that are typed with this type will extend the current set of modules, like [`extendModules`](#module-system-lib-evalModules-return-value-extendModules).
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However, the value returned from the type is just the [`config`](#module-system-lib-evalModules-return-value-config), like any submodule.
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If you're familiar with prototype inheritance, you can think of this `evalModules` invocation as the prototype, and usages of this type as the instances.
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This type is also available to the [`modules`](#module-system-lib-evalModules-param-modules) as the module argument `moduleType`.
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<!-- TODO: document the module arguments. Using moduleType is like saying: suppose this configuration was extended. -->
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#### `extendModules` {#module-system-lib-evalModules-return-value-extendModules}
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A function similar to `evalModules` but building on top of the already passed [`modules`](#module-system-lib-evalModules-param-modules). Its arguments, `modules` and `specialArgs` are added to the existing values.
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If you're familiar with prototype inheritance, you can think of the current, actual `evalModules` invocation as the prototype, and the return value of `extendModules` as the instance.
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This functionality is also available to modules as the `extendModules` module argument.
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::: {.note}
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**Evaluation Performance**
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`extendModules` returns a configuration that shares very little with the original `evalModules` invocation, because the module arguments may be different.
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So if you have a configuration that has been (or will be) largely evaluated, almost none of the computation is shared with the configuration returned by `extendModules`.
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The real work of module evaluation happens while computing the values in `config` and `options`, so multiple invocations of `extendModules` have a particularly small cost, as long as only the final `config` and `options` are evaluated.
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If you do reference multiple `config` (or `options`) from before and after `extendModules`, evaluation performance is the same as with multiple `evalModules` invocations, because the new modules' ability to override existing configuration fundamentally requires constructing a new `config` and `options` fixpoint.
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:::
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#### `_module` {#module-system-lib-evalModules-return-value-_module}
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A portion of the configuration tree which is elided from `config`.
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<!-- TODO: when markdown migration is complete, make _module docs visible again and reference _module docs. Maybe move those docs into this chapter? -->
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#### `_type` {#module-system-lib-evalModules-return-value-_type}
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A nominal type marker, always `"configuration"`.
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#### `class` {#module-system-lib-evalModules-return-value-_configurationClass}
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The [`class` argument](#module-system-lib-evalModules-param-class).
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## Module arguments {#module-system-module-arguments}
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Module arguments are the attribute values passed to modules when they are evaluated.
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They originate from these sources:
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1. Built-in arguments
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- `lib`,
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- `config`,
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- `options`,
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- `_class`,
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- `_prefix`,
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2. Attributes from the [`specialArgs`] argument passed to [`evalModules`] or `submoduleWith`. These are application-specific.
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3. Attributes from the `_module.args` option value. These are application-specific and can be provided by any module.
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The prior two categories are available while evaluating the `imports`, whereas
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the last category is only available after the `imports` have been resolved.
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[`lib`]{#module-system-module-argument-lib} [🔗](#module-system-module-argument-lib)
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: A reference to the Nixpkgs library.
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[`config`]{#module-system-module-argument-config} [🔗](#module-system-module-argument-config)
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: All option values.
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Unlike the `evalModules` [`config` return attribute](#module-system-lib-evalModules-return-value-config), this includes `_module`.
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[`options`]{#module-system-module-argument-options} [🔗](#module-system-module-argument-options)
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: All evaluated option declarations.
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[`_class`]{#module-system-module-argument-_class} [🔗](#module-system-module-argument-_class)
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: The [expected class](#module-system-lib-evalModules-param-class) of the loaded modules.
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[`_prefix`]{#module-system-module-argument-_prefix} [🔗](#module-system-module-argument-_prefix)
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: The location under which the module is evaluated.
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This is used to improve error reporting and to find the implicit `name` module argument in submodules.
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It is exposed as a module argument due to how the module system is implemented, which cannot be avoided without breaking compatibility.
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It is a good practice not to rely on `_prefix`. A module should not make assumptions about its location in the configuration tree.
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For example, the root of a NixOS configuration may have a non-empty prefix, for example when it is a specialisation, or when it is part of a larger, multi-host configuration such as a [NixOS test](https://nixos.org/manual/nixos/unstable/#sec-nixos-tests).
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Instead of depending on `_prefix` use explicit options, whose default definitions can be provided by the module that imports them.
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<!-- markdown link aliases -->
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[`evalModules`]: #module-system-lib-evalModules
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[`specialArgs`]: #module-system-lib-evalModules-param-specialArgs
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