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+# 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. This function is typically only used once per application (e.g. once in NixOS, once in Home Manager, ...).
+
+### Parameters {#module-system-lib-evalModules-parameters}
+
+#### `modules` {#module-system-lib-evalModules-param-modules}
+
+A list of modules. These are merged together to form the final configuration.
+<!-- TODO link to section about merging, TBD -->
+
+#### `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 after all `imports` have been resolved.
+
+#### `class` {#module-system-lib-evalModules-param-class}
+
+If the `class` attribute is set and non-`null`, the module system will reject `imports` with a different `_class` declaration.
+
+The `class` value should be a string in lower [camel case](https://en.wikipedia.org/wiki/Camel_case).
+
+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:
+
+ - `nixos` as in `flake.nixosModules`
+ - `nixosTest`: modules that constitute a [NixOS VM test](https://nixos.org/manual/nixos/stable/index.html#sec-nixos-tests)
+<!-- We've only just started with `class`. You're invited to add a few more. -->
+
+#### `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}
+
+The result is an attribute set with the following attributes:
+
+#### `options` {#module-system-lib-evalModules-return-value-options}
+
+The nested attribute set of all option declarations.
+
+#### `config` {#module-system-lib-evalModules-return-value-config}
+
+The nested attribute set of all option values.
+
+#### `type` {#module-system-lib-evalModules-return-value-type}
+
+A module system type. This type is an instance of `types.submoduleWith` containing the current [`modules`](#module-system-lib-evalModules-param-modules).
+
+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).
+
+However, the value returned from the type is just the [`config`](#module-system-lib-evalModules-return-value-config), like any submodule.
+
+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.
+
+This type is also available to the [`modules`](#module-system-lib-evalModules-param-modules) as the module argument `moduleType`.
+<!-- TODO: document the module arguments. Using moduleType is like saying: suppose this configuration was extended. -->
+
+#### `extendModules` {#module-system-lib-evalModules-return-value-extendModules}
+
+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.
+
+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.
+
+This functionality is also available to modules as the `extendModules` module argument.
+
+::: {.note}
+
+**Evaluation Performance**
+
+`extendModules` returns a configuration that shares very little with the original `evalModules` invocation, because the module arguments may be different.
+
+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`.
+
+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.
+
+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.
+:::
+
+#### `_module` {#module-system-lib-evalModules-return-value-_module}
+
+A portion of the configuration tree which is elided from `config`.
+
+<!-- TODO: when markdown migration is complete, make _module docs visible again and reference _module docs. Maybe move those docs into this chapter? -->
+
+#### `_type` {#module-system-lib-evalModules-return-value-_type}
+
+A nominal type marker, always `"configuration"`.
+
+#### `class` {#module-system-lib-evalModules-return-value-_configurationClass}
+
+The [`class` argument](#module-system-lib-evalModules-param-class).