The nixpkgs library provides a module system for Nix expressions. To learn it, see our tutorial: Introduction to module system.
NixOS makes use of the module system to provide various functionality including services and programs. See https://search.nixos.org/options for a list of all available options.
This module system is not natively supported in Flakes. However, flakes can define and use modules using flake-parts.
nixpkgs (https://github.com/NixOS/nixpkgs) is a monorepo containing a collection of Nix packages. It also includes various utility Nix functions (like writeShellApplication), as well as the NixOS Linux distribution (including the Module System).
flake-parts brings the NixOS module system to flakes, thus providing a cleaner and simpler way to write otherwise complex flakes.
By leveraging the module system, we can hide the implementation details and only expose the passetto service to the user, enabling its use as shown above.
Using the module system is a key stepping stone to writing maintainable and shareable Nix code. In this tutorial, we’ll write a configuration system for the simple lsd command, thus introducing the reader to the Nix module system, so that they benefit from features such as configuration type checking, option documentation, and modularity. To learn more about the module system, we recommend this video from Tweag as well the article “Module system deep dive” from nix.dev.
As a final step, let’s permanently enable Flakes on our system, which is particular useful if you do a lot of software development. This time, instead of editing configuration.nix again, let’s do it in a separate module (for no particular reasons other than pedagogic purposes). Remember the modules argument to nixosSystem function in our flake.nix? It is a list of modules, so we can add a second module there:
[!info] What is hardware-configuration.nix? Hardware specific configuration (eg: disk partitions to mount) are defined in /etc/nixos/hardware-configuration.nix which is imported, as a module, by configuration.nix.