The Nix Language versus the NixOS Module System

If you are configuring NixOS, it behooves you to understand the difference between the Nix language (hereafter: Nix) and the NixOS module system (hereafter: module system). The module system is implemented and configured using Nix.

Generally the module system way of doing things is preferred as it properly merges values, doesn't set empty settings and allows the module system to verify the structure even if conditions are false.

Here is a cheat sheet, with explanations below:

Feature	In Nix	In the module system
Merging two attrsets	lib.recurisveUpdate attrs1 attrs2	lib.mkMerge [ attrs1 attrs2 ]
If-then	if cond then values else { }	lib.mkIf cond values
Imports	import ./file.nix	imports = [ ./file.nix ];