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: NMS). This is a confusing topic because they are closely related:

• NMS is implemented in, and configured using, Nix.
• Many of the same things are possible in both.
• You can 80% get away with using the Nix way of doing things when you should be using NMS, but that 20% will bite you.

Here is a cheatsheet, with explanations below:

Feature	In Nix	In NMS
Merging	attrs1 // attrs2	lib.mkMerge [ attrs1 attrs2 ]
If-then	if cond then values else { } lib.optional cond values lib.optionals cond valueList	lib.mkIf cond values
If-then-else	if cond then v1 else v2	lib.mkMerge [ (lib.mkIf cond v1) (lib.mkIf (!cond) v2) ]
Imports	import ./file.nix	imports = [ ./file.nix ];