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 |
|---|---|---|
| Deep merging two attrsets | lib.recursiveUpdate attrs1 attrs2
|
lib.mkMerge [ attrs1 attrs2 ]
|
| If-then | if cond then values else { } |
lib.mkIf cond values
|
| Imports | import ./file.nix
|
imports = [ ./file.nix ]; |
| Recursive references | rec {
foo = 1;
bar = foo;
}
|
{ config, ... }:
{
foo = 1;
bar = config.foo;
}
|