how to add NixOS unstable channel declaratively in configuration.nix - nix

Question

The NixOS cheatsheet describes how to install packages from unstable in configuration.nix.
It starts off by saying to add the unstable channel like so:
$ sudo nix-channel --add https://nixos.org/channels/nixpkgs-unstable
$ sudo nix-channel --update
Then, it is easy to use this channel in configuration.nix (since it should now be on NIX_PATH):
nixpkgs.config = {
allowUnfree = true;
packageOverrides = pkgs: {
unstable = import <nixos-unstable> {
config = config.nixpkgs.config;
};
};
};
environment = {
systemPackages = with pkgs; [
unstable.google-chrome
];
};
I would like to not have to do the manual nix-channel --add and nix-channel --update steps.
I would like to be able to install my system from configuration.nix without first having to run the nix-channel --add and nix-channel --update steps.
Is there a way to automate this from configuration.nix?

Answer 1

I was able to get this working with a suggestion by #EmmanuelRosa.
Here are the relevant parts of my /etc/nixos/configuration.nix:
{ config, pkgs, ... }:
let
unstableTarball =
fetchTarball
https://github.com/NixOS/nixpkgs/archive/nixos-unstable.tar.gz;
in
{
imports =
[ # Include the results of the hardware scan.
/etc/nixos/hardware-configuration.nix
];
nixpkgs.config = {
packageOverrides = pkgs: {
unstable = import unstableTarball {
config = config.nixpkgs.config;
};
};
};
...
};
This adds an unstable derivative that can be used in environment.systemPackages.
Here is an example of using it to install the htop package from nixos-unstable:
environment.systemPackages = with pkgs; [
...
unstable.htop
];