how to add NixOS unstable channel declaratively in configuration.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

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-channels/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
  ];