configuration.nix

{ config, pkgs, ... }:

let
  # Import unstable channel.
  # sudo nix-channel --add http://nixos.org/channels/nixpkgs-unstable nixpkgs-unstable
  unstable = import <nixpkgs-unstable> {};
in

{
  environment.systemPackages =
    [ pkgs.hello  # regular channel
      unstable.spotify  # newer version from nixpkgs-unstable
    ];
}