unstable-overlay.nix

self: super:

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

{
  unstable = super.unstable or {} // {
    foo = unstable.hello.override { inherit (super) stdenv fetchurl; };
    bar = super.callPackage <nixpkgs-unstable/pkgs/applications/misc/hello> { };
  };
}