{ pkgs ? import <nixpkgs> {}}:
with pkgs;
let
  fractalsPkg = haskellPackages.callPackage ./fractals.nix { };
in
## This works:
# fractalsPkg.env
## This does not work:
  stdenv.mkDerivation {
    name = "fractals-env";
    buildInputs = [ fractalsPkg.env ];
  }