let nixpkgs_unstable = import (builtins.fetchTarball {
  # FIXME: upgrade to nixpkgs 19 when stable and stick to stable
  # versions
  name = "nixos-unstable-2018-12-08";
  url = https://github.com/nixos/nixpkgs/archive/61c3169a0e17d789c566d5b241bfe309ce4a6275.tar.gz;
  sha256 = "0qbycg7wkb71v20rchlkafrjfpbk2fnlvvbh3ai9pyfisci5wxvq";
}) {};
in
  with nixpkgs_unstable;
{
  inherit nixpkgs_unstable;
  fetchedGithub = path:
    let
      json = lib.importJSON path;
    in rec {
      version = json.tag;
      name = "${json.meta.name}-${version}";
      src = fetchFromGitHub json.github;
    };

  fetchedGit = path:
    let
      json = lib.importJSON path;
    in rec {
      version = json.tag;
      name = "${json.meta.name}-${version}";
      src = fetchgit json.git;
    };
}