test.nix

{ nixpkgs ? import <nixpkgs> {}}:
let
  lib = nixpkgs.lib;
  mkpkg = a: deps: { name = a; deps = deps;};
  testPkgs = {
    a = mkpkg "a" [ "b" "c" ];
    b = mkpkg "b" [ "a" ];
    c = mkpkg "c" [ "b" ];
  };

  buildPkgList = pkgs: list: lib.foldl' (l: i:
    l ++ (if builtins.hasAttr i pkgs
          then (let pkg = lib.getAttr i pkgs;
                in [ pkg ] ++ (buildPkgList pkgs (lib.subtractList list pkg.deps)))
          else [])
  ) [] (builtins.trace list) list;
in
{
  inherit testPkgs buildPkgList;
}