nixpkgs.nix

{ ... }@args:

let
  mapConfig = defaults: attrs: builtins.mapAttrs (getDefault defaults) (builtins.mapAttrs (n: builtins.getEnv) attrs);
  getDefault = defaults: n: v: if v != "" then v else defaults."${n}" or "";
  traceInputs = message: x: if builtins.getEnv "NIX_TRACE_INPUTS" == "0" then x else builtins.trace message x;

  defaults = {
    url = "ssh://git@gitlab.example.org/upstream/nixpkgs.git";
    ref = "foo";
  };

  cfg = mapConfig defaults {
    url = "FOO_NIXPKGS_URL";
    ref = "FOO_NIXPKGS_BRANCH";
    rev = "FOO_NIXPKGS_REVISION";
  };

  nixpkgs = builtins.fetchGit cfg;
in

traceInputs ''
fetched dynamic inputs

Use these variables to reproduce the current build.

    FOO_NIXPKGS_URL=${cfg.url}
    FOO_NIXPKGS_BRANCH=${cfg.ref}
    FOO_NIXPKGS_REVISION=${nixpkgs.rev}
''

import nixpkgs args