nix-config/lib/default.nix

{ lib, ... }:
{
  # use path relative to the root of the project
  relativeToRoot = lib.path.append ../.;
  scanPaths = path:
    builtins.map
      (f: (path + "/${f}"))
      (builtins.attrNames
        (lib.attrsets.filterAttrs
          (
            path: _type:
              (_type == "directory") # include directories
              || (
                # FIXME this barfs when child directories don't contain a default.nix
                # example:
                # error: getting status of '/nix/store/mx31x8530b758ap48vbg20qzcakrbc8 (see hosts/common/core/services/default.nix)a-source/hosts/common/core/services/default.nix': No such file or directory
                # I created a blank default.nix in hosts/common/core/services to work around
                (path != "default.nix") # ignore default.nix
                && (lib.strings.hasSuffix ".nix" path) # include .nix files
              )
          )
          (builtins.readDir path)
        )
      );

  mapPathsToPrefix = { prefix, list }:
    # Filter... sth like builtins.filter (path: builtins.pathExists path)
    builtins.map (path: prefix + path) list;
}