{ lib }:

let
  inherit (lib) hasSuffix;
  inherit (builtins) concatMap isPath filter readFileType;

  expandIfFolder = elem:
    if !isPath elem || readFileType elem != "directory"
      then [ elem ]
    else lib.filesystem.listFilesRecursive elem;

in
  list: filter
    # Filter out any path that doesn't look like `*.nix`. Don't forget to use
    # toString to prevent copying paths to the store unnecessarily
    (elem: !isPath elem || hasSuffix ".nix" (toString elem))
    # Expand any folder to all the files within it.
    (concatMap expandIfFolder list)