nomicon/infra/libkookie/nixpkgs/unstable/lib/tests/modules/freeform-unstr-dep-str.nix

{ lib, config, ... }: {
  options.value = lib.mkOption {
    type = lib.types.nullOr lib.types.str;
    default = null;
  };

  config.foo = lib.mkIf (config.value != null) config.value;
}