modules.nix

# nix eval -f modules.nix
let
  lib = import <nixpkgs/lib>;
  eval = (import <nixpkgs/lib/modules.nix> { inherit lib; }).evalModules {
    modules = [
      ({ lib, ... }: with lib; {
        options.roles.a = mkOption {
          type = types.str;
        };
      })
      ({ lib, ... }: with lib; {
        options.roles.b = mkOption {
          type = types.str;
        };
      })
      ({
        roles.a = "foo";
        roles.b = "bar";
      })
    ];
  };
in
  eval.config.roles.a + eval.config.roles.b