useSWRMutation for dynamic urls?

[FezVrasta]

The way useSWRMutation works makes it impossible to have properly typed conditional usages.

Take a REST endpoint: PUT books/:id.

If I wanted to create a reusable hook to update a book I have to do this:

export function useUpdateBookMutation(id: string) {
  return useSWRMutation(`books/${id}`, updateBook);
}

This means I can't use useUpdateBookMutation when I don't have an id always present.

If the useSWRMutation interface allowed to pass the url parameter as part of the trigger callback though, this would be trivial.

async function updateBook(urlGenerator, { arg }) {
  return fetch(urlGenerator(arg.id));
}

export function useUpdateBookMutation() {
  return useSWRMutation(id => `/books/${id}`, updateBook);
}

[...]

const App = () => {
  const updateBook = useUpdateBookMutation();
  const [bookId, setBookId] = useState(undefined);

  return <>
    <Dropdown books={books} onSelect={setBookId} />
    {bookId !== undefined &&
      <Button onClick={() => updateBook(trigger(bookId))}>Update selected book</Button>
    }
  </>
}

Is this impossible to achieve with SWR or am I missing something obvious?