fun book(
    id: Long,
    env: DataFetchingEnvironment,
): CompletionStage<DataFetcherResult<Book>> { // here
    val bookNameDataLoader = env.dataLoaderRegistry.getDataLoader<Long, String>("key")
    val bookNameCompletableFuture = bookNameDataLoader.load(id)

    return CompletableFuture.supplyAsync {
        DataFetcherResult.newResult<Book>()
            .localContext("additionalContext")
            .data(
                Book(
                    id = id,
                    name = bookNameCompletableFuture.get(),
                )
            )
            .build()
    }
}