simple_split %>%
dplyr::mutate(train_set = purrr::map(splits, training))
## # # Monte Carlo cross-validation (0.75/0.25) with 1 resamples
## # A tibble: 1 x 3
## splits id train_set
## * <list> <chr> <list>
## 1 <split [2.2K/731]> train/test split <tibble [2,199 × 81]>