Is nest() + mutate() + map() + unnest() really the best alternative to dplyr::do()

[alistaire]

@mauro_lepore

by_group <- function(.x, .f, ...) {
  grouped <- tidyr::nest(.x)
  out <- dplyr::mutate(grouped, data = purrr::map(.data$data, .f, ...))
  tidyr::unnest(out)
}

There used to be by_slice and by_row functions in purrr. They were super powerful, but complex and the implementation was slow compared to equivalent methods. They still exist in their new home in purrrlyr, but I don't believe they will see further development there, as the nest/list columns approach is now the preferred idiom.