`hoist()` on a zero length list-of col should utilize the ptype

[DavisVaughan]

Taken from: #997 (comment)

library(dplyr)
library(tidyr)

# no hoist column is returned at all
mtcars %>%
  group_nest(cyl) %>% # this gives us a list-of `data` col
  slice(0) %>%
  hoist(data, "mpg")
# A tibble: 0 x 2
# ... with 1 variable: cyl <dbl>

It ideally should instead return:

mtcars %>%
  group_nest(cyl) %>%
  slice(0) %>%
  hoist(data, "mpg")
# A tibble: 0 x 2
# ... with 2 variables: cyl <dbl>, mpg <dbl>

This currently doesn't work for two reasons:

• This double map() should take into account the fact that x might be a list-of col, in which case applying the plucker to each list element could always result in the same type of plucked result. If the list elements are list-ofs or data.frames, then I think every plucked result should always have the type of pluck(<ptype>, ...), but this isn't fully thought through.

  tidyr/R/rectangle.R

  Lines 197 to 201 in 511fc0f

  	pluck <- purrr::pluck
  	cols <- map(pluckers, function(idx) {
  	map(x, ~ pluck(.x, !!!idx))
  	})
  	cols <- new_data_frame(cols, n = vec_size(.data))

• Even if we fix the above point, hoist() isn't type stable due to purrr::pluck() behavior with empty elements #1203, would force pluck(<ptype>, ...) to always return NULL, which makes fixing the above point useless right now

[DavisVaughan]

#1203 seems fixed by dev purrr now

[DavisVaughan]

A few other cases to think about related to this

library(tidyr)
library(vctrs)

tibble(bar = list()) %>%
  hoist(bar, 'baz')
#> # A tibble: 0 × 0

tibble(bar = list_of(.ptype = integer())) %>%
  hoist(bar, 'baz')
#> # A tibble: 0 × 0

tibble(bar = list_of(.ptype = data_frame(baz = integer()))) %>%
  hoist(bar, 'baz')
#> # A tibble: 0 × 0

^{Created on 2022-12-21 with reprex v2.0.2.9000}