rowwise() and empty input

[krlmlr]

Should we guarantee that the functions used in a rowwise mutate() or summarize() or reframe() always see length-one input? This is currently the case for everything but empty input, which creates a corner case to be worked around in user code. The example below shows mutate(), but the behavior is very similar for summarize() and reframe()

options(conflicts.policy = list(warn = FALSE))

library(dplyr)

tibble(a = 1L) |>
  rowwise() |>
  mutate(print(a))
#> [1] 1
#> # A tibble: 1 × 2
#> # Rowwise: 
#>       a `print(a)`
#>   <int>      <int>
#> 1     1          1

tibble(a = 1:3) |>
  rowwise() |>
  mutate(print(a))
#> [1] 1
#> [1] 2
#> [1] 3
#> # A tibble: 3 × 2
#> # Rowwise: 
#>       a `print(a)`
#>   <int>      <int>
#> 1     1          1
#> 2     2          2
#> 3     3          3

# Should this print `NA_integer_` ?
tibble(a = integer()) |>
  rowwise() |>
  mutate(print(a))
#> integer(0)
#> # A tibble: 0 × 2
#> # Rowwise: 
#> # ℹ 2 variables: a <int>, print(a) <int>

^{Created on 2024-06-12 with reprex v2.1.0}