FR: Error on empty selection allow_empty = FALSE has informative class

[eutwt]

Many of the eval_select() errors have a class that lets you know what went wrong, but not the error you get on an empty selection with allow_empty = FALSE. This would be useful if people want to rethrow the error so that the "Must select at least one item" message is more informative in the context of their function, especially to disambiguate the source of the error if there are multiple tidy-select arguments (e.g. rethrow as "Must select at least one country").

library(tidyselect)

eval_select(quote(any_of('z')), list(x = 1), allow_empty = FALSE) |>
  rlang::catch_cnd() |> 
  class()
#> [1] "rlang_error" "error"       "condition"

eval_select(c(a = 'x'), list(x = 1), allow_rename = FALSE) |>
  rlang::catch_cnd() |> 
  class()
#> [1] "tidyselect:::error_disallowed_rename"
#> [2] "rlang_error"                         
#> [3] "error"                               
#> [4] "condition"

eval_select('z', list(x = 1)) |>
  rlang::catch_cnd() |> 
  class()
#> [1] "vctrs_error_subscript_oob" "vctrs_error_subscript"    
#> [3] "rlang_error"               "error"                    
#> [5] "condition"

eval_select(list(1), list(x = 1)) |>
  rlang::catch_cnd() |> 
  class()
#> [1] "vctrs_error_subscript_type" "vctrs_error_subscript"     
#> [3] "rlang_error"                "error"                     
#> [5] "condition"

^{Created on 2023-11-30 with reprex v2.0.2}

[olivroy]

Could also improve the base message?

f <- function(dat, cols) {
  tidyselect::eval_select(
    expr = rlang::enquo(cols),
    data = dat,
    allow_empty = FALSE
  )
}
mtcars |> f(c(vs, mpg2))
#> Error in `f()`:
#> ! Can't select columns that don't exist.
#> ✖ Column `mpg2` doesn't exist.
mtcars |> f(c(tidyselect::starts_with("vs")))
#> vs 
#>  8
mtcars |> f(c(tidyselect::starts_with("vs2")))
#> Error in `f()`:
#> ! Must select at least one item.


mtcars |> tidyr::pivot_longer(tidyselect::starts_with("vs2"))
#> Error in `tidyr::pivot_longer()`:
#> ! `cols` must select at least one column.

^{Created on 2024-04-25 with reprex v2.1.0}

Or add maybe an arg_nm?

so that it could throw a message like tidyr does natively?

cols must select at least one column

Currently, tidyr rethrows manually

https://github.com/tidyverse/tidyr/blob/c6c126a61f67a10b5ab9ce6bb1d9dbbb7a380bbd/R/pivot-long.R#L363-L373

But with my proposal,

# The call in tidyr could become 
 cols <- tidyselect::eval_select(
    expr = enquo(cols),
    data = data[unique(names(data))],
    allow_rename = FALSE,
    allow_empty = FALSE,
    error_arg = "cols",
    error_call = error_call
  )
#> `cols` must select at least one column 

error_arg was proposed in #282 (comment), is there plan to implement it?

Actually, the open issue is #327