Give warning for early atomic conditions in case_when

[eutwt]

Warning to alert user of single-valued conditions when not all conditions are single-valued. Does not warn for last case (TRUE ~ RHS is okay for last case).

For example, conditions resulting from mixing && and & as in tidyverse/dtplyr#300. Maybe should be an error at some point, but that would of course be a breaking change for code depending on current behavior.

Current dplyr:

library(dplyr, warn.conflicts = FALSE)

df <- data.frame(a = 1:5, b = 11:15)

df %>% 
  mutate(grp = 
    case_when(
      a == 3 && b < 15 ~ 'grp 1',
      b > 15 ~ 'grp 2',
      TRUE ~ 'other'
    )
  )
#>   a  b   grp
#> 1 1 11 other
#> 2 2 12 other
#> 3 3 13 other
#> 4 4 14 other
#> 5 5 15 other

^{Created on 2021-09-11 by the reprex package (v2.0.1)}

This PR:

library(dplyr, warn.conflicts = FALSE)

df <- data.frame(a = 1:5, b = 11:15)

df %>% 
  mutate(grp = 
    case_when(
      a == 3 && b < 15 ~ 'grp 1',
      b > 15 ~ 'grp 2',
      TRUE ~ 'other'
    )
  )
#> Warning: LHS of case 1 (`a == 3 && b < 15 ~ "grp 1"`) is a single value and will
#> be recycled.
#>   a  b   grp
#> 1 1 11 other
#> 2 2 12 other
#> 3 3 13 other
#> 4 4 14 other
#> 5 5 15 other

^{Created on 2021-09-11 by the reprex package (v2.0.1)}

Actually in RStudio I get the following additional warning lines added by mutate, not sure why they don't show in reprex

Problem with `mutate()` column `a`.
ℹ `a = case_when(1:32 == 1 ~ 1:32, 3 == 4 ~ 2L, TRUE ~ 3L)`.

[eutwt]

Currently the warning only references the first single-valued condition. Not sure whether to say something like LHS of cases 2, 3, and 4 .... With a warning listing all offending cases, should any expressions be shown?

Also, is it commonly known what LHS means? Is "condition" more widely understood? I think most reference material for SQL case statements uses the word "condition", but abort_case_when_logical uses "LHS".

[romainfrancois]

This looks good to me, however we will implement a newer case_when() in funs:: and perhaps this should be a feature request in tidyverse/funs#38

[eutwt]

Closing in favor of moving to feature request in tidyverse/funs#38