Writing a custom case_when function to use in dplyr mutate using tidyeval

Question

I'm trying to write a custom case_when function to use inside dplyr. I've been reading through the tidyeval examples posted in other questions, but still can't figure out how to make it work. Here's a reprex:

df1 <- data.frame(animal_1 = c("Horse", "Pig", "Chicken", "Cow", "Sheep"),
           animal_2 = c(NA, NA, "Horse", "Sheep", "Chicken"))


translate_title <- function(data, input_col, output_col) {
  mutate(data, 
    !!output_col := case_when(
    input_col == "Horse" ~ "Cheval",
    input_col == "Pig" ~ "Рorc",
    input_col == "Chicken" ~ "Poulet",
    TRUE ~ NA)
  )
}


df1 %>% 
  translate_title("animal_1", "animaux_1") %>% 
  translate_title("animal_2", "animaux_2")

When I try to run this, I'm getting Error in mutate_impl(.data, dots) : Evaluation error: must be type string, not logical.

Also I would actually like to rewrite the function so that it can be used like this:

df1 %>% 
  mutate(animaux_1 = translate_title(animal_1),
         animaux_2 = translate_title(animal_2)
         )

But not sure how.

Answer 1

Depending on how you want to pass your input to the function you can solve it in two ways :

1) Pass input as unquoted using {{}}

library(dplyr)

translate_title <- function(data, input_col, output_col) {

   mutate(data, 
        !!output_col := case_when(
        {{input_col}} == "Horse" ~ "Cheval",
        {{input_col}} == "Pig" ~ "Рorc",
        {{input_col}} == "Chicken" ~ "Poulet",
        TRUE ~ NA_character_)
  )
}

df1 %>% 
  translate_title(animal_1, "animaux_1") %>%
  translate_title(animal_2, "animaux_2")

#  animal_1 animal_2 animaux_1 animaux_2
#1    Horse     <NA>    Cheval      <NA>
#2      Pig     <NA>      Рorc      <NA>
#3  Chicken    Horse    Poulet    Cheval
#4      Cow    Sheep      <NA>      <NA>
#5    Sheep  Chicken      <NA>    Poulet

2) Pass input quoted using sym and !!

translate_title <- function(data, input_col, output_col) {
   mutate(data, 
         !!output_col := case_when(
          !!sym(input_col) == "Horse" ~ "Cheval",
          !!sym(input_col) == "Pig" ~ "Рorc",
          !!sym(input_col) == "Chicken" ~ "Poulet",
          TRUE ~ NA_character_)
  )
}

df1 %>% 
  translate_title("animal_1", "animaux_1") %>%
  translate_title("animal_2", "animaux_2")