Apply a custom function rowwise with a character vector of column names as an argument

ref_tables <- tibble(`col 1` = letters[1:5], `col 2` = letters[6:10], `col 3` = letters[11:15])

  `col 1` `col 2` `col 3`
  <chr>   <chr>   <chr>  
1 a       f       k      
2 b       g       l      
3 c       h       m      
4 d       i       n      
5 e       j       o  

index_selection <- c("col 1", "col 2", "col 3")

I want to create a custom function that applies to each row of ref_tables. I want the result in a fourth column called dfs.

This is a trivial function:

f <- function(x) {
  return(paste(x, collapse = " "))
}

If I explicitly type the column names using backticks like this, I get results I'm expecting.

d <- ref_tables %>%
  rowwise() %>%
  mutate(dfs = list(f(c(`col 1`, `col 2`, `col 3`))))

# d$dfs[1] returns the result i'm expecting "a f k".

But I want to use the character vector index_selection.

d <- ref_tables %>%
  rowwise() %>%
  mutate(dfs = list(f(index_selection)))

# d$dfs[1] returns a result I am not expecting. "col 1 col 2 col 3".

I've tried using a combination of !!, !!!, sym() and cannot figure out how to do this. Can anyone help?

You can use column names from another vector with the help of all_of() within a selecting function such as c_across():

ref_tables %>%
  rowwise() %>%
  mutate(dfs = list(f(c_across(all_of(index_selection)))))