Passing a list of arguments to a function with quasiquotation

I am trying to write a function in R that summarizes a data frame according to grouping variables. The grouping variables are given as a list and passed to group_by_at, and I would like to parametrize them.

What I am doing now is this:

library(tidyverse)

d = tribble(
  ~foo, ~bar, ~baz,
  1, 2, 3,
  1, 3, 5
  4, 5, 6,
  4, 5, 1
)

sum_fun <- function(df, group_vars, sum_var) {
  sum_var = enquo(sum_var)
  return(
    df %>% 
      group_by_at(.vars = group_vars) %>% 
      summarize(sum(!! sum_var))
  )
}

d %>% sum_fun(group_vars = c("foo", "bar"), baz)

However, I would like to call the function like so:

d %>% sum_fun(group_vars = c(foo, bar), baz)

Which means the grouping vars should not be evaluated in the call, but in the function. How would I go about rewriting the function to enable that?

I have tried using enquo just like for the summary variable, and then replacing group_vars with !! group_vars, but it leads to this error:

Error in !group_vars : invalid argument type

Using group_by(!!!group_vars) yields:

Column `c(foo, bar)` must be length 2 (the number of rows) or one, not 4

What would be the proper way to rewrite the function?

Solution

You can rewrite the function using a combination of dplyr::group_by(), dplyr::across(), and curly curly embracing {{. This works with dplyr version 1.0.0 and greater.

I've edited the original example and code for clarity.

library(tidyverse)

my_data <- tribble(
  ~foo, ~bar, ~baz,
   "A",  "B",    3,
   "A",  "C",    5,
   "D",  "E",    6,
   "D",  "E",    1
)

sum_fun <- function(.data, group, sum_var) {
    .data %>% 
      group_by(across({{ group }})) %>% 
      summarize("sum_{{sum_var}}" := sum({{ sum_var }}))
}

sum_fun(my_data, group = c(foo, bar), sum_var = baz)
#> `summarise()` has grouped output by 'foo'. You can override using the `.groups` argument.
#> # A tibble: 3 x 3
#> # Groups:   foo [2]
#>   foo   bar   sum_baz
#>   <chr> <chr>   <dbl>
#> 1 A     B           3
#> 2 A     C           5
#> 3 D     E           7

^{Created on 2021-09-06 by the reprex package (v2.0.0)}