rlang::sym in anonymous functions

Question

I recently notices that rlang::sym doesn't seem to work in anonymous functions and I don't understand why. Here an example, it's pretty clumsy and ugly but I think it illustrates the point

require(tidyverse)
data <- tibble(x1 = letters[1:3],
               x2 = letters[4:6],
               val = 1:3)

get_it <- function(a, b){
    data %>%
        mutate(y1 = !!rlang::sym(a)) %>%
        mutate(y2 = !!rlang::sym(b)) %>%
        select(y1, y2, val)
}
get_it("x1", "x2")

This defines some toy data and a (horrible) function that essentially renames the columns based on column names. Now I can do the same thing for different combinations of a and b:

d <- tibble(x = c("x1", "x2"),
            y = c("x2", "x1"))
d %>% mutate(tmp = map2(x, y, get_it))

However, if I try to do the exact same thing with an anonymous function it doesn't work:

d %>% mutate(tmp = map2(x, y, function(a, b){
data %>%
    mutate(y1 = !!rlang::sym(a)) %>%
    mutate(y2 = !!rlang::sym(b)) %>%
    select(y1, y2, val)
}))

This fails with object 'a' not found even though the functions are exactly the same just here it is anonymous. Can anyone explain why?

Answer 1

Note that the following code has rlang::sym() inside an anonymous function and works as intended:

map2(c("x1","x2"), c("x2","x1"), function(a, b){
  data %>%
    mutate(y1 = !!rlang::sym(a)) %>%
    mutate(y2 = !!rlang::sym(b)) %>%
    select(y1, y2, val)})

The original issue has to do with the unquoting operator !! and its precedence relative to anonymous function creation inside non-standard evaluation (NSE) functions, such as mutate. Consider a nested data frame

XX <- data_frame( dfs = list(
  data_frame( x = letters[1:3], y = 1:3 ),
  data_frame( x = letters[4:6], y = 4:6 )) )

If I attempt to select the first column inside each of the internal data frames using

XX %>% mutate( dfs1 = map( dfs, function(df) {
  i <- 1
  df %>% select(!!i)} ))

I get an object 'i' not found error, because the !! unquoting happens relative to the outer mutate, before the environment for the anonymous function (containing select) is created. If I move the anonymous function outside of mutate

map( XX$dfs, function(df) {
  i <- 1
  df %>% select(!!i)} )

it works without problems because map follows standard evaluation rules and the anonymous function environment is now created before the unquoting.

---

The help page for !! states that

The !! operator unquotes its argument. It gets evaluated immediately in the surrounding context.

This implies that when you write a complex NSE expression, such as select inside mutate, the unquoting will take place in the environment of the expression as a whole. As pointed out by @lionel in the comments, the unquoting in NSE takes precedence over other things, such as creation of anonymous function environments.