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

The issue is not anonymous functions, but the operator precedence of !! . 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, the unquoting then takes precedence over other things, such as creation of anonymous function environments.

In your case the !! unquoting is done with respect to the outer mutate() , which then attempts to find column x1 inside d , not data . There are two possible solutions:

1) Pull the expression involving !! into a standalone function (as you've done in your question):

res1 <- d %>% mutate(tmp = map2(x, y, get_it))

2) Replace !! with eval to delay expression evaluation:

res2 <- d %>% mutate(tmp = map2(x, y, function(a, b){
  data %>%
    mutate(y1 = eval(rlang::sym(a))) %>%
    mutate(y2 = eval(rlang::sym(b))) %>%
    select(y1, y2, val)
}))

identical(res1, res2)       #TRUE