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

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