In rlang: Functions for Base Types and Core R and 'Tidyverse' Features

The embrace operator r link("{{") should be used exclusively with function arguments:

fn <- function(arg) {
  quo(foo({{ arg }}))
}

fn(1 + 1)
#> <quosure>
#> expr: ^foo(^1 + 1)
#> env:  0x7ffd89aac518

However you may have noticed that it also works on regular objects:

fn <- function(arg) {
  arg <- force(arg)
  quo(foo({{ arg }}))
}

fn(1 + 1)
#> <quosure>
#> expr: ^foo(^2)
#> env:  0x7ffd8a633398

In that case, {{ captures the value of the expression instead of a defused expression. That's because only function arguments can be defused.

Note that this issue also applies to [enquo()] (on which {{ is based).

Why is this not an error?

Ideally we would have made {{ on regular objects an error. However this is not possible because in compiled R code it is not always possible to distinguish a regular variable from a function argument. See r link("topic_embrace_constants") for more about this.