```{r, child = "setup.Rmd", include = FALSE}
```

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

```r
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:

```r
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.