topic-embrace-non-args

The embrace operator <code>{{</code> should be used exclusively with function arguments:
<div class="sourceCode r"></div><pre><code>fn &lt;- function(arg) {
 quo(foo({{ arg }}))
}fn(1 + 1)
#&gt; &lt;quosure&gt;
#&gt; expr: ^foo(^1 + 1)
#&gt; env: 0x7ffd89aac518
</code></pre>
However you may have noticed that it also works on regular objects:
<div class="sourceCode r"></div><pre><code>fn &lt;- function(arg) {
 arg &lt;- force(arg)
 quo(foo({{ arg }}))
}fn(1 + 1)
#&gt; &lt;quosure&gt;
#&gt; expr: ^foo(^2)
#&gt; env: 0x7ffd8a633398
</code></pre>
In that case, <code>{{</code> 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 <code>enquo()</code> (on which <code>{{</code> is based).

internal

A toolbox for working with base types, core R features
like the condition system, and core 'Tidyverse' features like tidy
evaluation.

Lionel Henry

rlang

Functions for Base Types and Core R and 'Tidyverse' Features

Hadley Wickham

mikefc 

Yann Collet

Posit, PBC 

topic-embrace-non-args function

Ideally we would have made <code>{{</code> 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 Why are strings and other constants enquosed in the empty environment? for more about this.

Why is this not an error?

The embrace operator <code>{{</code> should be used exclusively with function arguments:
<div class="sourceCode r"><pre><code>fn &lt;- function(arg) {
 quo(foo({{ arg }}))
}

fn(1 + 1)
#&gt; &lt;quosure&gt;
#&gt; expr: ^foo(^1 + 1)
#&gt; env: 0x7ffd89aac518
</code></pre></div>
However you may have noticed that it also works on regular objects:
<div class="sourceCode r"><pre><code>fn &lt;- function(arg) {
 arg &lt;- force(arg)
 quo(foo({{ arg }}))
}

fn(1 + 1)
#&gt; &lt;quosure&gt;
#&gt; expr: ^foo(^2)
#&gt; env: 0x7ffd8a633398
</code></pre></div>
In that case, <code>{{</code> 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 <code>enquo()</code> (on which <code>{{</code> is based).

topic-embrace-non-args: Does `{{` work on regular objects?

Description

Arguments

Why is this not an error?