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</head>
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<body>
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<h1 class="title toc-ignore">How to write a function that wraps
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glue</h1>
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<div class="sourceCode" id="cb1"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb1-1"><a href="#cb1-1" tabindex="-1"></a><span class="fu">library</span>(glue)</span></code></pre></div>
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<p>Imagine that you want to call <code>glue()</code> repeatedly inside
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your own code (e.g. in your own package) with a non-default value for
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one or more arguments. For example, maybe you anticipate producing R
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code where <code>{</code> and <code>}</code> have specific syntactic
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meaning. Therefore, you’d prefer to use <code><<</code> and
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<code>>></code> as the opening and closing delimiters for
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expressions in <code>glue()</code>.</p>
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<p>Spoiler alert: here’s the correct way to write such a wrapper:</p>
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<div class="sourceCode" id="cb2"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb2-1"><a href="#cb2-1" tabindex="-1"></a>my_glue <span class="ot"><-</span> <span class="cf">function</span>(..., <span class="at">.envir =</span> <span class="fu">parent.frame</span>()) {</span>
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<span id="cb2-2"><a href="#cb2-2" tabindex="-1"></a> <span class="fu">glue</span>(..., <span class="at">.open =</span> <span class="st">"<<"</span>, <span class="at">.close =</span> <span class="st">">>"</span>, <span class="at">.envir =</span> .envir)</span>
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<span id="cb2-3"><a href="#cb2-3" tabindex="-1"></a>}</span></code></pre></div>
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<p>This is the key move:</p>
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<blockquote>
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<p>Include <code>.envir = parent.frame()</code> as an argument of the
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wrapper function and pass this <code>.envir</code> to the
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<code>.envir</code> argument of <code>glue()</code>.</p>
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</blockquote>
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<p>If you’d like to know why this is the way, keep reading. It pays off
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to understand this, because the technique applies more broadly than
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glue. Once you recognize this setup, you’ll see it in many functions in
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the withr, cli, and rlang packages (e.g. <code>withr::defer()</code>,
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<code>cli::cli_abort()</code>, <code>rlang::abort()</code>).</p>
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<div id="working-example" class="section level2">
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<h2>Working example</h2>
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<p>Here’s an abbreviated excerpt of the roxygen comment that generates
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the documentation for the starwars dataset in dplyr
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(<code>?dplyr::starwars</code>):</p>
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<div class="sourceCode" id="cb3"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb3-1"><a href="#cb3-1" tabindex="-1"></a><span class="co">#' \describe{</span></span>
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<span id="cb3-2"><a href="#cb3-2" tabindex="-1"></a><span class="co">#' \item{name}{Name of the character}</span></span>
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<span id="cb3-3"><a href="#cb3-3" tabindex="-1"></a><span class="co">#' \item{height}{Height (cm)}</span></span>
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<span id="cb3-4"><a href="#cb3-4" tabindex="-1"></a><span class="co">#' \item{mass}{Weight (kg)}</span></span>
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<span id="cb3-5"><a href="#cb3-5" tabindex="-1"></a><span class="co">#' \item{species}{Name of species}</span></span>
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<span id="cb3-6"><a href="#cb3-6" tabindex="-1"></a><span class="co">#' \item{films}{List of films the character appeared in}</span></span>
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<span id="cb3-7"><a href="#cb3-7" tabindex="-1"></a><span class="co">#' }</span></span></code></pre></div>
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<p>To produce such text programmatically, the first step might be to
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generate the <code>\item{}{}</code> lines from a named list of column
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names and descriptions. Notice that <code>{</code> and <code>}</code>
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are important to the <code>\describe{...}</code> and
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<code>\item{}{}</code> syntax, so this is an example where it is nice
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for glue to use different delimiters for expressions.</p>
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<p>Put the metadata in a suitable list:</p>
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<div class="sourceCode" id="cb4"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb4-1"><a href="#cb4-1" tabindex="-1"></a>sw_meta <span class="ot"><-</span> <span class="fu">list</span>(</span>
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<span id="cb4-2"><a href="#cb4-2" tabindex="-1"></a> <span class="at">name =</span> <span class="st">"Name of the character"</span>,</span>
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<span id="cb4-3"><a href="#cb4-3" tabindex="-1"></a> <span class="at">height =</span> <span class="st">"Height (cm)"</span>,</span>
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<span id="cb4-4"><a href="#cb4-4" tabindex="-1"></a> <span class="at">mass =</span> <span class="st">"Weight (kg)"</span>,</span>
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<span id="cb4-5"><a href="#cb4-5" tabindex="-1"></a> <span class="at">species =</span> <span class="st">"Name of species"</span>,</span>
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<span id="cb4-6"><a href="#cb4-6" tabindex="-1"></a> <span class="at">films =</span> <span class="st">"List of films the character appeared in"</span></span>
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<span id="cb4-7"><a href="#cb4-7" tabindex="-1"></a>)</span></code></pre></div>
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<p>Define a custom glue wrapper and use it inside another helper that
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generates <code>\item</code> entries<a href="#fn1" class="footnote-ref" id="fnref1"><sup>1</sup></a>:</p>
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<div class="sourceCode" id="cb5"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb5-1"><a href="#cb5-1" tabindex="-1"></a>my_glue <span class="ot">=</span> <span class="cf">function</span>(...) {</span>
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<span id="cb5-2"><a href="#cb5-2" tabindex="-1"></a> <span class="fu">glue</span>(..., <span class="at">.open =</span> <span class="st">"<<"</span>, <span class="at">.close =</span> <span class="st">">>"</span>, <span class="at">.envir =</span> <span class="fu">parent.frame</span>())</span>
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<span id="cb5-3"><a href="#cb5-3" tabindex="-1"></a>}</span>
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<span id="cb5-4"><a href="#cb5-4" tabindex="-1"></a></span>
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<span id="cb5-5"><a href="#cb5-5" tabindex="-1"></a>named_list_to_items <span class="ot"><-</span> <span class="cf">function</span>(x) {</span>
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<span id="cb5-6"><a href="#cb5-6" tabindex="-1"></a> <span class="fu">my_glue</span>(<span class="st">"</span><span class="sc">\\</span><span class="st">item{<<names(x)>>}{<<x>>}"</span>)</span>
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<span id="cb5-7"><a href="#cb5-7" tabindex="-1"></a>}</span></code></pre></div>
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<p>Apply <code>named_list_to_items()</code> to starwars metadata:</p>
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<div class="sourceCode" id="cb6"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb6-1"><a href="#cb6-1" tabindex="-1"></a><span class="fu">named_list_to_items</span>(sw_meta)</span>
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<span id="cb6-2"><a href="#cb6-2" tabindex="-1"></a><span class="co">#> \item{name}{Name of the character}</span></span>
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<span id="cb6-3"><a href="#cb6-3" tabindex="-1"></a><span class="co">#> \item{height}{Height (cm)}</span></span>
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<span id="cb6-4"><a href="#cb6-4" tabindex="-1"></a><span class="co">#> \item{mass}{Weight (kg)}</span></span>
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<span id="cb6-5"><a href="#cb6-5" tabindex="-1"></a><span class="co">#> \item{species}{Name of species}</span></span>
|
||
<span id="cb6-6"><a href="#cb6-6" tabindex="-1"></a><span class="co">#> \item{films}{List of films the character appeared in}</span></span></code></pre></div>
|
||
<p>Here’s how this would fail if we did <em>not</em> handle
|
||
<code>.envir</code> correctly in our wrapper function:</p>
|
||
<div class="sourceCode" id="cb7"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb7-1"><a href="#cb7-1" tabindex="-1"></a>my_glue_WRONG <span class="ot"><-</span> <span class="cf">function</span>(...) {</span>
|
||
<span id="cb7-2"><a href="#cb7-2" tabindex="-1"></a> <span class="fu">glue</span>(..., <span class="at">.open =</span> <span class="st">"<<"</span>, <span class="at">.close =</span> <span class="st">">>"</span>)</span>
|
||
<span id="cb7-3"><a href="#cb7-3" tabindex="-1"></a>}</span>
|
||
<span id="cb7-4"><a href="#cb7-4" tabindex="-1"></a></span>
|
||
<span id="cb7-5"><a href="#cb7-5" tabindex="-1"></a>named_list_to_items_WRONG <span class="ot"><-</span> <span class="cf">function</span>(x) {</span>
|
||
<span id="cb7-6"><a href="#cb7-6" tabindex="-1"></a> <span class="fu">my_glue_WRONG</span>(<span class="st">"</span><span class="sc">\\</span><span class="st">item{<<names(x)>>}{<<x>>}"</span>)</span>
|
||
<span id="cb7-7"><a href="#cb7-7" tabindex="-1"></a>}</span>
|
||
<span id="cb7-8"><a href="#cb7-8" tabindex="-1"></a></span>
|
||
<span id="cb7-9"><a href="#cb7-9" tabindex="-1"></a><span class="fu">named_list_to_items_WRONG</span>(sw_meta)</span></code></pre></div>
|
||
<p>It can be hard to understand why <code>x</code> can’t be found, when
|
||
it is clearly available inside <code>named_list_to_items_WRONG()</code>.
|
||
Why isn’t <code>x</code> available to <code>my_glue_WRONG()</code>?</p>
|
||
</div>
|
||
<div id="where-does-glue-evaluate-code" class="section level2">
|
||
<h2>Where does <code>glue()</code> evaluate code?</h2>
|
||
<p>What’s going on? It’s time to look at the (redacted) signature of
|
||
<code>glue()</code>:</p>
|
||
<div class="sourceCode" id="cb8"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb8-1"><a href="#cb8-1" tabindex="-1"></a><span class="fu">glue</span>(..., <span class="at">.envir =</span> <span class="fu">parent.frame</span>(), ...)</span></code></pre></div>
|
||
<p>The expressions inside a glue string are evaluated with respect to
|
||
<code>.envir</code>, which defaults to the environment where
|
||
<code>glue()</code> is called from.</p>
|
||
<p>Everything is simple when evaluating <code>glue()</code> in the
|
||
global environment:</p>
|
||
<div class="sourceCode" id="cb9"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb9-1"><a href="#cb9-1" tabindex="-1"></a>x <span class="ot"><-</span> <span class="dv">0</span></span>
|
||
<span id="cb9-2"><a href="#cb9-2" tabindex="-1"></a>y <span class="ot"><-</span> <span class="dv">0</span></span>
|
||
<span id="cb9-3"><a href="#cb9-3" tabindex="-1"></a>z <span class="ot"><-</span> <span class="dv">0</span></span>
|
||
<span id="cb9-4"><a href="#cb9-4" tabindex="-1"></a></span>
|
||
<span id="cb9-5"><a href="#cb9-5" tabindex="-1"></a><span class="fu">glue</span>(<span class="st">"{x} {y} {z}"</span>)</span>
|
||
<span id="cb9-6"><a href="#cb9-6" tabindex="-1"></a><span class="co">#> 0 0 0</span></span></code></pre></div>
|
||
<p>Now we wrap <code>glue()</code> in our own simple function,
|
||
<code>my_glue1()</code>. Notice that <code>my_glue1()</code> does not
|
||
capture its caller environment and pass that along.</p>
|
||
<p>When we execute <code>my_glue1()</code> in the global environment,
|
||
there’s no obvious problem.</p>
|
||
<div class="sourceCode" id="cb10"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb10-1"><a href="#cb10-1" tabindex="-1"></a>my_glue1 <span class="ot"><-</span> <span class="cf">function</span>(...) {</span>
|
||
<span id="cb10-2"><a href="#cb10-2" tabindex="-1"></a> x <span class="ot"><-</span> <span class="dv">1</span></span>
|
||
<span id="cb10-3"><a href="#cb10-3" tabindex="-1"></a> <span class="fu">glue</span>(...)</span>
|
||
<span id="cb10-4"><a href="#cb10-4" tabindex="-1"></a>}</span>
|
||
<span id="cb10-5"><a href="#cb10-5" tabindex="-1"></a></span>
|
||
<span id="cb10-6"><a href="#cb10-6" tabindex="-1"></a><span class="fu">my_glue1</span>(<span class="st">"{x} {y} {z}"</span>)</span>
|
||
<span id="cb10-7"><a href="#cb10-7" tabindex="-1"></a><span class="co">#> 1 0 0</span></span></code></pre></div>
|
||
<p>The value of <code>x</code> is found in the execution environment of
|
||
<code>my_glue1()</code>. The values of <code>y</code> and <code>z</code>
|
||
are found in the global environment. Importantly, this is because that
|
||
is the environment where <code>my_glue1()</code> is defined, not because
|
||
that is where <code>my_glue1()</code> is called.</p>
|
||
<p>However, if we call our <code>my_glue1()</code> inside another
|
||
function, we see that all is not well.</p>
|
||
<div class="sourceCode" id="cb11"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb11-1"><a href="#cb11-1" tabindex="-1"></a>my_glue2 <span class="ot"><-</span> <span class="cf">function</span>(...) {</span>
|
||
<span id="cb11-2"><a href="#cb11-2" tabindex="-1"></a> x <span class="ot"><-</span> <span class="dv">2</span></span>
|
||
<span id="cb11-3"><a href="#cb11-3" tabindex="-1"></a> y <span class="ot"><-</span> <span class="dv">2</span></span>
|
||
<span id="cb11-4"><a href="#cb11-4" tabindex="-1"></a> <span class="fu">my_glue1</span>(...)</span>
|
||
<span id="cb11-5"><a href="#cb11-5" tabindex="-1"></a>}</span>
|
||
<span id="cb11-6"><a href="#cb11-6" tabindex="-1"></a></span>
|
||
<span id="cb11-7"><a href="#cb11-7" tabindex="-1"></a><span class="fu">my_glue2</span>(<span class="st">"{x} {y} {z}"</span>)</span>
|
||
<span id="cb11-8"><a href="#cb11-8" tabindex="-1"></a><span class="co">#> 1 0 0</span></span></code></pre></div>
|
||
<p>Why do <code>x</code> and <code>y</code> not have the value 2?
|
||
Because <code>my_glue1()</code> and its eventual call to
|
||
<code>glue()</code> have no access to the execution environment of
|
||
<code>my_glue2()</code>, which is the caller environment of
|
||
<code>my_glue1()</code>.</p>
|
||
<p>If you want your glue wrapper to behave like <code>glue()</code>
|
||
itself and to work as expected inside other functions, make sure it
|
||
captures its caller environment and passes that along to
|
||
<code>glue()</code>.</p>
|
||
<div class="sourceCode" id="cb12"><pre class="sourceCode r"><code class="sourceCode r"><span id="cb12-1"><a href="#cb12-1" tabindex="-1"></a>my_glue3 <span class="ot"><-</span> <span class="cf">function</span>(..., <span class="at">.envir =</span> <span class="fu">parent.frame</span>()) {</span>
|
||
<span id="cb12-2"><a href="#cb12-2" tabindex="-1"></a> x <span class="ot"><-</span> <span class="dv">3</span></span>
|
||
<span id="cb12-3"><a href="#cb12-3" tabindex="-1"></a> <span class="fu">glue</span>(..., <span class="at">.envir =</span> .envir)</span>
|
||
<span id="cb12-4"><a href="#cb12-4" tabindex="-1"></a>}</span>
|
||
<span id="cb12-5"><a href="#cb12-5" tabindex="-1"></a></span>
|
||
<span id="cb12-6"><a href="#cb12-6" tabindex="-1"></a><span class="fu">my_glue3</span>(<span class="st">"{x} {y} {z}"</span>)</span>
|
||
<span id="cb12-7"><a href="#cb12-7" tabindex="-1"></a><span class="co">#> 0 0 0</span></span>
|
||
<span id="cb12-8"><a href="#cb12-8" tabindex="-1"></a></span>
|
||
<span id="cb12-9"><a href="#cb12-9" tabindex="-1"></a>my_glue4 <span class="ot"><-</span> <span class="cf">function</span>(...) {</span>
|
||
<span id="cb12-10"><a href="#cb12-10" tabindex="-1"></a> x <span class="ot"><-</span> <span class="dv">4</span></span>
|
||
<span id="cb12-11"><a href="#cb12-11" tabindex="-1"></a> y <span class="ot"><-</span> <span class="dv">4</span></span>
|
||
<span id="cb12-12"><a href="#cb12-12" tabindex="-1"></a> <span class="fu">my_glue3</span>(...)</span>
|
||
<span id="cb12-13"><a href="#cb12-13" tabindex="-1"></a>}</span>
|
||
<span id="cb12-14"><a href="#cb12-14" tabindex="-1"></a></span>
|
||
<span id="cb12-15"><a href="#cb12-15" tabindex="-1"></a><span class="fu">my_glue4</span>(<span class="st">"{x} {y} {z}"</span>)</span>
|
||
<span id="cb12-16"><a href="#cb12-16" tabindex="-1"></a><span class="co">#> 4 4 0</span></span></code></pre></div>
|
||
</div>
|
||
<div class="footnotes footnotes-end-of-document">
|
||
<hr />
|
||
<ol>
|
||
<li id="fn1"><p>Note that delimiters <code><<</code> and
|
||
<code>>></code> have special meaning in knitr (they are used for a
|
||
templating feature in knitr itself). So in code chunks inside RMarkdown
|
||
or Quarto documents, you may need to use different delimiters.<a href="#fnref1" class="footnote-back">↩︎</a></p></li>
|
||
</ol>
|
||
</div>
|
||
|
||
|
||
|
||
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