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<h1 class="title toc-ignore">Transformers</h1>
<h4 class="author"><em>Jim Hester</em></h4>
<h4 class="date"><em>2017-10-29</em></h4>



<p>Transformers allow you to apply functions to the glue input and output, before and after evaluation. This allows you to write things like <code>glue_sql()</code>, which automatically quotes variables for you or add a syntax for automatically collapsing outputs.</p>
<p>The transformer functions simply take two arguments <code>code</code> and <code>envir</code>, where <code>code</code> is the unparsed string inside the glue block and <code>envir</code> is the environment to execute the code in. Most transformers will then call <code>glue::evaluate()</code>, which takes <code>code</code> and <code>envir</code> and parses and evaluates the code.</p>
<p>You can then supply the transformer function to glue with the <code>.transformer</code> argument. In this way users can define manipulate the code before parsing and change the output after evaluation.</p>
<p>It is often useful to write a <code>glue()</code> wrapper function which supplies a <code>.transformer</code> to <code>glue()</code> or <code>glue_data()</code> and potentially has additional arguments. One important consideration when doing this is to include <code>.envir = parent.frame()</code> in the wrapper to ensure the evaluation environment is correct.</p>
<p>Some examples implementations of potentially useful transformers follow. The aim right now is not to include most of these custom functions within the <code>glue</code> package. Rather users are encouraged to create custom functions using transformers to fit their individual needs.</p>
<div id="collapse-transformer" class="section level3">
<h3>collapse transformer</h3>
<p>A transformer which automatically collapses any glue block ending with <code>*</code>.</p>
<div class="sourceCode"><pre class="sourceCode r"><code class="sourceCode r">collapse_transformer &lt;-<span class="st"> </span><span class="cf">function</span>(<span class="dt">regex =</span> <span class="st">&quot;[*]$&quot;</span>, ...) {
  <span class="cf">function</span>(code, envir) {
    <span class="cf">if</span> (<span class="kw">grepl</span>(regex, code)) {
        code &lt;-<span class="st"> </span><span class="kw">sub</span>(regex, <span class="st">&quot;&quot;</span>, code)
    }
    res &lt;-<span class="st"> </span><span class="kw">evaluate</span>(code, envir)
    <span class="kw">collapse</span>(res, ...)
  }
}

<span class="kw">glue</span>(<span class="st">&quot;{1:5*}</span><span class="ch">\n</span><span class="st">{letters[1:5]*}&quot;</span>, <span class="dt">.transformer =</span> <span class="kw">collapse_transformer</span>(<span class="dt">sep =</span> <span class="st">&quot;, &quot;</span>))
<span class="co">#&gt; 1, 2, 3, 4, 5</span>
<span class="co">#&gt; a, b, c, d, e</span>

<span class="kw">glue</span>(<span class="st">&quot;{1:5*}</span><span class="ch">\n</span><span class="st">{letters[1:5]*}&quot;</span>, <span class="dt">.transformer =</span> <span class="kw">collapse_transformer</span>(<span class="dt">sep =</span> <span class="st">&quot;, &quot;</span>, <span class="dt">last =</span> <span class="st">&quot; and &quot;</span>))
<span class="co">#&gt; 1, 2, 3, 4 and 5</span>
<span class="co">#&gt; a, b, c, d and e</span></code></pre></div>
</div>
<div id="emoji-transformer" class="section level3">
<h3>emoji transformer</h3>
<p>A transformer which converts the text to the equivalent emoji.</p>
<div class="sourceCode"><pre class="sourceCode r"><code class="sourceCode r">emoji_transformer &lt;-<span class="st"> </span><span class="cf">function</span>(code, envir) {
  <span class="cf">if</span> (<span class="kw">grepl</span>(<span class="st">&quot;[*]$&quot;</span>, code)) {
    code &lt;-<span class="st"> </span><span class="kw">sub</span>(<span class="st">&quot;[*]$&quot;</span>, <span class="st">&quot;&quot;</span>, code)
    <span class="kw">collapse</span>(<span class="kw">ji_find</span>(code)<span class="op">$</span>emoji)
  } <span class="cf">else</span> {
    <span class="kw">ji</span>(code)
  }
}

glue_ji &lt;-<span class="st"> </span><span class="cf">function</span>(..., <span class="dt">.envir =</span> <span class="kw">parent.frame</span>()) {
  <span class="kw">glue</span>(..., <span class="dt">.open =</span> <span class="st">&quot;:&quot;</span>, <span class="dt">.close =</span> <span class="st">&quot;:&quot;</span>, <span class="dt">.envir =</span> .envir, <span class="dt">.transformer =</span> emoji_transformer)
}
<span class="kw">glue_ji</span>(<span class="st">&quot;one :heart:&quot;</span>)
<span class="co">#&gt; one ❤️</span>
<span class="kw">glue_ji</span>(<span class="st">&quot;many :heart*:&quot;</span>)
<span class="co">#&gt; many 😍😻💘❤💓💔💕💟💌♥️❣️❤️</span></code></pre></div>
</div>
<div id="sprintf-transformer" class="section level3">
<h3>sprintf transformer</h3>
<p>A transformer which allows succinct sprintf format strings.</p>
<div class="sourceCode"><pre class="sourceCode r"><code class="sourceCode r">sprintf_transformer &lt;-<span class="st"> </span><span class="cf">function</span>(code, envir) {
  m &lt;-<span class="st"> </span><span class="kw">regexpr</span>(<span class="st">&quot;:.+$&quot;</span>, code)
  <span class="cf">if</span> (m <span class="op">!=</span><span class="st"> </span><span class="op">-</span><span class="dv">1</span>) {
    format &lt;-<span class="st"> </span><span class="kw">substring</span>(<span class="kw">regmatches</span>(code, m), <span class="dv">2</span>)
    <span class="kw">regmatches</span>(code, m) &lt;-<span class="st"> &quot;&quot;</span>
    res &lt;-<span class="st"> </span><span class="kw">evaluate</span>(code, envir)
    <span class="kw">do.call</span>(sprintf, <span class="kw">list</span>(<span class="kw">glue</span>(<span class="st">&quot;%{format}f&quot;</span>), res))
  } <span class="cf">else</span> {
    <span class="kw">evaluate</span>(code, envir)
  }
}

glue_fmt &lt;-<span class="st"> </span><span class="cf">function</span>(..., <span class="dt">.envir =</span> <span class="kw">parent.frame</span>()) {
  <span class="kw">glue</span>(..., <span class="dt">.transformer =</span> sprintf_transformer, <span class="dt">.envir =</span> .envir)
}
<span class="kw">glue_fmt</span>(<span class="st">&quot;π = {pi:.2}&quot;</span>)
<span class="co">#&gt; π = 3.14</span></code></pre></div>
</div>
<div id="safely-transformer" class="section level3">
<h3>safely transformer</h3>
<p>A transformer that acts like <code>purrr::safely()</code>, which returns a value instead of an error.</p>
<div class="sourceCode"><pre class="sourceCode r"><code class="sourceCode r">safely_transformer &lt;-<span class="st"> </span><span class="cf">function</span>(<span class="dt">otherwise =</span> <span class="ot">NA</span>) {
  <span class="cf">function</span>(code, envir) {
    <span class="kw">tryCatch</span>(<span class="kw">evaluate</span>(code, envir),
      <span class="dt">error =</span> <span class="cf">function</span>(e) <span class="cf">if</span> (<span class="kw">is.language</span>(otherwise)) <span class="kw">eval</span>(otherwise) <span class="cf">else</span> otherwise)
  }
}

glue_safely &lt;-<span class="st"> </span><span class="cf">function</span>(..., <span class="dt">.otherwise =</span> <span class="ot">NA</span>, <span class="dt">.envir =</span> <span class="kw">parent.frame</span>()) {
  <span class="kw">glue</span>(..., <span class="dt">.transformer =</span> <span class="kw">safely_transformer</span>(.otherwise), <span class="dt">.envir =</span> .envir)
}

<span class="co"># Default returns missing if there is an error</span>
<span class="kw">glue_safely</span>(<span class="st">&quot;foo: {xyz}&quot;</span>)
<span class="co">#&gt; foo: NA</span>

<span class="co"># Or an empty string</span>
<span class="kw">glue_safely</span>(<span class="st">&quot;foo: {xyz}&quot;</span>, <span class="dt">.otherwise =</span> <span class="st">&quot;Error&quot;</span>)
<span class="co">#&gt; foo: Error</span>

<span class="co"># Or output the error message in red</span>
<span class="kw">library</span>(crayon)
<span class="kw">glue_safely</span>(<span class="st">&quot;foo: {xyz}&quot;</span>, <span class="dt">.otherwise =</span> <span class="kw">quote</span>(<span class="kw">glue</span>(<span class="st">&quot;{red}Error: {conditionMessage(e)}{reset}&quot;</span>)))
<span class="co">#&gt; foo: Error: object 'xyz' not found</span></code></pre></div>
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