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Interpolation bitter desugaring #153

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Interpolation bitter desugaring #153

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60 changes: 60 additions & 0 deletions puzzlers/pzzlr-072.html
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<h1>Strung Along</h1>
<table class="table meta-table table-condensed">
<tbody>
<tr>
<td class="header-column"><strong>Contributed by</strong></td>
<td>A. P. Marki</td>
</tr>
<tr>
<td><strong>Source</strong></td>
<td><a target="_blank" href="https://gitter.im/scala/contributors?at=5b249065202c8f71f506067b">Gitter</a></td>
</tr>
<tr>
<td><strong>First tested with Scala version</strong></td>
<td>2.12.6</td>
</tr>
</tbody>
</table>
<div class="code-snippet">
<h3>What is the result of executing the following code?</h3>
<pre class="prettyprint lang-scala">
val StringContext: String = s"hello, world"
</pre>
<ol>
<li>It fails to compile with a type mismatch. It found a StringContext but requires a String.</li>
<li id="correct-answer">It fails to compile with a type mismatch. It found a String but requires an Int.</li>
<li>It compiles but at runtime it throws StringIndexOutOfBoundsException.</li>
<li>It fails to compile because method s is not a member of String.</li>
<li>It fails to compile by throwing scala.reflect.internal.MissingRequirementError: object scala.StringContext in compiler mirror not found.</li>
</ol>
</div>
<button id="show-and-tell" class="btn btn-primary" href="#">Display the correct answer, explanation and comments</button>
<div id="explanation" class="explanation" style="display:none">
<h3>Explanation</h3>
<p>
The interpolation desugars unsurprisingly to:
<pre class="prettyprint lang-scala">
StringContext("hello, world").s()
</pre>
</p>
<p>
<tt>StringContext</tt>, in this context, does not mean <tt>scala.StringContext</tt> but instead means
the current recursive definition, which is of type <tt>String</tt>.
</p>
<p>
<tt>String.apply</tt> takes an <tt>Int</tt> and performs lookup at that index.
</p>
<p>
The <tt>s</tt> comes first in the source but second in the expansion, so there is no error to indicate
that there is no "s-interpolator" available. That error could be made visible, in a way:
<pre class="prettyprint lang-scala">
scala> implicit def f(s: String): Int = s.length
f: (s: String)Int

scala> val StringContext: String = s"hello, world"
<console>:12: error: value s is not a member of Char
val StringContext: String = s"hello, world"
^
</pre>
</p>
</div>