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matching on objects with lower-case first letters #154

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70 changes: 70 additions & 0 deletions puzzlers/pzzlr-072.html
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<h1>Matching on <code>object</code>s with lower-case first letters</h1>
<table class="table meta-table table-condensed">
<tbody>
<tr>
<td class="header-column"><strong>Contributed by</strong></td>
<td><a href="https://github.com/ryan-williams">Ryan Williams</a>, <a href="https://github.com/martijnhoekstra">Martijn Hoekstra</a></td>
</tr>
<tr>
<td><strong>Source</strong></td>
<!--
If no source (your puzzler is not mentioned anywhere on the web), just say N/A
-->
<td><a target="_blank" href="https://gitter.im/scala/scala?at=5b68cf3085278d705e9332fc">scala/scala 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">
sealed trait obj
case object one extends obj
case object two extends obj
def test(o: obj) =
o match {
case one ⇒ 1
case two ⇒ 2
}
println(test(one))
println(test(two))
</pre>
<ol>
<!--
The correct answer should have id for corresponding li element equal to "correct-answer".
When placing code snippets wrap them with pre tag:
<pre class="prettyprint lang-scala">
...
</pre>
-->
<li>prints "1" and then "2"</li>
<li>throws <code>MatchError</code></li>
<li id="correct-answer">prints "1" and then "1"</li>
<li>compile error</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>
Identifiers that start with a lower-case letter are treated as a new variable with that name, whose value is the
input to the <code>match</code> expression.

Generally, you don't want to do this unless it's the last, "catch-all" <code>case</code>.

To correctly <code>match</code> on the <code>object</code>s above, you'd have to do:

<pre class="prettyprint lang-scala">
def ok(o: obj) =
o match {
case _: one.type ⇒ 1
case _: two.type ⇒ 2
}
</pre>
You can also <a href="https://gist.github.com/ryan-williams/16914966e656385108c75b681f9f3c5d">make them 0-arg
<code>case class</code>es, capitalize their first letter, or put their names in backticks</a>.
</p>
</div>