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Created puzzler: What's in a box. #127

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77 changes: 77 additions & 0 deletions puzzlers/pzzlr-056.html
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<h1>What's in a box</h1>
<table class="table meta-table table-condensed">
<tbody>
<tr>
<td class="header-column"><strong>Contributed by</strong></td>
<td>Tim van Heugten</td>
</tr>
<tr>
<td><strong>Source</strong></td>
<td>N/A</td>
</tr>
<tr>
<td><strong>First tested with Scala version</strong></td>
<td>2.11.4</td>
</tr>
</tbody>
</table>
<div class="code-snippet">
<h3>What is the result of executing the following code?</h3>
<pre class="prettyprint lang-scala">
case class Box(i: Int)

object Box {
def ->(i: Int) = Box(i)
}

val myBox = new Box(5)
val yourBox = 5 -> Box
val hisBox = Box -> 5

if (myBox == yourBox)
println("I know what's in your box!")
else
println("What's in your box?")

if (myBox == hisBox)
println("I know what's in his box!")
else
println("What's in his box?")
</pre>
<ol>
<li id="correct-answer">Prints:
<pre class="prettyprint lang-scala">
What's in your box?
I know what's in his box!
</pre>
</li>

<li>Prints:
<pre class="prettyprint lang-scala">
I know what's in your box!
What's in his box?
</pre>
</li>

<li>Prints:
<pre class="prettyprint lang-scala">
I know what's in your box!
I know what's in his box!
</pre>
</li>

<li>Does not compile.
</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 usual case class implements an equals based on the arguments. Therefore, <tt>myBox</tt> and <tt>hisBox</tt> are equal as expected. But what is that syntax of <tt>yourBox</tt> assignment?
</p>
<p>
There is no right-associativity going on, as it would suggest. But there is <a href="http://www.scala-lang.org/api/current/scala/Predef%24%24ArrowAssoc.html">ArrowAssoc</a> that defines an implicit conversion for invocation of the <tt>-&gt;</tt> method. As a result, <tt>yourBox</tt> is a <tt>Tuple</tt> of <tt>(5, Box)</tt>. That obviously cannot be equal to <tt>myBox</tt>.
</p>
</div>