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Tuple.x puzzler #131

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aadd265
Adding a WIP puzzler about Tuple2.x
Jul 11, 2015
b6f5454
Adding an explanation for the Tuple.x puzzler
Jul 26, 2015
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91 changes: 91 additions & 0 deletions puzzlers/pzzlr-tuple-x-puzzler.html
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<h1>Making a Point</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>2.11.6</td>
</tr>
</tbody>
</table>
<div class="code-snippet">
<h3>What is the result of executing the following code?</h3>
<pre class="prettyprint lang-scala">
object TwoD {
case class Point(x: Int, y: Int)
object Point {
def ->(coords: (Int, Int)) = apply(coords._1, coords._2)
}
}

val originMethod = TwoD.Point.->(0.->(0))
val zerooneMethod = TwoD.Point.->(0.->(1))
val originInfix = TwoD.Point -> 0 -> 0
val zerooneInfix = TwoD.Point -> 0 -> 1

println(originMethod.x == zerooneMethod.x)
println(originInfix.x == zerooneInfix.x)
</pre>
<ol>
<li>Prints:
<pre class="prettyprint lang-scala">
true
true
</pre>
</li>
<li id="correct-answer">Prints:
<pre class="prettyprint lang-scala">
true
false
</pre>
</li>
<li>Both <tt>println</tt> statements fail to compile</li>
<li>The first <tt>println</tt> statement prints:
<pre class="prettyprint lang-scala">
true
</pre>
and the second fails to 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>
Since method <tt>-></tt> is a single-argument method, it can be invoked
using <a href="http://docs.scala-lang.org/style/method-invocation.html" target="blank">
infix notation</a> <tt>A -> B</tt>, as well as via the &quot;standard&quot;
Java convention <tt>A.->(B)</tt>. In common with other Scala operators not
ending in a colon <tt>':'</tt> character, it is left-associative (SLS
&sect;6.12.3).
</p>
<p>
As a result, the definitions of <tt>originInfix</tt> and <tt>zerooneInfix
</tt> are parsed as <tt>(TwoD.Point -> 0) -> 0</tt> and
<tt>(TwoD.Point -> 0) -> 1</tt>, respectively. Both values are tuples:
<tt>originInfix</tt> becomes <tt>((TwoD.Point, 0), 0)</tt>;
<tt>zerooneInfix</tt> becomes <tt>((TwoD.Point, 0), 1)</tt>.
</p>
<p>
The expression <tt>originInfix.x == zerooneInfix.x</tt> still compiles,
however, because a value <tt>x</tt> actually is defined (via an implicit
conversion) on <a href="http://www.scala-lang.org/api/current/index.html#scala.Tuple2" target="blank">
<tt>Tuple2</tt></a>. This value is the tuple itself, so <tt>originInfix.x
== zerooneInfix.x</tt> returns false.
</p>
<p>
In the definitions of values <tt>originMethod</tt> and
<tt>zerooneMethod</tt>, the parentheses requires by the standard
&quot;Java-style&quot; method invocation mean that expressions are
parsed as <tt>TwoD.Point -> (0 -> 0)</tt> and <tt>TwoD.Point ->
(0 -> 1)</tt>. The two resulting <tt>Point</tt> instances have the same
x-coordinate, so <tt>originMethod.x == zerooneMethod.x</tt> returns true.
</p>
</div>