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POHow do I apply the enrich-my-library pattern to Scala collections?
 

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  1. POHow do I apply the enrich-my-library pattern to Scala collections?
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    <p>One of the most powerful patterns available in Scala is the enrich-my-library* pattern, which uses implicit conversions to <em>appear</em> to add methods to existing classes without requiring dynamic method resolution. For example, if we wished that all strings had the method <code>spaces</code> that counted how many whitespace characters they had, we could:</p> <pre><code>class SpaceCounter(s: String) { def spaces = s.count(_.isWhitespace) } implicit def string_counts_spaces(s: String) = new SpaceCounter(s) scala&gt; "How many spaces do I have?".spaces res1: Int = 5 </code></pre> <p>Unfortunately, this pattern runs into trouble when dealing with generic collections. For example, a number of questions have been asked about <a href="https://stackoverflow.com/questions/4761386/scala-list-function-for-grouping-consecutive-identical-elements">grouping items sequentially with collections</a>. There is nothing built in that works in one shot, so this seems an ideal candidate for the enrich-my-library pattern using a generic collection <code>C</code> and a generic element type <code>A</code>:</p> <pre><code>class SequentiallyGroupingCollection[A, C[A] &lt;: Seq[A]](ca: C[A]) { def groupIdentical: C[C[A]] = { if (ca.isEmpty) C.empty[C[A]] else { val first = ca.head val (same,rest) = ca.span(_ == first) same +: (new SequentiallyGroupingCollection(rest)).groupIdentical } } } </code></pre> <p>except, of course, it <em>doesn't work</em>. The REPL tells us:</p> <pre><code>&lt;console&gt;:12: error: not found: value C if (ca.isEmpty) C.empty[C[A]] ^ &lt;console&gt;:16: error: type mismatch; found : Seq[Seq[A]] required: C[C[A]] same +: (new SequentiallyGroupingCollection(rest)).groupIdentical ^ </code></pre> <p>There are two problems: how do we get a <code>C[C[A]]</code> from an empty <code>C[A]</code> list (or from thin air)? And how do we get a <code>C[C[A]]</code> back from the <code>same +:</code> line instead of a <code>Seq[Seq[A]]</code>?</p> <p>*<sup> Formerly known as pimp-my-library.</sup></p>
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    1. USRex Kerr
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    1. COGreat question! And, even better, it comes with an answer! :-)
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      1. POHow do I apply the enrich-my-library pattern to Scala collections?
      1. USDaniel C. Sobral
    2. CO@Daniel - I have no objection to it coming with two or more answers!
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    3. COForget it, buddy. I'm bookmarking this to look it up whenever I need to do something like this. :-)
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