One of the most powerful patterns available in Scala is the enrich-my-library* pattern, which uses implicit conversions to _appear_ to add methods to existing classes without requiring dynamic method resolution.  For example, if we wished that all strings had the method `spaces` that counted how many whitespace characters they had, we could:

    class SpaceCounter(s: String) {
      def spaces = s.count(_.isWhitespace)
    }
    implicit def string_counts_spaces(s: String) = new SpaceCounter(s)

    scala&gt; &quot;How many spaces do I have?&quot;.spaces
    res1: Int = 5

Unfortunately, this pattern runs into trouble when dealing with generic collections.  For example, a number of questions have been asked about [grouping items sequentially with collections][1].  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 `C` and a generic element type `A`:

    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
        }
      }
    }

except, of course, it _doesn&#39;t work_.  The REPL tells us:

    &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
                          ^

There are two problems: how do we get a `C[C[A]]` from an empty `C[A]` list (or from thin air)?  And how do we get a `C[C[A]]` back from the `same +:` line instead of a `Seq[Seq[A]]`?

*&lt;sup&gt; Formerly known as pimp-my-library.&lt;/sup&gt;


  [1]: https://stackoverflow.com/questions/4761386/scala-list-function-for-grouping-consecutive-identical-elements


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