SortedSet Collector

[benhutchison]

Immutable SortedSets deserve more usage than they get IMHO. Often, I see List where people really ought to use a Set or a SortedSet.

One use case I've recently hit, not for the first time, is a predictable result ordering even when the generating process is nondeterministic, as many CE3 / FS2 processes can be. Predictable results are nice because it means we can assert on them in unit tests. I was seeing a unit test fail because the List of results contained the right elements, but in a varying order.

Truly what I wanted was SortedSet, not List, semantics. But there didnt seem to be a stream collector so I had to make one.

  def SortedSetCollector[T: Ordering] = new Collector[T]:
    type Out = SortedSet[T]
    def newBuilder = 
      val scalaBuilder = SortedSet.newBuilder[T]
      new Collector.Builder[T, Out]:
        def +=(c: Chunk[T]): Unit = scalaBuilder ++= c.iterator
        def result: Out = scalaBuilder.result()

[mpilquist]

Thanks for calling this out. It was intended to work out of the box without the need for an explicitly defined collector. The expected derivation is something like this:

given [A: Ordering] => Collector.Aux[A, collection.immutable.SortedSet[A]] =
  Collector.supportsFactory(collection.immutable.SortedSet.evidenceIterableFactory[A])

Would be great to figure out exactly why this isn't happening automatically and fix it for the general case. I'm guessing it's because evidenceIterableFactory returns a Factory[SortedSet[A], A] and not an IterableFactory[SortedSet[A], A] and only the former has a materializer?

[mpilquist]

Fixed by #3613

[benhutchison]

Thanks Michael 🙂