how to support 2.12/2.13 cross-building?

[SethTisue]

hey @szeiger if I need import scala.collection.parallel.CollectionConverters._ before I can use .par then how are library maintainers supposed to cross-build their code in 2.12/2.13?

idea: should we add a dummy empty scala.collection.parallel.CollectionConverters class to Scala 2.12.2? if it has no runtime residue, doing so won't break binary compatibility as far as I can see

[retronym]

This might be a good usecase for -Ypredef, you could have Scala version dependent root imports.

[szeiger]

For now I would suggest to put a dummy CollectionConverters object into a scala-2.12 source folder in affected projects. It's a one-line change and sbt supports it out of the box.

In the long run we'll need some compatibility library for 2.12 -> 2.13 collections migration where we can add this object.

[sjrd]

@szeiger That solution is not really acceptable: it leaks that dummy object down to other projects depending on your library. And if there are two such libraries, you now inherit from twice this dummy object on the classpath. That's too ugly to be a recommended default solution.

[sjrd]

Here is a non-leaking solution, fully source compatible across Scala versions (no need for scala-2.12 or scala-2.13 folders).

CompatParColls.scala (private to the library)

package test

private[test] object CompatParColls {
  val Converters = {
    import Compat._

    {
      import scala.collection.parallel._

      CollectionConverters
    }
  }

  object Compat {
    object CollectionConverters
  }
}

Test.scala (actual code in the library that wants to use parallel collections)

package test

import test.CompatParColls.Converters._

object Main {
  def main(args: Array[String]): Unit = {
    val argsInt = args.par.map(_.toInt)
    argsInt.foreach(println(_))
  }
}

Gist: https://gist.github.com/sjrd/ccc2ec61a2ccbae1eb4ee9dea004b7a5

[SethTisue]

note that it's often possible to replace something like

Vector.fill(100)(...).par

with

collection.parallel.immutable.ParVector.fill(100)(...)

which is fully source-compatible.

[szeiger]

We should provide 2.11 & 2.12 builds with a dummy CollectionConverters objects. We're already using the same approach for https://github.com/scala/scala-collection-compat/