Use immutable Seq for repeated

[hseeberger]

Currently a repeated field of a message gets compiled to a scala.collection.Seq parameter or a case class. Case classes really should be immutable, which can be broken by passing a mutable Seq. Therefore I suggest to use scala.collection.immutable.Seq.

[xuwei-k]

related #218

[thesamet]

This should be an option users can choose once #218 is implemented since making scala.collection.immutable.Seq the default generated type would break users' existing code:

scala> case class Foo(x: collection.immutable.Seq[Int])
defined class Foo

scala> Foo(Seq(3,4,5))
<console>:14: error: type mismatch;
 found   : Seq[Int]
 required: scala.collection.immutable.Seq[Int]
       Foo(Seq(3,4,5))
              ^

[hseeberger]

Even better if it becomes an option. But for a pre-1.0 software I personally wouldn't be surprised by breaking changes.

[rogern]

I wonder, can't this default to immutable.Seq if generated on >=2.13 and collection.Seq on earlier versions?
A tad annoying with false positives in IDE wherever used :)
We will configure it for now though.

[thesamet]

ScalaPB generator code is meant to be used on all supported versions of Scala, incl. Scala 3. Been trying to avoid having language-version logic in the generator, though this is something that can be considered in the future if there's a real advantage. For this one, you can set collection_type as a package-scoped option in your project, and alter PB.protoSources in sbt to include that options file only in a certain Scala versions.