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https://issues.apache.org/jira/browse/SPARK-27681?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16839280#comment-16839280
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Lukas Rytz commented on SPARK-27681:
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I think the best solution for 2.13, if we forget about 2.12, would be to look
at uses of {{scala.Seq}} individually.
Methods with a Seq parameter can continue to accept {{collection.Seq}} if that
makes sense, i.e., if the implementation can work fine with mutable Seqs. If
that's not the case and {{collection.Seq}} was used accidentally, the method
should probably be deprecated.
When used as result type, it would be better to replace it by {{immutable.Seq}}
if the result is actually immutable. A more precise type is good in general,
and it makes sure that the result is still compatible with {{scala.Seq}}, which
a user might expect, as noted by [~vanzin] above.
Unfortunately, changing the result type is *not* binary compatible, so it
cannot be done in a 2.12 minor release of Spark. If there will be separate
source files for 2.12 and 2.13, you could use a type alias that points to
{{collection.Seq}} in 2.12, and to {{immutable.Seq}} in 2.13, and use it in
these cases. Otherwise, the only solution is to use {{collection.Seq}}
everywhere.
Or maybe the Spark version that will actually cross-build with 2.13 does need
to be binary compatible on 2.12? In this case, using a more precise return type
would be good for 2.12 as well.
> Use scala.collection.Seq explicitly instead of scala.Seq alias
> --------------------------------------------------------------
>
> Key: SPARK-27681
> URL: https://issues.apache.org/jira/browse/SPARK-27681
> Project: Spark
> Issue Type: Sub-task
> Components: ML, MLlib, Spark Core, SQL, Structured Streaming
> Affects Versions: 3.0.0
> Reporter: Sean Owen
> Assignee: Sean Owen
> Priority: Major
>
> {{scala.Seq}} is widely used in the code, and is an alias for
> {{scala.collection.Seq}} in Scala 2.12. It will become an alias for
> {{scala.collection.immutable.Seq}} in Scala 2.13. In many cases, this will be
> fine, as Spark users using Scala 2.13 will also have this changed alias. In
> some cases it may be undesirable, as it will cause some code to compile in
> 2.12 but not in 2.13. In some cases, making the type {{scala.collection.Seq}}
> explicit so that it doesn't vary can help avoid this, so that Spark apps
> might cross-compile for 2.12 and 2.13 with the same source.
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