aalexandrov edited a comment on pull request #11895:
URL: https://github.com/apache/flink/pull/11895#issuecomment-808167845
Just to clarify - after building a scala-shell with the changes from this
PR, and testing as follows:
1. Start scala-shell:
```bash
bin/start-scala-shell.sh local
```
2. Try the following Scala code:
```scala
scala> case class Pair(x: Int, y: Int)
defined class Pair
scala> benv.fromElements(1,2,3).map(x => Pair(x,x)).collect()
java.lang.IllegalArgumentException: requirement failed:
The class Pair is an instance class, meaning it is not a member of a
toplevel object, or of an object contained in a toplevel object,
therefore it requires an outer instance to be instantiated, but we don't
have a
reference to the outer instance. Please consider changing the outer class to
an object.
at scala.Predef$.require(Predef.scala:277)
at
org.apache.flink.api.scala.typeutils.ScalaCaseClassSerializer$.lookupConstructor(ScalaCaseClassSerializer.scala:97)
at
org.apache.flink.api.scala.typeutils.ScalaCaseClassSerializer.<init>(ScalaCaseClassSerializer.scala:46)
... 70 elided
```
The problem is that with `-Yrepl-class-based` enabled, UDTs (such as case
classes) are defined within a nested class, and as such cannot be easily
serialized (the `ScalaCaseClassSerializer` invoked by the
`org.apache.flink.api.scala.createTypeInformation` throws a very detailed error
explaining this).
I am not sure what were the use-cases that lead the Scala team (and Spark)
to adopt `-Yrepl-class-based` over the traditional object-based wrapping - it
seems that nesting objects in objects is more robust w.r.t. serialization.
FYI in Scala 2.13 the default is class-based and it needs to be explicitly
disabled with `-Yrepl-class-based:false`.
@aljoscha / @zjffdu: do you know any particular scenarios where object-based
REPL is problematic?
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