Github user aljoscha commented on the pull request:
https://github.com/apache/flink/pull/1723#issuecomment-189462738
The `PassThroughFunction` is created because at that point it is not yet
clear what type of `InternalWindowFunction` will be created. I can fix it by
duplicating all the WindowOperator creation code, but this applies to
reduce/fold both on the Java and Scala API.
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