[jira] [Created] (FLINK-9221) Add method SinkFunction[A]#contramap[B](f: B => A): SinkFunction[B]

["Josh Lemer (JIRA)" <ji...@apache.org>]

Josh Lemer created FLINK-9221:
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             Summary: Add method SinkFunction[A]#contramap[B](f: B => A): SinkFunction[B]
                 Key: FLINK-9221
                 URL: https://issues.apache.org/jira/browse/FLINK-9221
             Project: Flink
          Issue Type: Task
            Reporter: Josh Lemer


Just like it is very useful to use `DataStream[T]` as a sort of Functor or Monad with `map`/`flatMap`/`filter` methods, it would be extremely handy to have a `SinkFunction[A]#contramap[B](f: B => A): SinkFunction[B]` on `SinkFunctions`, so that you can reuse existing complex sink functions, but with a different input type. For example:
{code}
val bucketingStringSink: SinkFunction[String] = 
  new BucketingSink[String]("...")
    .setBucketr(new DateTimeBucketer("yyyy-MM-dd-HHmm")

val bucketingIntListSink: SinkFunction[List[Int]] =
  bucketingStringSink.contramap[List[Int]](_.mkString(","))
{code}

For some more formal motivation behind this, https://typelevel.org/cats/typeclasses/contravariant.html is definitely a great place to start!



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