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https://issues.apache.org/jira/browse/FLINK-9221?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16531680#comment-16531680
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Josh Lemer commented on FLINK-9221:
-----------------------------------
[~yanghua]You may be right. One reason why it may be wrong to put the method on
`SinkFunction` is that the opposite methods (`map`, `flatMap`) are not on
`SourceFunction` or `MapFunction`, but instead are on `DataStream`. However,
there isn't really any other abstraction in flink where contramap can go on,
since there's not an equivalent `DataSink` abstraction like there is in Akka
Streams or similar, so putting it on SinkFunction may be the best option unless
we come up with a proper `DataSink` abstraction.
> 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
> Components: DataSet API, DataStream API
> Affects Versions: 1.5.0
> Reporter: Josh Lemer
> Assignee: vinoyang
> Priority: Minor
> Labels: flink
>
> 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]("...")
> .setBucketer(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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