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From "vinoyang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (FLINK-9221) Add method SinkFunction[A]#contramap[B](f: B => A): SinkFunction[B]
Date Sun, 01 Jul 2018 09:04:00 GMT

    [ https://issues.apache.org/jira/browse/FLINK-9221?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16529020#comment-16529020
] 

vinoyang commented on FLINK-9221:
---------------------------------

[~joshlemer] I think we should not add this method to `SinkFunction` interface, it seems could
be implemented in a util class. What's your opinion? [~till.rohrmann] 

> 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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