I have an open PR that does everything we can do for preparing the code base for Scala 2.12 without breaking the API: https://github.com/apache/flink/pull/6784
> On 8. Oct 2018, at 09:56, Chesnay Schepler <ches...@apache.org> wrote: > > I'd rather not maintain 2 master branches. Beyond the maintenance overhead I'm > wondering about the benefit, as the API break still has to happen at some > point. > > @Aljoscha how much work for supporting scala 2.12 can be merged without > breaking the API? > If this is the only blocker I suggest to make the breaking change in 1.8. > > On 05.10.2018 10:31, Till Rohrmann wrote: >> Thanks Aljoscha for starting this discussion. The described problem brings >> us indeed a bit into a pickle. Even with option 1) I think it is somewhat >> API breaking because everyone who used lambdas without types needs to add >> them now. Consequently, I only see two real options out of the ones you've >> proposed: >> >> 1) Disambiguate the API (either by removing >> reduceGroup(GroupReduceFunction) or by renaming it to reduceGroupJ) >> 2) Maintain a 2.11 and 2.12 master branch until we phase 2.11 completely out >> >> Removing the reduceGroup(GroupReduceFunction) in option 1 is a bit >> problematic because then all Scala API users who have implemented a >> GroupReduceFunction need to convert it into a Scala lambda. Moreover, I >> think it will be problematic with RichGroupReduceFunction which you need to >> get access to the RuntimeContext. >> >> Maintaining two master branches puts a lot of burden onto the developers to >> always keep the two branches in sync. Ideally I would like to avoid this. >> >> I also played a little bit around with implicit conversions to add the >> lambda methods in Scala 2.11 on demand, but I was not able to get it work >> smoothly. >> >> I'm cross posting this thread to user as well to get some more user >> feedback. >> >> Cheers, >> Till >> >> On Thu, Oct 4, 2018 at 7:36 PM Elias Levy <fearsome.lucid...@gmail.com> >> wrote: >> >>> The second alternative, with the addition of methods that take functions >>> with Scala types, seems the most sensible. I wonder if there is a need >>> then to maintain the *J Java parameter methods, or whether users could just >>> access the functionality by converting the Scala DataStreams to Java via >>> .javaStream and whatever the equivalent is for DataSets. >>> >>> On Thu, Oct 4, 2018 at 8:10 AM Aljoscha Krettek <aljos...@apache.org> >>> wrote: >>> >>>> Hi, >>>> >>>> I'm currently working on >>> https://issues.apache.org/jira/browse/FLINK-7811, >>>> with the goal of adding support for Scala 2.12. There is a bit of a >>> hurdle >>>> and I have to explain some context first. >>>> >>>> With Scala 2.12, lambdas are implemented using the lambda mechanism of >>>> Java 8, i.e. Scala lambdas are now SAMs (Single Abstract Method). This >>>> means that the following two method definitions can both take a lambda: >>>> >>>> def map[R](mapper: MapFunction[T, R]): DataSet[R] >>>> def map[R](fun: T => R): DataSet[R] >>>> >>>> The Scala compiler gives precedence to the lambda version when you call >>>> map() with a lambda in simple cases, so it works here. You could still >>> call >>>> map() with a lambda if the lambda version of the method weren't here >>>> because they are now considered the same. For Scala 2.11 we need both >>>> signatures, though, to allow calling with a lambda and with a >>> MapFunction. >>>> The problem is with more complicated method signatures, like: >>>> >>>> def reduceGroup[R](fun: (scala.Iterator[T], Collector[R]) => Unit): >>>> DataSet[R] >>>> >>>> def reduceGroup[R](reducer: GroupReduceFunction[T, R]): DataSet[R] >>>> >>>> (for reference, GroupReduceFunction is a SAM with void >>>> reduce(java.lang.Iterable<T> values, Collector<O> out)) >>>> >>>> These two signatures are not the same but similar enough for the Scala >>>> 2.12 compiler to "get confused". In Scala 2.11, I could call >>> reduceGroup() >>>> with a lambda that doesn't have parameter type definitions and things >>> would >>>> be fine. With Scala 2.12 I can't do that because the compiler can't >>> figure >>>> out which method to call and requires explicit type definitions on the >>>> lambda parameters. >>>> >>>> I see some solutions for this: >>>> >>>> 1. Keep the methods as is, this would force people to always explicitly >>>> specify parameter types on their lambdas. >>>> >>>> 2. Rename the second method to reduceGroupJ() to signal that it takes a >>>> user function that takes Java-style interfaces (the first parameter is >>>> java.lang.Iterable while the Scala lambda takes a scala.Iterator). This >>>> disambiguates the code, users can use lambdas without specifying explicit >>>> parameter types but breaks the API. >>>> >>>> One effect of 2. would be that we can add a reduceGroup() method that >>>> takes a api.scala.GroupReduceFunction that takes proper Scala types, thus >>>> it would allow people to implement user functions without having to cast >>>> the various Iterator/Iterable parameters. >>>> >>>> Either way, people would have to adapt their code when moving to Scala >>>> 2.12 in some way, depending on what style of methods they use. >>>> >>>> There is also solution 2.5: >>>> >>>> 2.5 Rename the methods only in the Scala 2.12 build of Flink and keep the >>>> old method names for Scala 2.11. This would require some infrastructure >>> and >>>> I don't yet know how it can be done in a sane way. >>>> >>>> What do you think? I personally would be in favour of 2. but it breaks >>> the >>>> existing API. >>>> >>>> Best, >>>> Aljoscha >>>> >>>> >>>> >>>> >
