Breaking the API (or not breaking it but requiring explicit types when using Scala 2.12) and the Maven infrastructure to actually build a 2.12 release.
> On 8. Oct 2018, at 13:00, Chesnay Schepler <ches...@apache.org> wrote: > > And the remaining parts would only be about breaking the API? > > On 08.10.2018 12:24, Aljoscha Krettek wrote: >> 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 >>>>>> >>>>>> >>>>>> >>>>>> >> >
