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Re: [DISCUSS] Breaking the Scala API for Scala 2.12 Support


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.lucidity@xxxxxxxxx> 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 <aljoscha@xxxxxxxxxx> 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
>
>
>
>