gmethvin commented on PR #2684:
URL: https://github.com/apache/pekko/pull/2684#issuecomment-3978723228
I'm open to either approach. The naming consistency of reclaiming
`futureSink` in 2.x is appealing. But I think it's worth being precise about
the behavioral difference, since "lazy vs eager" is also about side effects.
For value-producing sinks, the eager approach is arguably what the user
wants anyway:
```scala
// Sink.seq on an empty stream
Source.empty[Int]
.toMat(Sink.futureSink(Future.successful(Sink.seq[Int])))(Keep.right)
.run().flatten
// Lazy: Future fails with NeverMaterializedException
// Eager: Future succeeds with Seq.empty
// Sink.fold on an empty stream
Source.empty[Int]
.toMat(Sink.futureSink(Future.successful(Sink.fold(0)(_ + _))))(Keep.right)
.run().flatten
// Lazy: NeverMaterializedException
// Eager: Future.successful(0)
```
I would expect that these users are already handling the
`NeverMaterializedException` and mapping to an empty value, in which case that
just becomes dead code, but it's not a breaking change.
But for side-effecting sinks, eager evaluation changes observable behavior:
```scala
// FileIO sink on an empty stream
Source.empty[ByteString]
.toMat(Sink.futureSink(Future.successful(FileIO.toPath(path))))(Keep.right)
.run().flatten
// Lazy: no file touched, NeverMaterializedException
// Eager: empty file created on disk, IOResult(0, Success(Done))
```
Anyone using `futureSink` with side-effecting sinks on potentially empty
streams today might rely (intentionally or not) on the fact that the inner sink
is never materialized. With eager semantics, those side effects would now fire.
The question is how likely it is that someone is depending on this
difference and also ignores both the 1.x deprecation warning and the 2.x
migration guide. My guess is this is rare but not zero.
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