pjfanning opened a new pull request, #3717:
URL: https://github.com/apache/pekko/pull/3717
### Motivation
Most stream javadsl operators take invariant `japi.function` types, so a
Java caller cannot pass an existing `Function<Object, String>`,
`Predicate<Number>` or `Function2<Number, Number, Integer>` where the element
or result type differs, even though the call is type safe. `java.util.stream`
uses `Function<? super T, ? extends R>`, `Predicate<? super T>` and `Supplier<?
extends T>` throughout, and #3713/#3714 already widened part of the javadsl,
leaving the remaining operators inconsistent (see the follow-up comment on
#3714).
Inline lambdas and method references already adapt to the target type, so
the practical gain is for function values that are already typed (fields,
helpers, functions from other APIs); the main argument is consistency with JDK
conventions.
### Modification
- Across the stream javadsl (`Source`, `Flow`, `Sink`, `SubFlow`,
`SubSource`, `FlowWithContext`, `SourceWithContext`, `BidiFlow`, `Graph`,
`Hub`, `Restart*`, `RetryFlow`, `Tcp`, `TLS`, `StreamConverters`, `Framing`),
widen method parameters:
- `Function[A, B]` → `Function[? >: A, ? <: B]`
- `Function2[A, B, C]` → `Function2[? >: A, ? >: B, ? <: C]`
- `Creator[T]` → `Creator[? <: T]`
- `Predicate[T]` → `Predicate[? >: T]`
- Type arguments that already had a wildcard are unchanged, and no bound is
added where it cannot widen anything (e.g. `? <:` on final types such as
`Boolean`, `String` or `NotUsed`).
- Add explicit type arguments at the few internal call sites where Scala can
no longer infer them (`combinerToScala`, `Flow.fromFunction`).
- Add MiMa excludes (`IncompatibleSignatureProblem` only) for the generic
signature changes.
### Result
Java callers can pass function values typed over a supertype of the input or
a subtype of the result without casts. Erasure is unchanged, so the change is
binary compatible.
### Tests
- `sbt "stream/compile" "++3.3.8 stream/compile"` - passed
- `sbt Test/compile` for `stream-tests`, `stream-typed`, `stream-testkit`,
`persistence-query`, `persistence-testkit`, `persistence-typed-tests` (Scala
2.13) - passed
- `sbt "stream/mimaReportBinaryIssues"` - passed with the added excludes
- New directional tests (`mustAcceptFunctionsOverWiderAndNarrowerTypes` in
`SourceTest`/`FlowTest`/`SinkTest`,
`mustAcceptConcreteSourceCreatorsInRestartSource`) not run locally - left to
CI, along with `docs`/`cluster-sharding-typed` compilation and Scala 3 test
compilation
### References
Refs #3712, #3713, #3714
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