Kotlin fold / reduce equivalent for multis

[MartinX3]

Or is it need to convert the Multi into a Uni<List<T>> first?

[jponge]

Do you have more context on what you are trying to achieve?

[jponge]

/cc @heubeck who's our Kotlin maintainer 😉

[Ladicek]

Perhaps stupid question, but isn't this maybe an ask for a little bit of streamlining of Multi.collect().in()? Managing the accumulator object is rather clunky in the common case, where you just want to write fold(initial, (accumulator, item) -> somehowCombine(accumulator, item)).

Something like this would be enough:

interface Multi<T> {
    ...

    default Uni<T> fold(T initialValue, BiFunction<T, T, T> function) {
        class Box {
            T value;

            Box(T value) {
                this.value = value;
            }
        }

        return collect().in(
                () -> new Box(initialValue),
                (accumulator, item) -> accumulator.value = function.apply(accumulator.value, item)
        ).map(box -> box.value);
    }

    ...
}

If I understand correctly, the Kotlin interop layer with Java would allow calling this in a Kotlin idiomatic way.

Obviously some more work would have to be put into this (perhaps move under collect(), null handling, documentation, testing), but I think this is what @MartinX3 asks for.

[MartinX3]

Thank you.

Googling for interop layer gave me this result
https://developer.android.com/kotlin/interop

Maybe it will help creating one for Smallrye?

[Ladicek]

I meant https://kotlinlang.org/docs/java-interop.html when I said "Kotlin interop layer with Java". That is, what would be multi.fold(0, (acc, i) -> acc + i) in Java could be written as multi.fold(0) { acc, i -> acc + i } in Kotlin with no extra work on any side.

[jponge]

Not sure you need a Box class in the sample above.