Kotlin and RxJava - Why is my Single.zip() not compiling?

Question

I'm going a little crazy here. I'm trying to create an Observable<BigDecimal> extension function (against RxJava 2.x) to emit the average of the emissions, but I'm getting a compile error with the Single.zip() function. Does anybody have any ideas what I'm doing wrong? I tried to be explicit with all my types too and that didn't work...

import io.reactivex.Observable
import io.reactivex.Single
import java.math.BigDecimal


fun Observable<BigDecimal>.sum() = reduce { total, next -> total + next }

//compile error
fun Observable<BigDecimal>.average() = publish().autoConnect(2).let {
    Single.zip(it.sum().toSingle(), it.count()) {
        sum, count -> sum / BigDecimal.valueOf(count)
    }
}

Ethnology

When asking about an error, post the error. – Thrall

I was going to do that but forgot, will update later for reference to others. – Ethnology

What's the error?? – Rolypoly

Added screen clip above with error popup. – Ethnology

Answer 1

Type inference mostly does not work for rxJava2. It's not a type inference problem actually. Kotlin usually generates extension methods to that replaces SAM with kotlin functional types, but this technic does not work for overridden methods for some reason.

More details here https://youtrack.jetbrains.com/issue/KT-13609

As an option, you could try to specify types for lambda arguments

fun Observable<BigDecimal>.average() = publish().autoConnect(2).let {
    Single.zip(it.sum().toSingle(), it.count(), BiFunction {
        sum: BigDecimal, count: Long ->
        sum / BigDecimal.valueOf(count)
    })
}

Unitary

Thanks, giving answer to Stepan since it maximizes inference. – Ethnology

What if i have three or more Singles in the zip, BiFunction doesn't seem to work since it takes only two – Tearing

@IzzoObella, in this case, you'll need to use Function3 instead of BiFunction – Unitary

Its okay, i got a solution by using RxKotlin which has an extension function for that – Tearing

nice. much cleaner looking that writing out the whole BiFunction declaration including generics like this. BiFunction<CurrencyData,CryptoCurrency, Pair<CurrencyData,CryptoCurrency>>{ a, b -> Pair(a,b)}) – Abstention

Answer 2

Type inference is failing for some reason, there must be somehow multiple combinations of types that could be inferred in this context.

You can specify the types explicitly with a more traditional (and unfortunately more verbose) syntax, like this:

fun Observable<BigDecimal>.average() = publish().autoConnect(2).let {
    Single.zip(it.sum().toSingle(), it.count(), BiFunction<BigDecimal, Long, BigDecimal> {
        sum, count ->
        sum / BigDecimal.valueOf(count)
    })
}

Update:

I've just found out while working on a similar problem that the actual problem here is that Kotlin isn't able to infer which Single.zip overload you're trying to call. From the official documentation:

If the Java class has multiple methods taking functional interfaces, you can choose the one you need to call by using an adapter function that converts a lambda to a specific SAM type. Those adapter functions are also generated by the compiler when needed.

So it turns out that using the more explicit SAM constructor solves this in itself, and gives you type inference back (basically, my previous answer was using a longer syntax than was actually required):

fun Observable<BigDecimal>.average(): Single<BigDecimal> = publish().autoConnect(2).let {
    Single.zip(it.sum().toSingle(), it.count(), BiFunction {
        sum, count ->
        sum / BigDecimal.valueOf(count)
    })
}

Aguascalientes

Ew... Okay I'll run this tomorrow and follow up if it works. If this can't be inferred that is unfortunate. – Ethnology

Hey, I've just updated my answer with a better solution. – Aguascalientes

Answer 3

If type inferencing is the problem, one thing you can do is use RxKotlin

implementation "io.reactivex.rxjava2:rxkotlin:$rxKotlinVersion"

RxKotlin specifically provides SAM helpers to help mitigate the issues with type inferencing issues.

In which case,

Singles.zip(..., ...)

would be able to work just fine without any ambiguity. Notice I am using Singles and not Single

Zulemazullo

Is it possible to use Singles.zip(..., ...) when I don't know the number of Singles (having them in a List)? – Misguided

@Misguided i think i am going to need some more context, it sounds like it would warrant its own question – Zulemazullo

I have N urls, I need to make N network calls and merge the result. I've created Singles and put them in the list. I couldn't use Singles.zip(..., ...), because it accepts exact number of Single objects. I finally solved it by using <T, R> Single<R> zip(final Iterable<? extends SingleSource<? extends T>> sources, Function<? super Object[], ? extends R> zipper) from RxJava2 library. Thanks anyway. ;-) – Misguided