It's not terrible that I have to use SAM syntax, and most of the time I can take a Kotlin function i...

# announcements

[Tim McCormack]

It's not terrible that I have to use SAM syntax, and most of the time I can take a Kotlin function instead; this is just me being curious about why the compiler is preventing the trailing lambda call in one place vs. another.

[karelpeeters]

https://kotlinlang.org/docs/reference/java-interop.html#sam-conversions

Also note that this feature works only for Java interop; since Kotlin has proper function types, automatic conversion of functions into implementations of Kotlin interfaces is unnecessary and therefore unsupported.

[karelpeeters]

However it's going to start working for Kotlin interfaces too in 1.4, see https://blog.jetbrains.com/kotlin/2019/12/what-to-expect-in-kotlin-1-4-and-beyond/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+kotlin+%28Project+Kotlin%29

[Tim McCormack]

I don't think this is the same thing. I'm not implementing a Kotlin interface. Function is from the Java stdlib.

[Tim McCormack]

(I've also looked at KT-7770, which again is only about Kotlin interfaces with a single abstract method.)

[Tim McCormack]

It makes me wonder if there's special-casing for the Streams API + Function, or something like that.

[Tim McCormack]

And that announcement you linked is also about defining your own interfaces, which is not what's going on in that code sample.

[Tim McCormack]

(although I do think that's good news)

[karelpeeters]

I thought you were accidentally using the Kotlin Function interface, my bad.

[karelpeeters]

It's not the variance either, f: Function<in Int, out Int> still doesn't work.

[Tim McCormack]

It's weird, right?

[Tim McCormack]

I've had a devil of a time asking people about this because everyone thinks I'm talking about KT-7770. 😆

[karelpeeters]

Yeah I apologize for annoying you even more :).

[karelpeeters]

It looks like Comparator.thenComparing works too, but that could just be special cased as well.

[Tim McCormack]

If I make a library that exposes something like takesSAM, it doesn't work. But that's still a Kotlin-compiled library, so maybe there's some trace of that in the compiled code...

[Tim McCormack]

I'll have to find a java library that takes Function and see what the compiler says about calling that.

[Tim McCormack]

Ah... Google Guava has some, and I can't use trailing lambda with those! So I think the Java stdlib is special-cased. Example:

ImmutableTable.toImmutableTable(
        Function<Int, Int> { it * 2 },
        Function<Int, Int> { it * 2 },
        Function<Int, Int> { it * 2 }
    )

[karelpeeters]

The only mention I could find in the compiler is this, but I'm not sure if it's related at all: https://github.com/JetBrains/kotlin/blob/master/core/descriptors.jvm/src/org/jetbrains/kotlin/load/java/FakePureImplementationsProvider.kt#L37

[karelpeeters]

I'll try setting up the compiler for debugging tomorrow, should be interesting.

[Tim McCormack]

Update: I've been told this is actually https://youtrack.jetbrains.com/issue/KT-11129