在reduce（）中使用扩展函数引用

Question

Suppose:

val sets = listOf(setOf(1, 2, 3), setOf(2, 3, 4), setOf(3, 4, 5))

Why is this illegal in Kotlin 1.2?

val unionOfSets = sets.reduce(Set<Int>::union)  // == setOf(1, 2, 3, 4, 5)

Shouldn't it be equivalent to this?

val unionOfSets = sets.reduce { acc, set -> acc.union(set) }

From https://kotlinlang.org/docs/reference/lambdas.html :

Non-literal values of function types with and without receiver are interchangeable, so that the receiver can stand in for the first parameter, and vice versa. For instance, a value of type (A, B) -> C can be passed or assigned where a A.(B) -> C is expected and the other way around

Answer 1

It appears that Kotlin doesn't do type inference successfully on the receiver, maybe because of all the templates (Kotlin doesn't have any guarantee on types).

But you can work around it by passing a reference to the invoke callable instead :

fun main(args: Array<String>) {
    val sets = listOf(setOf(1,2,3), setOf(4,5,6), setOf(7,8,9))
    val unionOfSets = sets.reduce(Set<Int>::union::invoke)

    println(unionOfSets)
}

Update : Here is the reason why sets.reduce(Set::union) doesn't work

Kotlin use a smart cast to resolve receivers (cf. https://github.com/JetBrains/kotlin/blob/143c3ccb95f93299233ade88c24b2fa2b9b29abf/compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/tower/PSICallResolver.kt at line 553)

But smart cast can work only if the type is guaranteed as mentionned in the documentation ( https://kotlinlang.org/docs/reference/typecasts.html#smart-casts ) :

Note that smart casts do not work when the compiler cannot guarantee that the variable cannot change between the check and the usage.

The compiler tries to find the right receiver (through a smart cast), but cannot do it because the compiler cannot guarantee the acc variable cannot change. This is due to the nested generic: you have a List<Set<Int>> which will be List<*> at runtime.

And that's why we can use invoke instead. Invoke is an operator method generated at build time and will be type safe (no smart cast needed).