Kotlin: how to pass a function as parameter to another?

Question

Given function foo :

fun foo(m: String, bar: (m: String) -> Unit) {
    bar(m)
}

We can do:

foo(\"a message\", { println         


        

Answer 1

About the member function as parameter:

1. Kotlin class doesn't support static member function, so the member function can't be invoked like: Operator::add(5, 4)
2. Therefore, the member function can't be used as same as the First-class function.
3. A useful approach is to wrap the function with a lambda. It isn't elegant but at least it is working.

code:

class Operator {
    fun add(a: Int, b: Int) = a + b
    fun inc(a: Int) = a + 1
}

fun calc(a: Int, b: Int, opr: (Int, Int) -> Int) = opr(a, b)
fun calc(a: Int, opr: (Int) -> Int) = opr(a)

fun main(args: Array<String>) {
    calc(1, 2, { a, b -> Operator().add(a, b) })
    calc(1, { Operator().inc(it) })
}

Answer 2

First-class functions are currently not supported in Kotlin. There's been debate about whether this would be a good feature to add. I personally think they should.

Answer 3

Kotlin 1.1

use :: to reference method.

like

    foo(::buz) // calling buz here

    fun buz() {
        println("i am called")
    }

Answer 4

Jason Minard's answer is a good one. This could also be achieved using a lambda.

fun foo(m: String, bar: (m: String) -> Unit) {
    bar(m)
}

val buz = { m: String ->
    println("another message: $m")
}

Which can be called with foo("a message", buz).

You can also make this a bit more DRY by using a typealias.

typealias qux = (m: String) -> Unit

fun foo(m: String, bar: qux) {
    bar(m)
}

val buz: qux = { m ->
    println("another message: $m")
}