Does Kotlin support partial application?

Question

Since Kotlin supports many concepts from functional programming, I was wondering if there is a way to do partial application of a function in Kotlin as well?

One such example of where partial application can be useful is:

// In one class
fun doSomething(cb: (a, b) -> Unit) {
    <some logic here to compute someField>
    doSomethingElse(cb.applyPartially(someField))
}

// In another class
fun doSomethingElse(cb: (b) -> Unit) {
    <some logic here to compute someOtherField>
    cb(someOtherField)
}

Answer 1

Out of the box, no. But it isn't too hard to do with a helper function:

    fun add(a: Int, b:Int): Int {
        return a + b
    }

    fun <A, B, C> partial2(f: (A, B) -> C, a: A): (B) -> C {
        return { b: B -> f(a, b)}
    }

    val add1 = partial2(::add, 1)

    val result = add1(2) //3

So partial2 takes a function of 2 arguments and the first argument and applies it to get a function of 1 argument. You would have to write such helpers for all arities you need.

Alternatively, you can do it with an extension method:

fun <A,B,C> Function2<A,B,C>.partial(a: A): (B) -> C {
    return {b -> invoke(a, b)}
}

val abc: (Int) -> Int = (::add).partial(1)

Answer 2

There is a very nice and light library to Kotlin: org.funktionale. In module funktionale-currying you'll find extenstion methods to lambdas: curried() and uncurried().

Example:

val add = { x: Int, y: Int -> x + y }.curried()
val add3 = add(3)

fun test() {
    println(add3(5)) // prints 8
}