Higher-order functions in Kotlin

As a Higher order functions In Kotlin, those functions are called that other functions accept as parameters or deliver functions as results. They are an important part of functional programming and offer greater flexibility and abstraction when developing programs.

Below are some examples of higher order functions in Kotlin.

Example 1: Function as a parameter

fun main() {
    val resultAddition = calculation(10, 2, ::addNumbers)
    val resultSubstruction= calculation(10, 2, ::subtractNumbers)

    println("Result of addition: $resultAddition")
    println("Result of substruction: $resultSubstruction")
}

fun calculation(a: Int, b: Int, mathOperation: (Int, Int) -> Int): Int {
    return mathOperation(a, b)
}

fun addNumbers(a: Int, b: Int): Int = a + b
fun subtractNumbers(a: Int, b: Int): Int = a - b

In this example, function is called calculating defined, the two Int-arguments a and b as well as a function mathOperation with the same argument types and a Int-Return type expected.

The functions addNumbers and subtractNumbers are simple functions that accept two integers as parameters and return their sum or difference.

Within the main()-Function becomes the function calculating called twice. The first time the function is called addNumbers as an argument using the reference operator :: pass the two numbers 10 and 2 to add. The reference operator :: also called a “member reference operator” and is used to reference a function or object without executing it directly. The second time the function is called subtractNumbers Passed as argument to the two numbers 10 and 2 to subtract. 

Example 2: Function as return value

fun main() {
    val addition = createMathOperation("add")
    val subtraction = createMathOperation("subtract")

    println("Result of addition: ${addition(5, 3)}")
    println("Result of subtraction: ${subtraction(5, 3)}")
}

fun createMathOperation(chosenOperation: String): (Int, Int) -> Int {
    return when (chosenOperation) {
        "add" -> { a, b -> a + b }
        "subtract" -> { a, b -> a - b }
        else -> throw IllegalArgumentException("Invalid operation")
    }
}

In this example the function returns createMathOperation returns a function based on the passed string chosenOperation based. The resulting functions add and abstract can then be used to perform calculations.

Higher-order functions are widely used in Kotlin and make it possible to develop flexible and abstract solutions to various problems.