Mastering High Performance with Kotlin

Declarative versus imperative

With functional programming, you can write code that allows you to concentrate on business logic instead of specific implementations. The following example is written in an imperative style:

val numbers = listOf(1, 2, 3, 4, 5, 6, 7)val odds = ArrayList<Int>()for (i in 0..numbers.lastIndex) {    val item = numbers[i]    if (item % 2 != 0) {        odds.add(item)    }}

If we rewrite this code in a declarative style, it will look as follows:

val numbers = listOf(1, 2, 3, 4, 5, 6, 7)val odds = numbers.filter { it % 2 != 0 }

This code describes what it does instead of how it does it, making it easier to concentrate on the main logic of the application.