Functional Kotlin

Logical complement

A logical complement takes any predicate (a function with a return Boolean type) and negates it. Let's look at the following code:

import arrow.core.Predicateimport arrow.syntax.function.complementfun main(args: Array<String>) {   val evenPredicate: Predicate<Int> = { i: Int -> i % 2 == 0 }   val oddPredicate: (Int) -> Boolean = evenPredicate.complement()   val numbers: IntRange = 1..10   val evenNumbers: List<Int> = numbers.filter(evenPredicate)   val oddNumbers: List<Int> = numbers.filter(oddPredicate)   println(evenNumbers)   println(oddNumbers)}

Notice that we use a Predicate<T> type, but it is just an alias for (T) -> Boolean. There are complement extension functions for predicates from 0 to 22 parameters.