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.Predicate
import arrow.syntax.function.complement


fun 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.