Understanding functions as first-class citizens

Since its first release, Swift has been a multiparadigm programming language, and one of its supported programming paradigms is functional programming. Functional programming favors immutable data and, therefore, avoids state changes. The code written with a functional programming style is as declarative as possible, and it is focused on what it does instead of how it must do it.

As it happens in many modern programming languages, functions are first-class citizens in Swift 3. You can use functions as arguments for other functions or methods. We can easily understand this concept with a simple example: array filtering. However, take into account that we will start by writing imperative code with functions as first-class citizens, and then, we will create a new version for this code that uses a functional approach in Swift through a filter operation.

The following lines declare the applyFunctionTo function that receives an array of Int, numbers...