Building blocks of functional programming in Swift

The first thing to realize is that Swift is not a functional programming language. At its core, it will always be an object-oriented programming language. However, since functions in Swift are first-class citizens, we can use some of the core techniques. Swift provides some built-in methods to get us started.

Filter

The first method we are going to discuss is called filter. As the name suggests, this method is used to filter elements in a list. For example, we can filter our numbers array to include only even numbers:

var evenNumbers = numbers.filter({ element in
    element % 2 == 0
}) // [2, 4]

The closure we provide to filter will be called once for each element in the array. It is tasked with returning true if the element needs to be included in the result and false otherwise. The preceding closure takes advantage of the implied return value and simply returns true if the number has a remainder of zero when being divided by two.

Note that...