The join function takes an array of objects and joins them with a provided separator. The following example presents a simple version of join:

func join<Element: Equatable>(elements: [Element], separator: String) -> String { 

    return elements.reduce("") { 
        initial, element in 
        let aSeparator = (element == elements.last) ? "" : separator 
        return "(initial)(element)(aSeparator)" 
    } 
} 

This function takes an array with a separator, joins elements in an array, and provides a single String. We can test it as follows:

let items = ["First", "Second", "Third"] 
let commaSeparatedItems = join(elements: items, separator: ", ") 

The result will be "First, Second, Third".