It is also possible to develop a filter function in terms of reduce:

func filterIntermsOfReduce<Element>(elements: [Element], predicate: (Element) -> Bool) -> [Element] { 

    return reduce(elements: elements, initial: []) { 
        predicate($1) ? $0 + [ $1 ] : $0 
    } 
} 

let result = filterIntermsOfReduce(elements: numbers) { $0 % 2 == 0 } 

Again, the result is identical to our previously developed filter function.

In the function body, we provide elements, an empty initial array, and finally, predicate as a combinator.