Swift Functional Programming - Second Edition

The flatMap function in terms of reduce

To understand the power of reduce, we can implement the flatMap function in terms of reduce as well:

func flatMapIntermsOfReduce<Element>(elements: [Element], transform: (Element) -> Element?) -> [Element] {   return reduce(elements: elements, initial: []) {         guard let transformationResult = transform($1) else {             return $0         }         return $0 + [transformationResult]     } } let anArrayOfNumbers = [1, 3, 5] let oneDimensionalArray = flatMapIntermsOfReduce(elements: anArrayOfNumbers) { $0 + 5 } 

Also, we can flatten a two-dimensional array using the reduce method as follows:

func flatMapIntermsOfReduce<Element>(elements: [[Element]], transform: (Element) -> Element) -> [Element] {  return elements.reduce([]) ...