Map, FlatMap, Reduce & More


released Wed, 20 Feb 2019
Swift Version 5.0


Reduce is sort of a generalized version of map, compactMap, flatMap, or filter. The basic idea is to reduce a sequence into a different shape utilizing an accumulator that can keep incremental state. To do this, we hand the function a combinator closure/function/method which is called once for each item in the sequence. This may sound complicated but becomes really easy with a couple of examples.

It is a method on SequenceType and looks like this (simplified):

func reduce<T>(_ initialResult: T, _ nextPartialResult: (T, Self.Generator.Element) -> T) -> T

There're two parameters here:

  1. A initialResult value of generic type T
  2. A closure nextPartialResult that receives two parameters and returns the generic type T. The two parameters are T, once again, and the element type of the array. Imagine we had an array of String and we'd like to reduce it to the count of elements Int. Reduce would look like this:
func reduce(_ initialResult: Int, _ nextPartialResult: (Int, String) -> Int) -> Int

So here we have an initial value Int, and we have a closure which expects us to return the same type as the initial value (Int). The final value of the operation is also of the same type as the initial value.

If we take a very simple reduce operation - counting the elments in an array of String - , the evaluation will look like this:

func count(accumulator: Int, current: Int) -> Int {

    return accumulator += 1


[1, 2, 3].reduce(0, count)

// The following steps will be performed

count(0, 1) { return 0 + 1 } = 1

count(1, 2) { return 1 + 1 } = 2

count(2, 3) { return 2 + 1 } = 3

= 6

And if we want to sum up the integer elements in an array of Int, we'd write this:

func sum(accumulator: Int, current: Int) -> Int {

    return accumulator + current


[1, 2, 3].reduce(0, sum)

// The following steps will be performed

sum(0, 1) { return 0 + 1 } = 1

sum(1, 2) { return 1 + 2 } = 3

sum(3, 3) { return 3 + 3 } = 6

= 6

The nextPartialResult closure (sum in our example) will be called once for each item in the list [1, 2, 3]. The state will be kept in the accumulator variable which is just an Integer.

Let's start re-implementing some of our other, trusted, functional programming friends. In order to keep things simple for now, all these functions will operate on Int or Optional<Int>; i.e. we will ignore generics in here. Also, keep in mind that the implementations below exist to explain the behaviour of reduce. The native Swift implementations are usually much faster compared to the reduce versions below. Reduce shines in a different set of problems, which will be explained further below.