What is a lazy sequence?

Swift version: 5.1

Paul Hudson @twostraws May 28th 2019

Lazy sequences are regular sequences where each item is computed on demand rather than up front. For example, consider this array of numbers:

let numbers = Array(1...100000)

That will hold 100,000 numbers. Now, if we wanted to double all those numbers, we’d write something like this:

let doubled = numbers.map { $0 * 2 }

That will cause Swift to double all 100,000 numbers, and sometimes that’s exactly what you want. However, if you know you intend to use only a handful of them, you can make the calculation lazy instead, like this:

let lazyDoubled = numbers.lazy.map { $0 * 2 }

Now that map() call won’t do any work up front – it just stores the original array (numbers 1 to 100,000) alongside the transformation closure (double each number). So, when you request item 5,000 it can calculate just that one for you and return it in a split second – a significant time saving.