How to use flatMap() with an optional value

Swift version: 5.1

Paul Hudson    @twostraws    May 28th 2019

The flatMap() method of optionals allows you to transform the optional if it has a value, or do nothing if it is empty. This makes for shorter and more expressive code than doing a regular unwrap, and doesn’t require you to change your data type.

Using flatMap() with optionals is similar to using map(), with one important difference: if your transformation closure returns an optional, flatMap() will combine that optional with the existing optional, whereas map() will keep them both.

Here’s a practical example so you can see the difference:

let stringNumber: String? = "5"
let intNumber = stringNumber.map { Int($0) }

When that code runs, intNumber will be an Int?? – an optional optional integer. This is because we already have optionality from stringNumber, and the Int initializer from a string also returns an optional, so map() just puts them together.

In comparison, flatMap() acts differently:

let flatMapNumber = stringNumber.flatMap { Int($0) }

That will return a regular Int?, meaning that either the whole thing exists or nothing exists – it’s easier to work with.

Available from iOS – learn more in my book Pro Swift

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