Gold Challenge
This challenge is unique in that it does not have a specific problem or solution. Instead, it is an encouragement to spend some time reading interfaces written by the Swift team at Apple.
You first encountered the map(_:) method in Chapter 13, where you called it on arrays.
Then in Chapter 19, you called map(_:) on a range constructed with the ..< operator.
Both of these were actually calling map(_:) methods defined in protocol extensions by the Swift standard library.
The Swift standard library contains a large number of properties and methods provided by protocol extensions.
Many of them also include where clauses that restrict their use based on various criteria.
Remember that you can Command-click on a type, ...
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