The flatMap() method transforms optionals and arrays

Paul Hudson      @twostraws

The flatMap() method is designed to allow you to transform optionals and elements inside a collection while also decreasing the amount of containment that happens. For example, if you transform an optional in a way that will also return an optional, using map() would give you an optional optional (a double optional), whereas flatMap() is able to combine those two optionals into a single optional.

let lengthOfFirst = names.first.flatMap { count($0) }

Decreasing the amount of containment also makes flatMap() a simple way of converting multi-dimensional arrays into single-dimensional arrays:

[[1, 2], [3, 4], [5, 6]].flatMap { $0 }

There is no "map" operation there, so we're just left with the flattening behavior – that will result in a single array containing the value [1, 2, 3, 4, 5, 6].

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