#75: map vs flatMap vs compactMap

These three methods are easy to confuse, but their jobs are different.

[1, 2, 3].map { $0 * 2 }
[[1, 2], [3]].flatMap { $0 }
["1", "x", "3"].compactMap(Int.init)

map transforms each element. flatMap flattens nested sequences. compactMap transforms and drops nil results.

If you keep those three ideas in mind, choosing the right one becomes much easier.