Is flatMap deprecated or not in Swift?

Question

print([ [1, 2], [3, 4]].flatMap{_ in })

Answer 1

One particular overload of flatMap has been deprecated. It's the flatMap method of Sequence with this signature:

func flatMap<ElementOfResult>(
    _ transform: (Self.Element) throws -> ElementOfResult?
) rethrows -> [ElementOfResult]

And it has been deprecated since Swift 4.1. You can learn more about why it was deprecated by watching episode 10 of Point-Free, “A Tale of Two Flat-Maps”. (That episode is free to watch.)

However, the compiler is forced to pick that deprecated overload of flatMap here.

For the non-deprecated version of flatMap, the transform function must return some type of Sequence. That's the whole point of (non-deprecated) flatMap: each element of the input is turned into some Sequence, and those Sequences are concatenated (flattened) into a single output array.

But your transform returns Void, and Void (which is an alias for the empty tuple, ()) is not a Sequence. So the compiler can't use the non-deprecated flatMap.

The compiler can, however, implicitly promote your transform to return an Optional<Void>, which then lets the compiler use the deprecated flatMap to compile the code.

The correct way to write your statement is to use map instead of flatMap, because you're returning a single value (()) rather than a Sequence:

[[1, 2], [3, 4]].map { _ in }