Open Radar

 
Summary:
Adding an overload for `flatMap` that takes a function from `T -> U?` and then removes `.None` values resulting in value of the type `[U]` is super convenient to have in the standard lib, but _calling_ it `flatMap` is confusing and unnecessary. `flatMap` has a very specific meaning for a lot of people, and overloading it to mean something _completely different_ is adding a completely unneeded amount of complexity to a language trying to find its bearings.

If we keep the word `flatMap` limited to it’s application in other languages (cough Monads cough), we only have to learn/teach that version of `flatMap`. What you’ve introduced here is essentially `map` composed with what would be called `catMaybes` in Haskell (possibly `catOptionals` in Swift). `catOptionals` would be useful in the stdlib, and I can even see the use in creating a convenience function for `catOptionals . map`, but it _should not_ be overloading `flatMap`.

Steps to Reproduce:
Look in the standard lib for implementations of `flatMap` on `Array`

Expected Results:
Expect to see the common function signature of `[T] -> (T -> [U]) -> [U]`
Expect _not_ to see a new function with a different signature

Actual Results:
See a function with a signature closer to `catOptionals . map` instead of `flatten . map`

Regression:
This did not exist in Swift 1.2

Notes:
Provide additional information, such as references to related problems, workarounds and relevant attachments.