[swift-evolution] [SR-933] Rename flatten to flattened

[Haravikk]

> On 7 Apr 2016, at 19:12, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> 
> We kept flatten as is because it is part of a suite of methods that are terms of art from functional programming (map, filter, flatMap, reduce) that don't follow the naming guidelines

I think it makes sense to just rename them; it’s not as though flattened or mapped is somehow far removed from the original meaning as the actual action is the same, it’s just changing tense.

For users coming to Swift as a first language I think that consistent naming is more important, and while terms of art are useful, it’s not as if changing the tense is changing the meaning or making them hard to find; anyone that types myArray.map… will have Xcode present them with .mapped. If we want mutating forms of this methods then I much prefer .map() and .mapped() to .map() and .mapInPlace() or whatever, as the latter contradicts the naming convention used everywhere else which only adds confusion.

Besides, I’d say the terms of art in these cases are more about what the basic operation does, whether or not the method manipulates the original is something that a developer should really check, i.e- when I look for .map in a new language I’m looking for a transformation method, I’ll then check what exactly that transformation does to the original collection. Plus features like unused result and type checks should keep people right about wether they’re using it correctly, so any mistake should be caught pretty easily.