Add sort overload to Sequence to sort by a Comparable attribute of elements

[xwu (Xiaodi Wu)]

Hi! Something like this has been discussed several times. You might find the following pitch to be edifying:

Map Sorting

Map Sorting Authors: Anthony Latsis, Cal Stephens Status: Awaiting Review Implementation: apple/swift#23156 Introduction This proposal presents an addition to the Standard Library that makes it easy to sort a collection over some set of mapped values, provided via a transform closure or KeyPath, in a way that is ergonomic, idiomatic, and performant. Motivation To sort a Collection, you're currently required to specify an areInIncreasingOrder predicate of the form (Element, Element) -> B…

Similarly:

Sort `Collection` using `KeyPath`

Hey everybody — I'd like to pitch and socialize a new method for sorting Collections using a KeyPath. Today, to sort a Collection of non-Comparable elements, we write a closure that returns whether or not two elements areInIncreasingOrder: [[1, 2, 3], [1], [1, 2]].sorted(by: { $0.count < $1.count }) // returns [[1], [1, 2], [1, 2, 3]] This has a few features that you could consider "pain points": The call site doesn't form a grammatical english phrase. [1] It's relatively easy to mix up < …

In the latter thread, @Ben_Cohen (from the core team) writes:

I think there is a lot of room for ergonomic improvements to sorting, so it's great to see this!

I would suggest, though, taking a step back and thinking about all the different aspects of this as a a whole. Individual targeted fixes are nice, but risk API sprawl when they handle some cases not others, leading to other targeted fixes.

Some topics to consider:

• key paths vs closures
  • note, localized/caseless sorting is also painful, but at least doable, with closure-based arguments
  • including weighing the pros/cons of parametrizing with ascending/descending choice (not a technique that sits well with the existing idioms used in the std lib)
• sorting by a sequence of comparators, lexicographically
• partial sorting/partitioning

A great place to start would be a comparison with the existing APIs to be found in other places i.e. Java, Ruby, Cocoa (which has all sorts of different ways to sort, sortUsingDescriptors being particularly of interest).