[swift-evolution] Add stableSort() to the standard library.

Tyler Cloutier cloutiertyler at aol.com
Wed Jan 20 14:19:14 CST 2016

> On Jan 20, 2016, at 9:58 AM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> on Sun Jan 10 2016, Davide Italiano via swift-evolution <swift-evolution-m3FHrko0VLzYtjvyW6yDsg-AT-public.gmane.org <http://swift-evolution-m3fhrko0vlzytjvyw6ydsg-at-public.gmane.org/>> wrote:
>> Feedback very appreciated.
>> Add stableSort() to the standard library.
>> ####
>> Introduction
>> ####
>> At the time of writing the Swift standard library provides two way two
>> different versions of sort.
>> The first one is sort(), which provides a regular unstable[1] sorting algorithm.
>> The second one, sortInPlace(), which also provides unstable sorting with the
>> additional guarantee that the algorithm will operate in-place, i.e., it will
>> use only a constant amount of extra space (independent from the input size).
>> The aim of this proposal is to implement a third variant, stableSort(), in order
>> to provide a stable version of sorting.
> A few thoughts:
> 1. The C++ stable sort algorithm draws on just nearly every other
>   algorithm in the entire standard library, many of which we don't
>   currently have in Swift.  Doing something similar would expand the
>   algorithms we support, which IMO would be a good thing.
> 2. I am very intrigued with Python's argument that, for a
>   general-purpose audience, stable sorting should be the default.  It's
>   certainly more widely-useful than unstable sorting.
> 3. There are several interesting and fast stable sorting algorithms out
>   there, including Python's timsort; it would take some experimentation
>   to discover which one(s) to use for Swift.
> 4. I'm not a fan of adding a parameter to sort to select stability,
>   because of the added API complexity.  People who want a
>   faster-but-unstable sort can call unstableSort, right?

I think the potential here is that people might not even be aware there is such an option. unstableSort() not terribly autocomplete friendly which I think adds to complexity. It seems like a parameter would be in line with API’s like print(_:separator:terminator:), even though the print arguments are not constants. I think the print API is very intuitive, but perhaps I’m misunderstanding what you mean by complexity?


> -Dave
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