[swift-evolution] Revisiting SE-0110

Paul Cantrell cantrell at pobox.com
Fri Jun 16 13:13:37 CDT 2017

> On Jun 15, 2017, at 7:17 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
> On Thu, Jun 15, 2017 at 19:03 Víctor Pimentel <vpimentel at tuenti.com <mailto:vpimentel at tuenti.com>> wrote:
> On 16 Jun 2017, at 01:55, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> On Thu, Jun 15, 2017 at 17:43 David Hart <david at hartbit.com <mailto:david at hartbit.com>> wrote:
>> By the way, I’m not attempting to deduce that nobody uses this feature by the fact I didn’t know about it. But I think it’s one interesting datapoint when comparing it to SE-0110.
>> SE-0110, **in retrospect**, has had impacts on a lot of users; prospectively, it was thought to be a minor change, even after review and acceptance.
>> Keep in mind that this proposed change would also eliminate inline tuple shuffle. For instance, the following code will cease to compile:
>> let x = (a: 1.0, r: 0.5, g: 0.5, b: 0.5)
>> func f(color: (r: Double, g: Double, b: Double, a: Double)) {
>>   print(color)
>> }
>> f(color: x)
>> It is an open question how frequently this is used. But like implicit tuple destructuring, it currently Just Works(TM) and users may not realize they’re making use of the feature until it’s gone.
> It's much much less used, by looking at open source projects I doubt that a significant portion of projects would have to change code because of this.
> The reason that I’m urging caution is because, if I recall correctly, that is also what we said about SE-0110 on this list. Then, as now, we were discussing an issue with something left over from the Swift 1 model of tuples. Then, as now, we believed that the feature in question was rarely used. Then, as now, we believed that removing that feature would improve consistency in the language, better both for the compiler and for users. Then, as now, leaving it in was thought to prevent moving forward with other features that could improve Swift.


I hacked up a regexp that will catch most uses of labeled tuples in pattern matches, e.g. “let (foo: bar) = baz”. That’s what we’re talking about, right?

I ran that against all 55 projects in swift-source-compat-suite, comprising about over 400,000 lines of Swift code, and found … drumroll … exactly one match:

neota (swift-source-compat-suite)$ find project_cache -name '*.swift' -print0 | xargs -0 pcregrep -M '(for|case|let|var|catch)\s+\([a-zA-Z0-9_]+\s*:'
project_cache/RxSwift/RxExample/RxExample-iOSTests/TestScheduler+MarbleTests.swift:                let (time: _, events: events) = segments.reduce((time: 0, events: [RecordedEvent]())) { state, event in

Caveats about this method:

• My regexp won’t match second and third patterns in a comma-separated let or case, e.g.:

   let a = b, (c: d) = e

• It doesn’t match non-ascii identifiers.

• This experiment only considers labeled tuples in pattern matches, what I took Chris’s original puzzler to be about. Label-based tuple shuffling is a separate question.

Still, even if it’s undercounting slightly, one breakage in half a million lines of code should put to rest concerns about unexpected widespread impact.

(Anything else I’m missing?)

• • •

Aside for those who know the tools out there: what would it take to run inspections like this against ASTs instead of using a regex? Could we instrument the compiler as Brent suggested? Or can SourceKit / SourceKitten give a full AST? Or has anybody written a Swift parser in Swift?



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