Keep in mind that once the latest proposal about enum cases is implemented, these will be at least notionally no longer tuple labels but rather a sugared way of spelling part of the case name. The rules surrounding labels during case matching have only just been revised and approved and have not even yet been implemented. I don’t think it would be wise to fiddle with them again.
On Fri, Jun 16, 2017 at 21:21 Paul Cantrell <cantr...@pobox.com> wrote: > On Jun 16, 2017, at 5:23 PM, Mark Lacey <mark.la...@apple.com> wrote: > > > On Jun 16, 2017, at 2:09 PM, Paul Cantrell <cantr...@pobox.com> wrote: > > > On Jun 16, 2017, at 3:43 PM, Mark Lacey <mark.la...@apple.com> wrote: > > > On Jun 16, 2017, at 1:21 PM, Mark Lacey <mark.la...@apple.com> wrote: > > > On Jun 16, 2017, at 11:13 AM, Paul Cantrell via swift-evolution < > swift-evolution@swift.org> wrote: > > > On Jun 15, 2017, at 7:17 PM, Xiaodi Wu via swift-evolution < > swift-evolution@swift.org> wrote: > > > On Thu, Jun 15, 2017 at 19:03 Víctor Pimentel <vpimen...@tuenti.com> > wrote: > >> On 16 Jun 2017, at 01:55, Xiaodi Wu via swift-evolution < >> swift-evolution@swift.org> wrote: >> >> On Thu, Jun 15, 2017 at 17:43 David Hart <da...@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. > > > Data: > > 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? > > > That’s the obvious example that people find confusing. > > Less obvious places that labeled tuple patterns show up are ‘case let’ and > ‘case’ (see below). > > > Okay, I should have looked at your regex and read further. It looks like > you were already trying to match these. > > > I did walk the grammar for all occurrences of _pattern_. > > I’m only matching named tuple patterns that immediately follow one of the > keywords which a pattern follows (for, case, let, var, and catch). As I > mentioned, I’m not matching patterns that come later in comma-separated > lists. I’m also not matching named tuples inside nested patterns, e.g. let > ((a: b), (c: d)). > > But again, if even the most basic form of this construct is so rare, I > doubt more robust matching would turn up that much more usage. > > I’m surprised you’re not seeing any uses of ‘case’ with labels. > > > Me too. But I just verified that my pattern does match them. > > > Are you sure? It doesn’t look like it’s going to match the example I gave > due to the leading ‘.’ on the enum case. > > > Ah! I should have read your original message more carefully. You’re quite > right, I only was checking case statements for raw tuples like this: > > case let (i: a, f: b): > > …and not for anything involving associated values. I hadn’t even > considered that associated values would be affected by this, but looking at > the grammar it seems they would indeed be. > > Another clumsy regex search, this time for patterns with tuple labels on > associated values, turned up 111 results (one per ~3800 lines). Not super > common, but certainly nothing to sneeze at. Here they are: > > https://gist.github.com/pcantrell/d32cdb5f7db6d6626e45e80011163efb > > Looking through that gist, these usages mostly strike me as being just > fine: > > case .cover(from: .bottom): > > case .reference(with: let ref): > > case .update(tableName: let tableName, columnNames: _): > > I’d even say that removing the tuple labels would make things worse. > Consider: > > case .name(last: let firstName, first: _): // mistake is clear > case .name(let firstName, _): // mistake is buried > > In Chris’s original brain-bending example, the confusion is that there’s > no “let” after the colon, so Int and Float look like types instead of > variable names: > > let (a : Int, b : Float) = foo() > > However, in the examples in the gist above, most of the patterns either > (1) declare variables using a `let` after the colon: > > case .reference(with: let ref): > > …or (2) don’t declare a variable at all: > > case .string(format: .some(.uri)): > > What if we allowed labels on associated values, but required a `let` after > the colon to bind a variable? > > case let .a(b: c): // disallowed > case .a(b: let c): // OK > > Only 15 of those 111 run afoul of _that_ rule. Here they are: > > https://gist.github.com/pcantrell/9f61045d7d7c8d18eeec8ebbef6cd8f8 > > That’s one breakage every ~28000 lines, which seems much more acceptable. > The drawback is that you can’t declare variables for a bunch of associated > value en masse anymore; you need one let per value. (See line 2 in that > gist.) > > You might want to try the patch I sent as it will definitely catch any > tuple pattern that makes it to the verifier and does have labels. > > > I’m not set up to build the compiler, unfortunately. One of these days. > > P > > > Mark > > > P > > > Mark > > Fortunately we do not appear to allow shuffling in these cases. I’m not > sure if the human disambiguation is easier here because of the context > (‘case let’ and ‘case’), but I don’t recall seeing complain about these > being confusing (having said that it’s entirely possible they are very > confusing the first time someone sees them, in particular ‘cast let’ and > the binding form of ‘case’. > > enum X { > case e(i: Int, f: Float) > } > > let x = X.e(i: 7, f: 12) > > if case let X.e(i: hi, f: bye) = x { > print("(i: \(hi), f: \(bye))") > } > > func test(_ x: X, _ a: Int, _ b: Float) { > switch x { > case .e(i: a, f: b): > print("match values") > case .e(i: let _, f: let _): > print("bind values") > default: > break > } > } > > test(X.e(i: 1, f: 2), 1, 2) > test(X.e(i: 1, f: 2), 3, 4) > > > > 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? > > > If you want to catch *all* of these cases then the patch below will do it > by failing the AST verifier when it hits a pattern with labels. If you only > want to find the plain let-binding versions of this and not the ‘case let’ > and ‘case’ ones, I’d suggest looking at the parser to see if there’s an > easy place to instrument (I don’t know offhand). > > Mark > > diff --git a/lib/AST/ASTVerifier.cpp b/lib/AST/ASTVerifier.cpp > index b59a7ade23..ba4b2a245d 100644 > --- a/lib/AST/ASTVerifier.cpp > +++ b/lib/AST/ASTVerifier.cpp > @@ -2772,6 +2772,13 @@ public: > } > > void verifyParsed(TuplePattern *TP) { > + for (auto &elt : TP->getElements()) { > + if (!elt.getLabel().empty()) { > + Out << "Labeled tuple patterns are offensive!\n"; > + abort(); > + } > + } > + > PrettyStackTracePattern debugStack(Ctx, "verifying TuplePattern", > TP); > verifyParsedBase(TP); > } > > > > > Or can SourceKit / SourceKitten give a full AST? Or has anybody written a > Swift parser in Swift? > > Cheers, > > Paul > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution > > >
_______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution