See: https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170417/035972.html
On Fri, Jun 16, 2017 at 22:32 Paul Cantrell <cantr...@pobox.com> wrote: > Under these not-yet-implemented plans, if associated value labels are no > longer tuple labels, then how will pattern matching work? And what existing > pattern matching code will break / continue to work? > > P > > On Jun 16, 2017, at 10:22 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote: > > 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 >> >> >> >
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