Fine by me as well, but to be clear this will be a non-trivial source breaking change.
On Sat, Jun 17, 2017 at 11:09 Matthew Johnson <matt...@anandabits.com> wrote: > > > Sent from my iPad > > On Jun 17, 2017, at 10:20 AM, Paul Cantrell via swift-evolution < > swift-evolution@swift.org> wrote: > > Yes, agreed, the fix for Chris’s brain-bender shouldn’t revisit any of > SE-0155’s matching & labeling rules. > > How about: > > 1. Disallow labels for bare tuples in patterns. (By “bare tuples” I mean > “not representing associated values on an enum.”) > > let (a: x, b: y) = foo // disallowed > let (x, y) = foo // OK > > 2. Maybe require “let” before individual identifiers when pattern matching > on associated values, while preserving SE-0155’s rules for when labels may > appear: > > case let .foo(a: x, b: y) // disallowed > case .foo(a: let x, b: let y) // OK > > #2 is debatable. It would solve an enum-based parallel to Chris’s original: > > case let .foo(a: Int, b: String) // disallowed > case .foo(a: let Int, b: let String) // allowed, and Int/String no longer > look like types > > > Doing this with #2 is what I suggested earlier. I like this because I > find the disallowed style to have too much cognitive load anyway. > > P > > On Jun 16, 2017, at 10:55 PM, Xiaodi Wu <xiaodi...@gmail.com> wrote: > > 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 >>> >>> >>> >> > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution > >
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