<html><head><style>body{font-family:Helvetica,Arial;font-size:13px}</style></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div id="bloop_customfont" style="font-family:Helvetica,Arial;font-size:13px; color: rgba(0,0,0,1.0); margin: 0px; line-height: auto;">Inline:</div> <br> <div id="bloop_sign_1487182636134899968" class="bloop_sign"><div style="font-family:helvetica,arial;font-size:13px">-- <br>Adrian Zubarev<br>Sent with Airmail</div></div> <br><p class="airmail_on">Am 15. Februar 2017 um 19:11:12, Rien (<a href="mailto:rien@balancingrock.nl">rien@balancingrock.nl</a>) schrieb:</p> <div><div><blockquote type="cite" class="clean_bq" style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;"><span><div><div></div><div><br>> On 15 Feb 2017, at 18:09, Adrian Zubarev <adrian.zubarev@devandartist.com> wrote:<span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>> As I already said:<span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>> • To make that feature happen, we need the protocol to be public (regardless if you can conform to it or not).<span class="Apple-converted-space"> </span><br>> • Today you can conform to every protocol because in reality they are open today.<span class="Apple-converted-space"> </span><br>> • If we remove the property requirement from the protocol the client can conform it to any type and break my API by calling the subscript with a wrong type: document["something", NSObject()] (assuming extension NSObject : SubscriptParameterType).<span class="Apple-converted-space"> </span><br>> • That forces me to create a requirement for the protocol which solves the issue in my cases, however there might exist other issues, which could be far more complicated than mine is.<span class="Apple-converted-space"> </span><br>> • That also implies I have make the enum public.<span class="Apple-converted-space"> </span><br>> • Which follows by the fact that I’m creating unnecessary copy operations to wrap every instance that conforms to my protocol into an enum case. From internal API perspective I also have to unwrap the enum case.<span class="Apple-converted-space"> </span><br>> Instead if we had consistent public vs. open behavior, I could make the protocol public (but-not-open), remove the requirement from it completely, remove the enum completely and simply cast to Int or String because as the author of the library I would know that the client won’t be able to conform to my protocol.<span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>> Importing my library will show the client only this:<span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>> extension Int : SubscriptParameterType {}<span class="Apple-converted-space"> </span><br>> extension String : SubscriptParameterType {}<span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br><br>Ok, thanks, I think I get it now. So you showed the work-around rather than the problem :-)<span class="Apple-converted-space"> </span><br>However you could also implement this by creating two subscript operations, one for integer and one for string.<span class="Apple-converted-space"> </span></div></div></span></blockquote></div><p>No not really, the idea behind this approach is carefully crafted. Plus I do not want optional chaining between subscripts, this is messy `document[“string”]?[10]?[“key”]`.</p><div><div><blockquote type="cite" class="clean_bq" style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;"><span><div><div><br><br>So while I agree that this might serve as an example on why a non-confirmable protocol might be useful, my original question still stands: how can subclassing create “brittle” libraries?<span class="Apple-converted-space"> </span></div></div></span></blockquote></div><p>If one would want the client user only to use subclasses of a certain superclass of the same library for whatever reasons. These subclasses might interact with each other internally (which is not meant to be public by any means). `final public` will not make it happen because then you couldn’t create any of these subclasses at all.</p><p>You’re asking here such a general question that simply cannot be answered easily. I also could ask everyone why on earth would be want to hide anything from the client? Lets make everything public, if the client breaks something it’s his fault. <— Clearly not the way to go right? A distinction between `public` and `open` adds more flexibility to solve issues that previously might not had any good solutions.</p><div><blockquote type="cite" class="clean_bq" style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;"><span><div><div><br><br>Also note that adding an extra access level did nothing to prevent brittle libraries… that task is still with the developer.<span class="Apple-converted-space"> </span><br><br>PS, this is not an educational list, so … :-)<span class="Apple-converted-space"> </span></div></div></span></blockquote></div></div></div><p>Being sarcastic? </p><div><blockquote type="cite" class="clean_bq" style="font-family: Helvetica, Arial; font-size: 13px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px;"><span><div><div><br><br>Regards,<span class="Apple-converted-space"> </span><br>Rien.<span class="Apple-converted-space"> </span><br><br>><span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>> --<span class="Apple-converted-space"> </span><br>> Adrian Zubarev<span class="Apple-converted-space"> </span><br>> Sent with Airmail<span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>> Am 15. Februar 2017 um 17:50:41, Rien (rien@balancingrock.nl) schrieb:<span class="Apple-converted-space"> </span><br>><span class="Apple-converted-space"> </span><br>>><span class="Apple-converted-space"> </span><br>>> > On 15 Feb 2017, at 17:22, Adrian Zubarev via swift-evolution <swift-evolution@swift.org> wrote:<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > A short example where I personally wanted a public-but-not-open protocol:<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > public protocol SubscriptParameterType {<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > // This property was needed to prevent the client from breaking<span class="Apple-converted-space"> </span><br>>> > // the library by conforming to the protocol, but I'd like to<span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > // keep it invisible for the client, or even better prevent the<span class="Apple-converted-space"> </span><br>>> > // client from conforming to the protocol.<span class="Apple-converted-space"> </span><br>>> > var parameter: Document.SubscriptParameter { get }<span class="Apple-converted-space"> </span><br>>> > }<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > extension Document {<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > public enum SubscriptParameter {<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > case string(String)<span class="Apple-converted-space"> </span><br>>> > case integer(Int)<span class="Apple-converted-space"> </span><br>>> > }<span class="Apple-converted-space"> </span><br>>> > }<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > extension String : SubscriptParameterType {<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > public var parameter: Document.SubscriptParameter {<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > return .string(self)<span class="Apple-converted-space"> </span><br>>> > }<span class="Apple-converted-space"> </span><br>>> > }<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > extension Int : SubscriptParameterType {<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > public var parameter: Document.SubscriptParameter {<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > return .integer(self)<span class="Apple-converted-space"> </span><br>>> > }<span class="Apple-converted-space"> </span><br>>> > }<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > // Somewhere inside the `Document` type<span class="Apple-converted-space"> </span><br>>> > public subscript(firstKey: String, parameters: SubscriptParameterType...) -> Value? { … }<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > That implementation enables more safe queries of my Document type like document["key1", intIndexInstance, stringKeyInstance, 10, "key"] rather than document["key1/\(intIndexInstance)/\(stringKeyInstance)/10/key”]<span class="Apple-converted-space"> </span><br>>><span class="Apple-converted-space"> </span><br>>> I see how that makes queries better.<span class="Apple-converted-space"> </span><br>>> However what I do not see is how making the protocol “open” would make this less safe.<span class="Apple-converted-space"> </span><br>>> (I do not see a reason to make it open either, but that is not the question)<span class="Apple-converted-space"> </span><br>>><span class="Apple-converted-space"> </span><br>>> It may be obvious to everyone else, but I don’t see it. Am I suffering from a brain freeze?.<span class="Apple-converted-space"> </span><br>>><span class="Apple-converted-space"> </span><br>>> Regards,<span class="Apple-converted-space"> </span><br>>> Rien.<span class="Apple-converted-space"> </span><br>>><span class="Apple-converted-space"> </span><br>>><span class="Apple-converted-space"> </span><br>>> > .<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>><span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > --<span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > Adrian Zubarev<span class="Apple-converted-space"> </span><br>>> > Sent with Airmail<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > Am 15. Februar 2017 um 17:03:32, Matthew Johnson via swift-evolution (swift-evolution@swift.org) schrieb:<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>> On Feb 15, 2017, at 9:59 AM, Rien <Rien@Balancingrock.nl> wrote:<span class="Apple-converted-space"> </span><br>>> >>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>> On 15 Feb 2017, at 16:45, Matthew Johnson <matthew@anandabits.com> wrote:<span class="Apple-converted-space"> </span><br>>> >>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>> On Feb 15, 2017, at 9:35 AM, Rien <Rien@Balancingrock.nl> wrote:<span class="Apple-converted-space"> </span><br>>> >>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>> On 15 Feb 2017, at 16:11, Matthew Johnson via swift-evolution <swift-evolution@swift.org> wrote:<span class="Apple-converted-space"> </span><br>>> >>>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>>> On Feb 15, 2017, at 5:59 AM, Jeremy Pereira via swift-evolution <swift-evolution@swift.org> wrote:<span class="Apple-converted-space"> </span><br>>> >>>>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>>>> On 15 Feb 2017, at 11:11, Brent Royal-Gordon via swift-evolution <swift-evolution@swift.org> wrote:<span class="Apple-converted-space"> </span><br>>> >>>>>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>>>> Our philosophy in general, however, is to default to the behavior which preserves the most flexibility for the library designer.<span class="Apple-converted-space"> </span><br>>> >>>>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>>> Actually, I thought the philosophy was to preserver type safety. When did that change?<span class="Apple-converted-space"> </span><br>>> >>>>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>>> Also, when was the library designer prioritised ahead of the application developer?<span class="Apple-converted-space"> </span><br>>> >>>>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>>>> Both open and non-open classes are common, but we chose to give non-open classes the `public` keyword because that's the flexibility-preserving option.<span class="Apple-converted-space"> </span><br>>> >>>>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>>> No it isn’t, it’s the flexibility restricting option. The consumer of an open class can subclass it. The consumer of a public class cannot subclass it. How is the second more flexible than the first?<span class="Apple-converted-space"> </span><br>>> >>>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>> It reduces complexity for the library author by allowing them to opt-out of the complexity involved in supporting unknown, user-defined subclasses. It is important to allow libraries to have this flexibility. They are free to declare a class `open` if they want to allow subclassing. It’s even possibly for a library to declare all classes `open` if it wishes to do so. But *requiring* that would reduce the design space libraries are allowed to explore and / or introduce fragility by moving the subclass restriction to a comment.<span class="Apple-converted-space"> </span><br>>> >>>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>> Why would a library author want to prohibit subclasses?<span class="Apple-converted-space"> </span><br>>> >>>>> A library user can always wrap the class and subclass the wrapper.<span class="Apple-converted-space"> </span><br>>> >>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>> This is composition, not inheritance. The most important difference is that a wrapper cannot override methods, it can only wrap and / or forward them. This means that when the superclass calls a method on `self` that method *always* invokes its version of that method rather than a subclass override. This is a very important difference.<span class="Apple-converted-space"> </span><br>>> >>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>> Agreed, however that does not answer the question why would a library developer want to disallow subclassing?<span class="Apple-converted-space"> </span><br>>> >>> I do not see a use case for that. I.e. a feature that cannot be implemented without it. (without “open”)<span class="Apple-converted-space"> </span><br>>> >><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >> The feature it enables is more robust libraries and the ability for library authors to better reason about their code. You may not find this benefit enough to be worth a language feature, but many of us do.<span class="Apple-converted-space"> </span><br>>> >><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>> Rien.<span class="Apple-converted-space"> </span><br>>> >>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>> There are cases where subclassing does not make sense. And thus preventing subclasses adds information for those users that don’t RTFM. But that imo is not worth the impact extra complexity places on all other users.<span class="Apple-converted-space"> </span><br>>> >>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>> Rien.<span class="Apple-converted-space"> </span><br>>> >>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>>> _______________________________________________<span class="Apple-converted-space"> </span><br>>> >>>>>>> swift-evolution mailing list<span class="Apple-converted-space"> </span><br>>> >>>>>>> swift-evolution@swift.org<span class="Apple-converted-space"> </span><br>>> >>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution<span class="Apple-converted-space"> </span><br>>> >>>>>><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >>>>>> _______________________________________________<span class="Apple-converted-space"> </span><br>>> >>>>>> swift-evolution mailing list<span class="Apple-converted-space"> </span><br>>> >>>>>> swift-evolution@swift.org<span class="Apple-converted-space"> </span><br>>> >>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution<span class="Apple-converted-space"> </span><br>>> >><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> >> _______________________________________________<span class="Apple-converted-space"> </span><br>>> >> swift-evolution mailing list<span class="Apple-converted-space"> </span><br>>> >> swift-evolution@swift.org<span class="Apple-converted-space"> </span><br>>> >> https://lists.swift.org/mailman/listinfo/swift-evolution<span class="Apple-converted-space"> </span><br>>> ><span class="Apple-converted-space"> </span><span class="Apple-converted-space"> </span><br>>> > _______________________________________________<span class="Apple-converted-space"> </span><br>>> > swift-evolution mailing list<span class="Apple-converted-space"> </span><br>>> > swift-evolution@swift.org<span class="Apple-converted-space"> </span><br>>> > https://lists.swift.org/mailman/listinfo/swift-evolution<span class="Apple-converted-space"> </span><br>>><span class="Apple-converted-space"> </span><br><br></div></div></span></blockquote></div></body></html>