Interesting. So you’d want `newtype Foo = String` to start off with no members on Foo? On Fri, Jun 9, 2017 at 15:18 Matthew Johnson <matt...@anandabits.com> wrote:
> On Jun 9, 2017, at 12:09 PM, Xiaodi Wu via swift-evolution < > swift-evolution@swift.org> wrote: > > On Fri, Jun 9, 2017 at 12:44 Robert Bennett via swift-evolution < > swift-evolution@swift.org> wrote: > >> Somewhat related to this, shouldn’t it be possible to sub-struct a struct >> as long as you only add functions and computed properties (i.e., no stored >> properties)? Traditionally structs cannot be subtyped because their size >> must be known at compile time. I don’t know the implementation details of >> where functions and computed properties live, but something tells me they >> belong to the type and not the object (although I’ve never really made the >> effort to sit down and fully understand Swift’s type model), in which case >> adding them to a struct’s definition would not change the size of the >> object on the stack. Thus it should be possible to make custom substructs >> of String that add additional functionality but no new stored properties. >> Thoughts? >> > > Value subtyping is a large subject and, IIUC, newtype would be a subset of > that topic. Unlikely to be in scope for Swift 5, though, but that’s up to > the core team. > > > I see newtype as being more related to forwarding than subtyping. Usually > you want to hide significant parts of the interface to the wrapped type. > > > > On Jun 9, 2017, at 12:12 PM, Jacob Williams via swift-evolution < >> swift-evolution@swift.org> wrote: >> >> On Jun 9, 2017, at 9:53 AM, Charlie Monroe <char...@charliemonroe.net> >> wrote: >> >> -1 - this would disallow e.g. to share UI code between iOS and macOS: >> >> #if os(iOS) >> typealias XUView = UIView >> #else >> typealias XUView = NSView >> #endif >> >> extension XUView { >> ... >> } >> >> >> I really don’t see how this disallows code sharing between the two >> systems? Could you explain further? Based on my understanding of the pitch, >> this is valid code still. (Although I do like the suggestion of a new >> keyword rather than just limiting type alias). >> >> Even if your example was invalid, you could also just do something like >> this: >> >> #if os(iOS) >> typealias XUView = UIView >> extension XUView { >> //extension code here >> } >> #if os(macOS) >> typealias XUView = UIView >> extension XUView { >> // extension code here >> } >> #endif >> >> While not as pretty, still just as effective if you have to deal with >> different types based on the system being compiled for and you could easily >> still make the type alias extensions for each type work the same. >> >> On Jun 9, 2017, at 9:53 AM, Charlie Monroe <char...@charliemonroe.net> >> wrote: >> >> -1 - this would disallow e.g. to share UI code between iOS and macOS: >> >> #if os(iOS) >> typealias XUView = UIView >> #else >> typealias XUView = NSView >> #endif >> >> extension XUView { >> ... >> } >> >> or with any similar compatibility typealiases. >> >> On Jun 9, 2017, at 5:38 PM, Jacob Williams via swift-evolution < >> swift-evolution@swift.org> wrote: >> >> +1 from me. >> >> There have been times I’ve wanted to subclass an object (such as String) >> but since it is a non-class, non-protocol type you can only extend Strings >> existing functionality which adds that same functionality to Strings >> everywhere. It would be nice if we could either extend type aliases (and >> only the type alias), or if it were possible to inherit from structs so >> that we could create a custom string type like so: >> >> struct HeaderKey: String { >> static var lastModified: String { return “Last-Modified” } >> static var host: String { return “Host” } >> } >> >> I realize that struct inheritance is far less likely, since that defeats >> one of the main pieces of what makes a struct a struct. So I’m all for this >> proposal of allowing type aliases to be extended as though they were their >> own struct/class. >> >> Unfortunately, I’m not sure how feasible this kind of functionality would >> actually be, but if it’s possible then I’m in favor of implementing it. >> >> On Jun 8, 2017, at 10:14 PM, Yvo van Beek via swift-evolution < >> swift-evolution@swift.org> wrote: >> >> Typealiases can greatly reduce the complexity of code. But I think one >> change in how the compiler handles them could make them even more powerful. >> >> Let's say I'm creating a web server framework and I've created a simple >> dictionary to store HTTP headers (I know that headers are more complex than >> that, but as an example). I could write something like this: >> >> typealias HeaderKey = String >> >> var headers = [HeaderKey: String]() >> headers["Host"] = "domain.com" >> >> Now I can define a couple of default headers like this: >> >> extension HeaderKey { >> static var lastModified: String { return "Last-Modified" } >> static var host: String { return "Host" } >> } >> >> After that I can do this: >> >> var headers = [HeaderKey: String]() >> headers[.host] = "domain.com" >> headers[.lastModified] = "some date" >> headers["X-MyHeader"] = "This still works too" >> >> But unfortunately the extension is also applied to normal strings: >> >> var normalString: String = .host >> >> Perhaps it would be better if the extension would only apply to the parts >> of my code where I use the HeaderKey typealias and not to all Strings. This >> could be a great tool to specialize classes by creating a typealias and >> adding functionality to it. Another example I can think of is typealiases >> for dictionaries or arrays with added business logic through extensions >> (especially since you can't inherit from structs). >> >> If you want to create an extension that adds functionality to all Strings >> you could have created an extension for String instead of HeaderKey. >> >> Please let me know what you think. I'm not sure how complex this change >> would be. >> I could write a proposal if you're interested. >> >> Kind regards, >> Yvo >> _______________________________________________ >> 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 >> >> >> >> _______________________________________________ >> 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 >> > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution > >
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