[swift-evolution] Pitch: Limit typealias extensions to the typealias

[Xiaodi Wu]

On Fri, Jun 9, 2017 at 12:44 Robert Bennett via swift-evolution <
swift-evolution at 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.


On Jun 9, 2017, at 12:12 PM, Jacob Williams via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> On Jun 9, 2017, at 9:53 AM, Charlie Monroe <charlie at 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 <charlie at 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 at 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 at 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
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