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

Tony Allevato tony.allevato at gmail.com
Fri Jun 9 15:16:30 CDT 2017 

Makes sense. I guess from an implementation point of view, the compiler
only needs to be able to see the signatures of the members to synthesize
the calls to them. My thinking was that conforming the original type would
remove the need for the compiler to verify separately that the members
exist on the type (because of it conforms, they must, so you get it for
free), but I suppose in this case the compiler would just do a separate
validation pass.

I'm really looking forward to reading your draft!
On Fri, Jun 9, 2017 at 12:58 PM Matthew Johnson <matthew at anandabits.com>
wrote:

> On Jun 9, 2017, at 2:53 PM, Tony Allevato <tony.allevato at gmail.com> wrote:
>
> +1 to this. My ideal imagining of this behavior is to:
>
> 1) Define a protocol containing the precise operations you want to support
> via forwarding.
> 2) Internally/fileprivately extend the original type to conform to this
> protocol.
> 3) In your new type, tell it to forward protocol members for a particular
> property (the underlying String, or whatever) and the compiler will
> synthesize the protocol member stubs on the new type.
>
> There are definitely finer details to be worked out but that's a rough
> sketch of how I've been imagining it could work. Nice advantages include
> (1) if there already exists a protocol that defines the exact operations
> you want to forward, you can skip step 2, and (2) it's explicit/opt-in
> synthesis where you have to provide the members to forward (this is
> expected, at a minimum), but you don't have to write all the boilerplate
> implementations.
>
>
> I have a mostly finished second draft of a protocol-oriented forwarding
> proposal which is pretty similar to what you describe here.  The most
> important difference is that while protocols are used to perform forwarding
> neither type is required to actually conform to the protocol.
>
> That turns out to be pretty important when you think about edge cases.
> Sometimes you don’t actually want the forwardee to conform (that’s probably
> why you said internal / fileprivate).  Sometimes you also don’t want the
> forwarder to conform.  IIRC, there are also cases where it cannot conform
> (I would have to dig up the proposal to recall the details).
>
>
>
> On Fri, Jun 9, 2017 at 12:47 PM Matthew Johnson via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>> On Jun 9, 2017, at 2:39 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>
>> Interesting. So you’d want `newtype Foo = String` to start off with no
>> members on Foo?
>>
>>
>> Yeah.  Previous discussions of newtype have usually led to discussion of
>> ways to forward using a protocol-oriented approach.  Nothing has gotten too
>> far, but it usually comes up that suppressing undesired members is
>> important.
>>
>> It is also important to have some way to distinguish between members with
>> a parameter of the underlying type from members that should be treated by
>> newtype as Self parameters.  The mechanism we have for doing that in Swift
>> happens to be a protocol.
>>
>> On Fri, Jun 9, 2017 at 15:18 Matthew Johnson <matthew at anandabits.com>
>> wrote:
>>
>>> On Jun 9, 2017, at 12:09 PM, Xiaodi Wu via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>>
>>> 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.
>>>
>>>
>>> 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 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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>>>>
>>>>
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