[swift-users] Workaround for generics not currently supporting conditional conformance to a protocol

[Howard Lovatt]

That's my plan (to replace my wrappers once conditional conformance is
available). Good news that Conditional Conformance has just been approved.

On Thu., 17 Nov. 2016 at 1:30 pm, David Sweeris <davesweeris at mac.com> wrote:

>
> On Nov 16, 2016, at 16:35, Jordan Rose <jordan_rose at apple.com> wrote:
>
>
> On Nov 16, 2016, at 7:35, David Sweeris via swift-users <
> swift-users at swift.org> wrote:
>
>
> On Nov 15, 2016, at 11:55 PM, Howard Lovatt <howard.lovatt at gmail.com>
> wrote:
>
> @Dave,
>
> How do I write that though.
>
> I can't write:
>
>     extension Array: Equatable {
>         static func ==(lhs: Array, rhs: Array) -> Bool {
>             let size = lhs.count
>             precondition(rhs.count == size, "The arrays must be the same
> length")
>             for i in 0 ..< size {
>                 if (lhs[i] as! Equatable) != (rhs[i] as! Equatable) {
>                     return false
>                 }
>             }
>             return true
>         }
>     }
>
> Because I can't cast to an Equatable, because Equatable uses Self.
>
> Am I missing something?
>
>  -- Howard.
>
>   -- Howard.
>
> On 16 November 2016 at 16:35, David Sweeris <davesweeris at mac.com> wrote:
>
>
> > On Nov 15, 2016, at 21:39, Howard Lovatt via swift-users <
> swift-users at swift.org> wrote:
> >
> > Hi All,
> >
> > Does anyone have a good workaround for generics not currently supporting
> conditional conformance to a protocol. As stated in the Generics Manifesto
> something like this would be nice:
> >
> >   extension Array: Equatable where Element: Equatable {
> >         static func ==(lhs: Array, rhs: Array) -> Bool { ... }
> >     }
> >
> > But I would currently write a wrapper, something like:
> >
> >     struct ArrayE<T: Equatable> {
> >         var elements: [T]
> >     }
> >     extension ArrayE: Equatable {
> >         static func ==(lhs: ArrayE, rhs: ArrayE) -> Bool { ...  }
> >     }
> >
> > This can get unwieldy when there are a lot of conditional protocol
> extensions required, i.e. wrappers round wrappers.
> >
> > Is there a better way?
> >
> > Thanks for any tips,
> >
> >   -- Howard.
> > _______________________________________________
> > swift-users mailing list
> > swift-users at swift.org
> > https://lists.swift.org/mailman/listinfo/swift-users
>
> Can you make Array conform to Equatable for any T and then in the ==
> function, if T conforms to Equatable loop the Arrays to check if they're
> equal, and if it doesn't conform just return false?
>
> I mean, it's still "wrong", but at least you won't get any false positives.
>
> - Dave Sweeris
>
>
>
> You are correct. The work-around is to use two extensions and overload the
> == operator:
>
> extension Array: Equatable {
>     public static func == (lhs: Array, rhs: Array) -> Bool {
>         return false
>     }
> }
> extension Array where Element : Equatable {
>     public static func == (lhs: Array, rhs: Array) -> Bool {
>         return lhs.count == rhs.count && {
>             for i in 0..<lhs.count {
>                 if lhs[i] != rhs[i] {
>                     return false
>                 }
>             }
>             return true
>         }()
>     }
> }
>
> It works in playgrounds (Xcode 8.1 (8B62)), but I haven’t tested it
> outside a few trivial cases.
>
>
> This does not work. The == dispatch for Arrays is static in this case, not
> dynamic. You can test this by writing something generic on Equatable.
>
> func same<T: Equatable>(_ x: T, _ y: T) -> Bool { return x == y }
> print(same([1], [2]))
>
>
> Rule of thumb: overloads are resolved statically, protocol requirements
> are invoked dynamically. You cannot get dynamic behavior out of just
> overloads, ever.
>
> I don't think there's a way to get this behavior today, unfortunately.
>
>
> Aw, nuts! I forgot to try adding another level of, um... genericititty?
>
> Yeah, I think you're right... On the plus side, when conditional
> conformance hits the wrapper structs can be backed out just by searching
> your project for their name and replacing it with "Array" (or "Set", etc),
> right?
>
> - Dave Sweeris
>
-- 
-- Howard.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-users/attachments/20161117/8a04ae2f/attachment.html>