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

[David Sweeris]

> 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 <mailto:davesweeris at mac.com>> wrote:
> 
> > On Nov 15, 2016, at 21:39, Howard Lovatt via swift-users <swift-users at swift.org <mailto: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 <mailto:swift-users at swift.org>
> > https://lists.swift.org/mailman/listinfo/swift-users <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.

- Dave Sweeris
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