You know… if protocols could define which types can implement them, that could
be made to be the same thing:
protocol _String {…} `syntax which restricts instances of _String to be a`
String
extension Dictionary where Key: _String {…} //Same as "where Key ==
String”, because that’s the only type allowed to implement _String
Is that worth writing proposal over? Or is just saying this:
protocol _String { func asString() -> String }
extension String : _String { func asString -> String { return self } }
close enough?
- Dave Sweeris
> On Apr 14, 2016, at 4:31 PM, Brent Royal-Gordon via swift-users
> <swift-users@swift.org> wrote:
>
>> It appears that you can’t extend a generic class/struct with a requirement
>> that a type parameter inherit from a non-protocol:
>> extension Dictionary where Key : String { … }
>> The above produces the error “Type ‘Key’ constrained to non-protocol type
>> ‘String’”.
>
> You can't do `Key: String` because `String`, as a struct, cannot have any
> subtypes. `Key: NSString`, for instance, works.
>
> What you really want to do is say `Key == String`, but this isn't supported
> right now. (My understanding is that this is basically just an implementation
> shortcut they took.)
>
> One workaround is to define your own protocol and constrain to that:
>
> protocol _StringType: Hashable {
> // Include the String APIs you need to use here.
> }
> extension String: _StringType {}
> extension Dictionary where Key: _StringType { … }
>
> --
> Brent Royal-Gordon
> Architechies
>
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