How to extend protocol Optional, where Wrapped item is Array of Equatable generic elements?

问题

I would say this problem is about proper declaration of extension.

I would like to extend Array filled with generic Elements, where Element conforms to Equatable. I've managed to do that by :

    extension Array where Element: Equatable{
        // my code
    }

However I'd like to know how to properly declare extension when the Array filled with Equatable elements is inside an Optional? I know that in this case I am actually extending protocol Optional, but I can't figure out the rest

I was thinking something like:

    extension Optional where Wrapped: Array  & Equatable {
      // my code
    }

Can't figure it out. Any ideas ?

回答1:

Cristik provided a good solution. An alternative is to write an extension to an optional collection of equatable elements:

extension Optional where Wrapped: Collection, Wrapped.Element: Equatable {
    func foo() { }
}

This would be applicable to arrays, array slices, and other collections.

Depending on what the extension does, you may want to use a combination of Collection, MutableCollection, BidirectionalCollection, RandomAccessCollection.

回答2:

I don't think you can't specify this constraint at the extension level, however you should be able to specify it at the function level:

extension Optional {
    func myFunc<T: Equatable>() where Wrapped == [T] {
        // do your thing
    }
}

来源：https://stackoverflow.com/questions/54143757/how-to-extend-protocol-optional-where-wrapped-item-is-array-of-equatable-generi