Swift: Conforming a protocol to a protocol.

In some cases you want to conform a protocol to another protocol that defines an associated type.

protocol CollectionSlice: Collection {
    func prefix(_ maxLength: Int) -> CollectionSlice
}

It’s not uncommon to run into an error like this:

Protocol ‘CollectionSlice’ can only be used as a generic constraint because it has Self or associated type requirements

This is because the compiler can’t make sure that the returned CollectionSlice will result in the same underlying associated type as the defined protocol. Therefore, we need to setup a constraint that makes sure both types are equal:

protocol CollectionSlice: Collection {
    associatedtype Slice: CollectionSlice where Slice.Item == Item
    func prefix(_ maxLength: Int) -> Slice
}

Implementors of this protocol are now required to return a slice of the same type as its parent collection:

extension UppercaseStringsCollection: CollectionSlice {
    func prefix(_ maxLength: Int) -> UppercaseStringsCollection {
        var collection = UppercaseStringsCollection()
        for index in 0..<min(maxLength, count) {
            collection.append(self[index])
        }
        return collection
    }
}

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Associated types in Swift Protocols.
Declaration
Example
Contraints
Conforming a protocol to a protocol

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