How to constrain a protocol associated type

Swift version: 5.10

Paul Hudson @twostraws May 28th 2019

Protocol associated types let you add a huge amount of flexibility to your protocols, but sometimes you want a little less flexibility. For example, you might say that all types conforming to a protocol must specify the id of their object and also what type that ID must be:

protocol Identifiable1 {
    associatedtype ID
    var id: ID { get set }
}

With that code, ID could be anything – a String, an Int, a UILabel, and so on. However, you might find you need to apply some constraints to that type: perhaps you need to use it as a dictionary key (Hashable), or sort it in an array (Comparable).

To make this work, Swift lets us apply constraints to associated types: “it can be any type, as long as that type conforms to…”. For example, this forces ID to conform to Hashable:

protocol Identifiable2 {
    associatedtype ID: Hashable
    var id: ID { get set }
}

Because Hashable inherits from Equatable we can now be sure that any types used for ID can be compared using == and also used as keys in dictionaries.