What does "Protocol ... can only be used as a generic constraint because it has Self or associated type requirements" mean?

Question

I am trying to create a Dictionary (actually a HashSet) keyed on a custom protocol in Swift, but it is giving me the error in the title:

Protocol 'myProtocol' can only be used as a generic constraint because it has Self or associated type requirements

and I can't make heads nor tails of it.

protocol Observing: Hashable { }

var observers = HashSet<Observing>()

Solution 1

Protocol Observing inherits from protocol Hashable, which in turn inherits from protocol Equatable. Protocol Equatable has the following requirement:

func ==(lhs: Self, rhs: Self) -> Bool

And a protocol that contains Self somewhere inside it cannot be used anywhere except in a type constraint.

Here is a similar question.

Solution 2

To solve this you could use generics. Consider this example:

class GenericClass<T: Observing> {
   var observers = HashSet<T>()
}

Comments

[devios1]

...because the compiler has to ensure it's the same type on either side, but the protocol only ensures it meets the contract. I see. Still, it seems like Equatable ought not to necessarily imply Equatable since that's not strictly necessary to generate a hash code.

[devios1]

Oh no wait it does, because Dictionary needs to be able to know if a given object is indeed the correct key, because it's possible for two different objects to generate the same hash code. Hmm, that is a bit tricky. So the problem here is really with Equatable.

[finneycanhelp]

Look at 0:56 into the great presentation by Alexis Gallagher titled: Protocols with Associated Types and how they got that way (maybe) youtu.be/XWoNjiSPqI8

[devios1]

@finneycanhelp Thanks for this 👍👍. Great video!

[Era]

Swift is truly one of the most poorly designed languages I have ever seen...