Swift - Typealias dictionary with value that implements a generic protocol

Question

I want to typealias a dictionary of String keys and values of objects/structs that implements the Equatable protocol. So I wrote this line of code but it gave me error that I didn't know how to go on to fix.

typealias Storage = [String: Equatable]

I want to use the type [String: Equatable] as a variable in a protocol, e.g:

protocol StorageModel {
    var storage: Storage { get set }
    init(storage: Storage)
}

Error:

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

Can anyone suggest a solution?

Answer 1

Generally speaking, the protocol tag isn't required, protocol names are first-class type names and can be used directly:

typealias Storage = [String:Equatable]

In this case, what the error is telling you is that because Equatable includes func == (lhs:Self, rhs:Self) -> Bool and specifically lhs:Self, Equatable can't be used except as a constraint on a generic:

class Generic<T:Equatable> { ... }

Without more details about what you're trying to achieve and how you're trying to use StorageModel, the best I can come up with is:

protocol Matches {
    typealias T
    func matches(t:T) -> Bool
}

protocol StorageModel {
    typealias T
    var storage: [String:T] { get set }
    init(storage:[String:T])
}

extension Int : Matches {
    func matches(target:Int) -> Bool {
        return self == target
    }
}

class MyClass <T:Matches> {
    var model = [String:T]()


}

Another possibility is to use a generic instead of a protocol:

class StorageModel <T:Equatable> {
    var storage: [String:T]

    init(storage:[String:T]) {
        self.storage = storage
    }
}

From there you'll need to do some research, dig into the Swift manual, do some googling and see what solves your problem.