CODEWITHSUNDEEP
swiftprotocolsassociated-types
BenJacob
BenJacob

Reputation: 987

Heterogeneous array that conforms to protocol with associated type in Swift

I have a protocol AProtocol with an associated type AType and a function aFunc. I want to extend Array such that it conforms to the protocol by using the result of its elements aFunc function. Clearly this is only possible if elements of the array conform to Aprotocol and have the same associated type so I have set this toy example:

protocol AProtocol {
    associatedtype AType
    func aFunc(parameter:AType) -> Bool
}

extension Array : AProtocol where Element : AProtocol, Element.AType == Int {
    func aFunc(parameter: Int) -> Bool {
        return self.reduce(true, { r,e in r || e.aFunc(parameter: parameter) })
    }
}

extension String : AProtocol {
    func aFunc(parameter: Int) -> Bool {
        return true
    }
}

extension Int : AProtocol {
    func aFunc(parameter: Int) -> Bool {
        return false
    }
}

This works fine for arrays which contain only one type:

let array1 = [1,2,4]
array1.aFunc(parameter: 3)

However for heterogeneous arrays, I get the error Heterogeneous collection literal could only be inferred to '[Any]'; add explicit type annotation if this is intentional and then Value of type '[Any]' has no member 'aFunc' if annotate it as follows:

let array2 = [1,2,"Hi"] as [Any]
array2.aFunc(parameter: 3)

Is it possible to extend Array as I wish such that heterogeneous arrays are allowed so long as they conform to AProtocol and have the same AType?

Upvotes: 1

Views: 451

Answers (1)

user1046037
user1046037

Reputation: 17685

See if this fits your needs.

Approach:

Remove the associated type

Implementation:

protocol BProtocol {
    func aFunc(parameter: BProtocol) -> Bool
}

extension String : BProtocol {
    func aFunc(parameter: BProtocol) -> Bool {
        return true
    }
}

extension Int : BProtocol {
    func aFunc(parameter: BProtocol) -> Bool {
        return false
    }
}

extension Array : BProtocol where Element == BProtocol {

    func aFunc(parameter: BProtocol) -> Bool {
        return self.reduce(true, { r,e in r || e.aFunc(parameter: parameter) })
    }
}

Invoking:

let a1 : [BProtocol] = [1, 2, 3, "Hi"]

let boolean = a1.aFunc(parameter: 1)

Upvotes: 2

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