Extend Array to conform to protocol if Element conforms to given protocol

Question

I'd like to do something like this, but can't get the syntax right or find anywhere on the web that gives the right way to write it:

protocol JSONDecodeable {
    static func withJSON(json: NSDictionary) -> Self?
}

protocol JSONCollectionElement: JSONDecodeable {
    static var key: String { get }
}

extension Array: JSONDecodeable where Element: JSONCollectionElement {
    static func withJSON(json: NSDictionary) -> Array? {
        var array: [Element]?
        if let elementJSON = json[Element.key] as? [NSDictionary] {
            array = [Element]()
            for dict in elementJSON {
                if let element = Element.withJSON(dict) {
                    array?.append(element)
                }
            }
        }
        return array
    }
}

So I want to conform Array to my protocol JSONDecodeable only when the elements of this array conform to JSONCollectionElement.

Is this possible? If so, what's the syntax?

Use generics something like Array<JSONCollectionElement>

Omar Al-Shammary
– Omar Al-Shammary

2015-10-19 16:43:34 +00:00
Commented Oct 19, 2015 at 16:43 — Omar Al-Shammary
– Omar Al-Shammary, Commented Oct 19, 2015 at 16:43

Nate Cook · Accepted Answer · 2015-10-19 18:19:46Z

5

This isn't possible yet in Swift. You can see the same thing happen in the standard library: Array doesn't gain Equatable conformance when it's declared with Equatable elements.

answered Oct 19, 2015 at 18:19

Nate Cook

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1 Comment

doplumi Over a year ago

@OrkhanAlikhanov still not possible. I am getting a Extension of type 'Array' with constraints cannot have an inheritance clause build time error from XCode.

the Reverend · Accepted Answer · 2018-10-21 18:15:52Z

1

Swift 4.2

In Swift 4.2 I was able to extend an array with the elements conforming to a protocol like this:

public extension Array where Element: CustomStringConvertible{
    public var customDescription: String{
        var description = ""
        for element in self{
            description += element.description + "\n"
        }

        return description
    }
}

answered Oct 21, 2018 at 18:15

the Reverend

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2 Comments

kylejs Over a year ago

Confirmed! Thank you! I had a quick look around for some info on this change in Swift 4.2, but couldn't find any. If anyone's got it please share!

firetrap Over a year ago

And for optional array? Where Element must conform some protocol

Vyacheslav · Accepted Answer · 2018-01-19 10:06:59Z

1

I would suggest using a wrapper. For instance

struct ArrayContainer<T: Decodable>: Container {
    let array: [T]
}

answered Jan 19, 2018 at 10:06

Vyacheslav

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Comments

Jacob · Accepted Answer · 2018-12-01 17:59:15Z

1

I don't know if this is the best approach or if apple intended it to be used this way. I used this once and it worked well for me:

Say you have the following protocol

protocol MyProtocol {
    var test: Bool { get }
}

You can do this for Arrays

extension Array: MyProtocol where Element: MyProtocol {
    var test: Bool {
        return self.allSatisfy({ $0.test })
    }
}

And this for Dictionaries

extension Dictionary: MyProtocol where Value: MyProtocol {
    var test: Bool {
        return self.values.allSatisfy({ $0.test })
    }
}

edited Dec 1, 2018 at 17:59

answered Dec 1, 2018 at 17:47

Jacob

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