Erica Sadun

Where technology meets something or other

Initializing Typed Arrays

Foundation’s URLQueryItem is just a stringly-typed key-value pair. You create one with a name and value:

public init(name: String, value: String?)

Since Swift supports literal initialization, you’d think you could use a dictionary to set up a [URLQueryItem] array, right? Well, yes and no.

You can’t just conform Array where Element == URLQueryItem to ExpressibleByDictionaryLiteral. Array extensions with constraints cannot have inheritance clauses. There are several ways around this limitation.

First (and best), you can just map an initializer across a dictionary literal:

let result = ["key1": "value1", "key2": "value2", "key3": "value3"]
    .map({ URLQueryItem(name: $0.key, value: $0.value) })

Second, you could make URLQueryItem itself dictionary initializable, but that starts to get ugly:

extension URLQueryItem: ExpressibleByDictionaryLiteral {
    public typealias Key = String
    public typealias Value = String
    public init(dictionaryLiteral elements: (String, String)...) {
        guard elements.count == 1
            else { fatalError("URLQueryItem requires single key-value pair") }
        self.init(name: elements.last!.0, value: elements.last!.1)
    }
}

let uq: URLQueryItem = ["key": "value"] // okay
let uqs: [URLQueryItem] = [["key1": "value1"], ["key2": "value2"], ["key3": "value3"]] // bleh

Third, you could use some kind of intermediate type to produce a URL query item array using Swift shortcuts. For example, you can set up a struct that builds the query item array and then pull from there:

struct URLQueryItems: ExpressibleByDictionaryLiteral {
    public typealias Key = String
    public typealias Value = String
    let items: [URLQueryItem]
    public init(dictionaryLiteral elements: (String, String)...) {
        items = elements.map({ URLQueryItem(name: $0.0, value: $0.1) })
    }
}

let uqis: URLQueryItems = ["key1": "value1", "key2": "value2", "key3": "value3"]
uqis.items

But again, it’s really not an improvement on using a mapped dictionary.

In the best of all worlds, which doesn’t exist, you’d be able to do something like this, but I don’t think there’s a way to accomplish this in modern Swift. Solution 2 is about as close as you get.

myRequest.queryItems = ["key1": "value1", "key2": "value2", "key3": "value3"]

Is there something I’m overlooking? If so, let me know. Drop a comment, mail, or tweet. Thanks.

Update:

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