How to make an extension for dictionary that allows use of the subscript with a dynamic key?

Question

extension Dictionary where Key: ExpressibleByStringLiteral, Value: Any {   

    func date(forKey key: String) -> Date? {

        return self[key] as? Date

    }

}

let dictionary: [String : Any] = ["mydate" : Date(), "otherkey" : "Rofl"]

dictionary.date(forKey:"mydate")  // should return a Date? object

// i get the error ambiguous reference to member 'subscript'

How can i make my extension allow me to give a key and use the subscript with not a literal, but a "dynamic" key in the form of String?

Note that `Value: Any` is a redundant constraint. I also see no reason to constrain the `Key` to `ExpressibleByStringLiteral` – what difference does it make if `Key` isn't `ExpressibleByStringLiteral`? — Hamish, Dec 07 '16 at 13:26

score 4 · Accepted Answer · answered Dec 07 '16 at 13:39

4

Remove the unneeded constraints and directly use Key or Value types wherever you see fit.

extension Dictionary {
    func date(forKey key: Key) -> Date? {
        return self[key] as? Date
    }
}

answered Dec 07 '16 at 13:39

Cœur

37,241
25
195
267

score 2 · Answer 2 · answered Dec 07 '16 at 13:20

2

Just replace key: String with key: Key:

extension Dictionary where Key: ExpressibleByStringLiteral, Value: Any {

    func date(forKey key: Key) -> Date? {

        return self[key] as? Date

    }

}

answered Dec 07 '16 at 13:20

user28434'mstep

6,290
2
20
35

score 1 · Answer 3 · answered Dec 07 '16 at 15:32

You can obtain a little sugar syntax by "proxy-ing" the date query to something like this:

struct DictionaryValueProxy<DictKey: Hashable, DictValue, Value> {
    private let dictionary: [DictKey:DictValue]

    init(_ dictionary: [DictKey:DictValue]) {
        self.dictionary = dictionary
    }

    subscript(key: DictKey) -> Value? {
        return dictionary[key] as? Value
    }
}

extension Dictionary {
    var dates: DictionaryValueProxy<Key, Value, Date> { return DictionaryValueProxy(self) }
}

You can then ask the dictionary for it's dates in an seamless way:

let dict: [Int:Any] = [1: 2, 3: Date()]
dict.dates[1]                            // nil
dict.dates[3]                            // "Dec 7, 2016, 5:23 PM"

How to make an extension for dictionary that allows use of the subscript with a dynamic key?

3 Answers3