Swift Dictionary access value using key within extension

Question

It turns out that within a Dictionary extension, the subscript is quite useless since it says Ambiguous reference to member 'subscript'. It seems I'll either have to do what Swift does in its subscript(Key) or call a function. Any ideas?

For example,

public extension Dictionary {
    public func bool(_ key: String) -> Bool? {
        return self[key] as? Bool
    }
}

won't work, since the subscript is said to be ambiguous.

ADDED My misunderstanding came from the fact that I assumed that Key is an AssociatedType instead of a generic parameter.

Answer 1

Swift type Dictionary has two generic parameters Key and Value, and Key may not be String.

This works:

public extension Dictionary {
    public func bool(_ key: Key) -> Bool? {
        return self[key] as? Bool
    }
}

let dict: [String: Any] = [
    "a": true,
    "b": 0,
]
if let a = dict.bool("a") {
    print(a) //->true
}
if let b = dict.bool("b") {
    print(b) //not executed
}

---

For ADDED part.

If you introduce a new generic parameter T in extension of Dictionary, the method needs to work for all possible combination of Key(:Hashable), Value and T(:Hashable). Key and T may not be the same type.

(For example, Key may be String and T may be Int (both Hashable). You know you cannot subscript with Int when Key is String.)

So, you cannot subscript with key of type T.

---

For updated ADDED part.

Seems to be a reasonable misunderstanding. And you have found a good example that explains protocol with associated type is not just a generic protocol.