Fallbackable.swift

protocol Fallbackable {
    associatedtype Value
    static var fallback: Value { get }
}

struct DefaultDict<K: Hashable, V, F: Fallbackable where V == F.Value> {
    var dict: [K:V]
    
    subscript(key: K) -> V {
        get {
            return dict[key] ?? F.fallback
        }
        set {
            dict[key] = newValue
        }
    }
}

extension Dictionary {
    func defaulting<F: Fallbackable where Value == F.Value>(f: F.Type) -> DefaultDict<Key, Value, F> {
        return DefaultDict<Key, Value, F>(dict: self)
    }
}

struct IntZero : Fallbackable {
    static var fallback: Int { return 0 }
}

let dict = ["x": 3, "y": 4]
let dd = dict.defaulting(IntZero.self)
dd["x"] // 3
dd["y"] // 4
dd["z"] // 0