Fallbacks.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>(_ 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

struct EmptyArray<Element>: Fallbackable {
  static var fallback: [Element] { return [] }
}

var d2: [String: [Int]] = [:]
var d2d = d2.defaulting(EmptyArray.self)

d2d["a"].append(3)
d2d // "a":[3]