ImplicitDefaults.swift

/// the protocol
protocol ImplicitDefault {
  
  static var implicitDefault: Self { get }
}

/// the implementation for Strings
extension String: ImplicitDefault {
  
  /// i dont care i take the empty string as default
  static var implicitDefault: String { return "" }
  
}

/// kinda forced unwrap operator
postfix operator ~! {}

/// postfix function for the ImplicitDefault Optional
postfix func ~! <T: ImplicitDefault>(lhs: T?) -> T {
  guard let t = lhs else { return T.implicitDefault }
  return t
}


let s1: String? = nil
let s2: String? = "a"

let r1 = s1~! + s2~!
// -> "a"
let r2 = (s1 ?? "") + (s2 ?? "")
// -> "a"

struct A {
  let a: String?
}

let a1 = A(a: nil)
let a2 = A(a: nil)

let r3 = a1.a~! + a2.a~!
// -> ""
let r4 = (a1.a ?? "") + (a2.a ?? "")
// -> ""