unwrap.swift

func ??<T>(lhs: T?, rhs: @autoclosure () throws -> Never) rethrows -> T {
  guard let value = lhs else { try rhs() }
  return value
}

func ??<T>(lhs: T?, rhs: () throws -> Never) rethrows -> T {
  guard let value = lhs else { try rhs() }
  return value
}

enum GimmeA: Error {
  case B
}

func f(_ a: Int?, _ b: String?) throws {
  let a = a ?? fatalError("Gimme an A!")
  let b = try b ?? { throw GimmeA.B }
  print(a, b)
}

do {
  try f(42, "forty-two")
  try f(42, nil)
} catch {
  print("Threw", error)
}

do {
  try f(nil, nil)
} catch {}