enum Result<T> {

  case success(T)
  case failure(Error)

  func map<A>(_ transform: (T) throws -> A) rethrows -> Result<A> {

    switch self {
    case let .success(value):
      return Result<A>.success(try transform(value))
    case let .failure(error):
      return Result<A>.failure(error)
    }

  }

}