Result.swift

public enum Result<Value> {
  case success(Value)
  case failure(Error)
  
  
  public init(_ value: Value) {
    self = .success(value)
  }
  
  
  public init(_ error: Error) {
    self = .failure(error)
  }
  
  
  public func flatMap<U>(transform ƒ: (Value)->Result<U>) -> Result<U> {
    switch self {
    case .success(let value):
      return ƒ(value)
    case .failure(let e):
      return Result<U>.failure(e)
    }
  }
  
  
  public func asyncFlatMap<U>(completion: (Result<U>)->Void, transform ƒ: (Value)->Result<U>) {
    completion(flatMap(transform: ƒ))
  }
}