[英]Combining Futures (Twitter) and Either in Scala
We use Twitter futures (as part of the Finagle stack) and I don't like the concept of using (business) exceptions to control the flow of our application, because exceptions don't show up in method signatures. 我们使用Twitter期货(作为Finagle堆栈的一部分),我不喜欢使用(业务)异常来控制应用程序流的概念,因为异常不会出现在方法签名中。
So I had the idea to use Future[Either[A,B]] as a replacement. 所以我有了使用Future [Aither [A,B]]作为替代品的想法。
But I have some problems in using for comprehensions over futures with this concept: 但是我使用这个概念来使用对期货的理解存在一些问题:
Eg we have a repository method: 例如,我们有一个存储库方法:
def getUserCredentialsByNickname(nickname: String): Future[Either[EntityNotFound, UserCredentials]]
and a handler method which uses this repo and does some other checks and also creates a token 和一个处理程序方法,它使用这个repo并做一些其他检查,并创建一个令牌
def process(request: LoginRequest): Future[Either[Failure, Login]] = {
for {
credentialsEither <- userRepository.getUserCredentialsByNickname(request.username)
...several other calls/checks which should 'interrupt' this for comprehension
token <- determineToken(credentials)
} yield token
The calls in the for comprehension after the getUserCredentialsByNickname(..) should only be executed if this call returns a Right[UserCredentials], but also the detailed error information from each returned Either should be returned from the handler. getUserCredentialsByNickname(..)之后的for comprehension中的调用只应在此调用返回Right [UserCredentials]时执行,而且应从处理程序返回每个返回的Either的详细错误信息。
So now I've tried to use Scalaz Either (which is a right biased Either compared to the neutral scala Either) and the Monad Transformer EitherT and it seems it does exactly what I want. 所以现在我已经尝试使用Scalaz Either(这是一个正确的偏向Either与中性scala Either相比)和Monad Transformer EitherT,它似乎完全符合我的要求。 Thanks to Huw and especially Lars Hupel for hinting me in the right direction.
感谢Huw,特别是Lars Hupel暗示我正确的方向。
Here is working a sample for Twitter futures and Scalaz Either and EitherT: 这里有一个Twitter期货和Scalaz Either和EitherT的样本:
import com.twitter.util.{Await, Future}
import scalaz.{Monad, Functor, EitherT, \/}
import scalaz.syntax.ToIdOps
object EitherTest extends App with ToIdOps{
// make Twitter futures work with EitherT
implicit val FutureFunctor = new Functor[Future] {
def map[A, B](a: Future[A])(f: A => B): Future[B] = a map f
}
implicit val FutureMonad = new Monad[Future] {
def point[A](a: => A): Future[A] = Future(a)
def bind[A, B](fa: Future[A])(f: (A) => Future[B]): Future[B] = fa flatMap f
}
// The example begins here:
case class InvalidInfo(error: String)
case class Response(msg: String)
class ComponentA {
def foo(fail: Boolean): Future[\/[InvalidInfo, Response]] = {
if(fail) Future(InvalidInfo("Error A").left) else Future(Response("ComponentA Success").right)
}
}
class ComponentB {
def bar(fail: Boolean): Future[\/[InvalidInfo, Response]] = {
if(fail) Future(InvalidInfo("Error B").left) else Future(Response("ComponentB Success").right)
}
}
val a = new ComponentA
val b = new ComponentB
val result = for {
resultA <- EitherT(a.foo(false))
resultB <- EitherT(b.bar(false))
} yield (resultA, resultB)
println(Await.result(result.run))
}
You could extend the Future class by implicitly adding a method that handles Either
, instead of having to match it by yourself every time: 您可以通过隐式添加一个处理
Either
的方法来扩展Future类,而不是每次都必须自己匹配它:
implicit class EitherHandlingFuture[Exception, Value](future: Future[Either[Exception, Value]]) {
def mp[Return](fn: Value => Return) = {
future.map { eth: Either[Exception, Value] =>
eth match {
case Left(ex: Exception) => { print("logging the exception") /* handle or rethrow */ }
case Right(res: Value) => fn(res)
}
}
}
}
Then, this would be possible: 然后,这将是可能的:
def someComputation: Future[Either[Exception, Int]] = Future.value(Right(3))
someComputation mp { res: Int =>
println(res)
}
Note that the snippet above doesn't play with for
comprehensions, because to support them, it would be necessary to fully implement map/flatMap. 请注意,上面的代码段并不
for
理解,因为为了支持它们,有必要完全实现map / flatMap。 For that, you'd probably want to subclass Future
. 为此,你可能想要继承
Future
。
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