为什么for-comp内部的for-comp不起作用 [英] Why does a for-comp inside of a for-comp not work

问题描述

以下可以正常工作:

def show(id: Int) = myAction asyc {

  for {
    users <- userService.getAll()
    locations <- locationService.getByUsers(users)
  } yield {
    Ok("hi")
  }

}

但是，如果我这样做:

 for {
    users <- userService.getAll()
    locations <- locationService.getByUsers(users)
  } yield {
    for {
      f1 <- ...
      f2 <- ....
    } yield {
      Ok("hi")
    }
  }

我收到类型不匹配错误:

I get a type mismatch error:

found   : scala.concurrent.Future[play.api.mvc.Result]
[error]  required: play.api.mvc.Result
[error]           f1 <- ....

希望有人可以帮我解释一下.

Hoping someone can explain this for me.

推荐答案

添加类型注释可能有助于更清楚地了解正在发生的事情:

Adding type annotations might help make clearer what's going on:

val result: Future[Future[Result]] = for {
  users <- userService.getAll()
  locations <- locationService.getByUsers(users)
} yield {
  val innerResult: Future[Result] = for {
    f1 <- ...
    f2 <- ....
  } yield {
    Ok("hi")
  }
  innerResult
}

通过产生Future[Result](innerResult)，您使最外面的for成为Future[Future[Result]]类型.

By yielding a Future[Result] (innerResult), you're causing the outermost for to be of type Future[Future[Result]].

我们知道result必须是Future [???]类型(由第一个生成器的类型userService.getAll()确定).它最终会成为Future[Future[Result]]，因为您会产生Future[Result].

We know that result is going to have to be of type Future[???] (determined by the type of the first generator, userService.getAll()). It winds up being a Future[Future[Result]] because you yield a Future[Result].

解决方案是生成Result intead:

The solution is to yield a Result intead:

def foo(users: Seq[User], locations: Seq[Location]): Future[Result] = {
  for {
    f1 <- ...
    f2 <- ....
  } yield {
    Ok("hi")
  }
}

for {
  users <- userService.getAll()
  locations <- locationService.getByUsers(users)
  result <- foo(users, locations)
} yield result

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