Scala未来与过滤器在理解 [英] Scala Future with filter in for comprehension
问题描述
在下面的例子中,我得到了异常 java.util.NoSuchElementException:Future.filter谓词不满足
当检查 if(i == 2)
失败时,我希望得到结果 Future(Test2)
如何处理过滤器/如果在处理组合期货的理解?
下面是一个在Scala REPL中工作的简单示例。
代码:
import scala.concurrent.Future
import scala.util。{Try,Success,Failure}
import scala.concurrent.ExecutionContext.Implicits.global
val f1 = Future(1)
val f2 = for(
i< - f1
if(i == 2)
} yieldTest1
f2.recover {case _ => Test2}
f2.value
在 for-comprehension
中,您按照 i == 2
过滤。因为 f1
的值不是两个,所以不会产生成功
,而是一个失败
。过滤器的谓词不满足,因为你的错误信息告诉你。但是, f2.recover
会返回一个新的 Future
。 f2
的值不被操纵。它仍然存储失败
。这就是当您调用 f2.value
时出现错误消息的原因。
我能想到的唯一选择就是在 for-comprehension中使用
如图所示此处。 (i < - f1)yield {b $ b $ if(i == 2)Test1 else
elseTest2
}
f2.value
将会返回 Some(成功(Test2))
作为您的 f3.value
。
In the example below I get the exception java.util.NoSuchElementException: Future.filter predicate is not satisfied
I want to have the result Future( Test2 )
when the check if( i == 2 )
fails. How do I handle filter/if within a for comprehension that deals with composing futures?
Below is a simplified example that works in the Scala REPL.
Code:
import scala.concurrent.Future
import scala.util.{ Try, Success, Failure }
import scala.concurrent.ExecutionContext.Implicits.global
val f1 = Future( 1 )
val f2 = for {
i <- f1
if( i == 2 )
} yield "Test1"
f2.recover{ case _ => "Test2" }
f2.value
In your for-comprehension
, you are filtering by i == 2
. Because the value of f1
is not two, it will not yield a Success
but instead a Failure
. The predicate of the filter is not satisfied, as your errror message tells you. However, f2.recover
returns a new Future
. The value of f2
is not manipulated. It still stores the Failure
. That is the reason you get the error message when you call f2.value
.
The only alternative I can think of would be using an else
in your for-comprehension
as shown here.
val f2 = for ( i <- f1) yield {
if (i == 2) "Test1"
else "Test2"
}
f2.value
This will return Some(Success(Test2))
as your f3.value
does.
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