相关疑难解决方法(0)

是否有详细的指南从filter()移动到withFilter()？现在我收到有关使用filter()实现的警告,但找不到移动到withFilter()的简单指南...

for {
  a <- Some(1)
  b <- Some(2)
} yield (a, b)

回报 Some((1, 2))

for {
  a <- Right(1).right
  b <- Left(2).left
} yield (a, b)

回报 Left((1, 2))

现在我想在理解中分解元组.

for {
  (a, b) <- Some((1, 2))
  (c, d) <- Some((3, 4))
} yield (a, b, c, d)

回报 Some((1, 2, 3, 4))

for {
  (a, b) <- Right((1, 2)).right
  (c, d) <- Left((3, 4)).left
} yield (a, b, c, d)

无法编译:

error: constructor cannot be instantiated to expected …

给出以下代码snippelt:

import scala.util.Try

def foo(x:Int) : (Int, String) = {
  (x+1, x.toString)
}

def main(args: Array[String]) : Unit = {
  val r1: Try[(Int, String)] = for {
    v <- Try { foo(3) }
  } yield v

  val r2: Try[(Int, String)] = for {
    (i, s)  <- Try { foo(3) } // compile warning refers to this line
  } yield (i, s)
}

1.为什么编译上面的代码会抛出以下警告？

`withFilter' method does not yet exist on scala.util.Try[(Int, String)], using `filter' method instead
[warn]       (i, s)  <- …

Scala 2.10似乎更新了对Either的理解.在2.10:

scala> val a = Right(5)
a: scala.util.Right[Nothing,Int] = Right(5)

scala> for {aa: Int <- a.right} yield {aa}
<console>:9: error: type mismatch;
 found   : Int => Int
 required: scala.util.Either[Nothing,Int] => ?
          for {aa: Int <- a.right} yield {aa}
                       ^

在2.9.3中,以上是可以的.

scala> val a = Right(5)
a: Right[Nothing,Int] = Right(5)

scala> for {aa: Int <- a.right} yield {aa}
res0: Product with Serializable with Either[Nothing,Int] = Right(5)

只需在2.10中删除aa的类型即可轻松修复.但我想知道为什么行为会在2.9和2.10之间发生变化.