Question

如何在Scala中执行此类操作？

case class Foo[A](x: A) {

  def get[T]: Option[T] = x match {
    case x: T => Some(x)    // if x is of type T i.e. T =:= A
    case _ => None
  }
}

val test = Foo("hi")
assert(test.get[Int] == None)
assert(test.get[String] == Some("hi"))

我试过这个并且遇到了一些奇怪的时间推断失败：

import scala.util.{Try, Success}
import reflect._

case class Foo[A](x: A) extends Dynamic {

  def get[T: ClassTag]: Option[T] = Try(x.asInstanceOf[T]) match {
    case Success(r) => Some(r) 
    case _ => None
  }
}

object Foo extends App {
  val test = Foo("hi")
  val wtf: Option[Int] = test.get[Int]
  assert(wtf.isInstanceOf[Option[String]])
  assert(wtf == Some("hi"))     // how????
  // val wtf2: Option[String] = wtf  // does not compile even if above assert passes!!
}

Answer 1

当然是一个骗局，但是仓促地说：

scala> :pa
// Entering paste mode (ctrl-D to finish)

import reflect._
case class Foo[A](x: A) {

  def get[T: ClassTag]: Option[T] = x match {
    case x: T => Some(x)    // if x is of type T i.e. T =:= A
    case _ => None
  }
}

// Exiting paste mode, now interpreting.

import reflect._
defined class Foo

scala> val test = Foo("hi")
test: Foo[String] = Foo(hi)

scala> test.get[Int]
res0: Option[Int] = None

scala> test.get[String]
res1: Option[String] = Some(hi)

Answer 2

如果你可以扔掉get：

case class Foo[A](x: A)
Seq(Foo("hi"), Foo(1), Foo(2d)).collect { case f @ Foo(x: String) => f }.foreach(println)

结果：

Foo(hi)

其他人同样微不足道。

Scala检查泛型的类型

2 个答案: