Scala 中的类型擦除 [英] Type Erasure in Scala
问题描述
我对这里发生的事情感到很困惑:
I'm rather confused about what's happening here:
import scala.collection.immutable._
object Main extends App {
sealed trait Node
sealed trait Group
case class Sheet(
val splat: String,
val charname: String,
val children: ListMap[String, Node],
val params0: ListMap[String, Param], //params0 to separate sheet-general parameters
val note: Option[Note]
) extends Node with Group
case class Attributes(val name: String) extends Node with Group
case class Param(val name: String, val value: String) extends Node
case class Note(val note: String) extends Node
我有三个版本的替换函数——最后一个是我真正要编写的版本,其他的只是调试.
I've got three versions of a replace function - the last one is the one I'm actually trying to write, the others are just debugging.
class SheetUpdater(s: Sheet) {
def replace1[T <: Group](g: T): Unit = {
s.children.head match {
case (_, _:Sheet) =>
case (_, _:Attributes) =>
}
}
}
这个版本没有抛出任何警告,所以显然我可以在运行时访问 s.children
的类型.
This version throws no warnings, so apparently I have access to the type of s.children
at runtime.
class SheetUpdater(s: Sheet) {
def replace2[T <: Group](g: T): Unit = {
g match {
case _:Sheet =>
case _:Attributes =>
}
}
}
这个版本也没有,所以显然g
的类型的细节在运行时也可用...
Neither does this version, so apparently the details of g
's type are also available at runtime...
class SheetUpdater(s: Sheet) {
def replace3[T <: Group](g: T): Unit = {
s.children.head match {
case (_, _:T) => //!
case (_, _:Attributes) =>
}
}
}
...但即便如此,这最终还是让我感到可怕的抽象类型模式 T 未被检查,因为它被擦除
警告消除了.这是怎么回事?
... but even so, this ends up throwing me the dreaded Abstract type pattern T is unchecked since it is eliminated by erasure
warning. What's going on here?
推荐答案
在 Scala 中,泛型在运行时被擦除,这意味着 List[Int]
和 List[Boolean]
其实是一样的.这是因为 JVM 作为一个整体擦除了泛型类型.这一切都是因为 JVM 希望在首次引入泛型时保持向后兼容......
In Scala, generics are erased at runtime, which means that the runtime type of List[Int]
and List[Boolean]
is actually the same. This is because the JVM as a whole erases generic types. All this is due because the JVM wanted to remain backwards compatible way back when generics were first introduced...
在 Scala 中有一种使用 ClassTag
的方法来解决这个问题,这是一个隐式参数,然后可以与您正在使用的任何泛型进行线程化.
There is a way around this in Scala using a ClassTag
, which is an implicit parameter that then can be threaded around with whatever generic you are using.
您可以将 : ClassTag
视为将泛型类型作为参数传递.(它是传递 ClassTag[T]
类型的隐式参数的语法糖.)
You can think of : ClassTag
as passing the type of the generic as an argument. (It is syntactic sugar for passing an implicit parameter of type ClassTag[T]
.)
import scala.reflect.ClassTag
class SheetUpdater(s: Sheet) {
def replace3[T <: Group : ClassTag](g: T): Unit = {
s.children.head match {
case (_, _:T) => //!
case (_, _:Attributes) =>
}
}
}
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