Scala 的类型擦除如何用于更高种类的类型参数？

Question

我不明白Scala 删除了哪些 泛型类型参数。以前觉得应该把all个泛型类型参数抹掉，但是好像不是这样

如果我错了请指正：如果我在代码中实例化了一个 Map[Int, String] 类型的实例，那么在运行时，该实例只知道它是 Map[_, _] 类型，并且对其泛型类型参数一无所知。这就是为什么以下命令能够成功编译和执行而没有错误的原因：

val x: Map[Int, String] = Map(2 -> "a")
val y: Map[String, Int] = x.asInstanceOf[Map[String, Int]]

现在我希望所有更高种类的类型参数也被删除，也就是说，如果我有一个

class Foo[H[_, _], X, Y]

我希望类型 Foo[Map, Int, String] 的实例对 Map 一无所知。但现在考虑以下类型转换序列 "experiments":

import scala.language.higherKinds

def cast1[A](a: Any): A = a.asInstanceOf[A]
def cast2[H[_, _], A, B](a: Any) = a.asInstanceOf[H[A, B]]
def cast3[F[_[_, _], _, _], H[_, _], X, Y](a: Any) = a.asInstanceOf[F[H, X, Y]]
class CastTo[H[_, _], A, B] {
  def cast(x: Any): H[A, B] = x.asInstanceOf[H[A, B]]
}

ignoreException {
  val v1 = cast1[String](List[Int](1, 2, 3))                      
  // throws ClassCastException
}

ignoreException {
  val v2 = cast2[Map, Int, Long](Map[String, Double]("a" -> 1.0)) 
  // doesn't complain at all, `A` and `B` erased
}

ignoreException {
  // right kind
  val v3 = cast2[Map, Int, Long]((x: Int) => x.toLong)            
  // throws ClassCastException
}

ignoreException {
  // wrong kind
  val v4 = cast2[Map, Int, Long]("wrong kind")                     
  // throws ClassCastException
}

ignoreException {
  class Foo[H[_, _], X, Y](h: H[X, Y])
  val x = new Foo[Function, Int, String](n => "#" * n)
  val v5 = cast3[Foo, Map, Int, Long](x)
  // nothing happens, happily replaces `Function` by `Map`
}

ignoreException {
  val v6 = (new CastTo[Map, Int, Long]).cast(List("hello?"))       
  // throws ClassCastException
}

ignoreException {
  val castToMap = new CastTo[Map, Int, Long]
  val v7 = castToMap.cast("how can it detect this?")               
  // throws ClassCastException
}

ignoreException {
  val castToMap = new CastTo[Map, Int, Long]
  val madCast = castToMap.asInstanceOf[CastTo[Function, Float, Double]]
  val v8 = madCast.cast("what does it detect at all?")
  // String cannot be cast to Function???
  // Why does it retain any information about `Function` here?
}


// --------------------------------------------------------------------
var ignoreBlockCounter = 0
/** Executes piece of code, 
  * catches an exeption (if one is thrown),
  * prints number of `ignoreException`-wrapped block,
  * prints name of the exception.
  */
def ignoreException[U](f: => U): Unit = {
  ignoreBlockCounter += 1
  try {
    f
  } catch {
    case e: Exception =>
      println("[" + ignoreBlockCounter + "]" + e)
  }
}

这是输出（scala -version 2.12.4）：

[1]java.lang.ClassCastException: scala.collection.immutable.$colon$colon cannot be cast to java.lang.String
[3]java.lang.ClassCastException: Main$$anon$$Lambda3/1744347043 cannot be cast to scala.collection.immutable.Map
[4]java.lang.ClassCastException: java.lang.String cannot be cast to scala.collection.immutable.Map
[6]java.lang.ClassCastException: scala.collection.immutable.$colon$colon cannot be cast to scala.collection.immutable.Map
[7]java.lang.ClassCastException: java.lang.String cannot be cast to scala.collection.immutable.Map
[8]java.lang.ClassCastException: java.lang.String cannot be cast to scala.Function1

情况1、3、4表明asInstanceOf[Foo[...]]确实关心Foo，这是意料之中的。
情况2表明asInstanceOf[Foo[X,Y]]不关心X和Y，这也是意料之中的。
案例 5 表明 asInstanceOf 不关心更高种类的类型参数 Map，与案例 2 类似，这也是预期的。

到目前为止一切顺利。但是，案例 6、7、8 表明了不同的行为：在这里，类型 CastTo[Foo, X, Y] 的实例似乎出于某种原因保留了有关泛型类型参数 Foo 的信息。更准确地说，CastTo[Map, Int, Long] 似乎携带了足够的信息，可以知道字符串不能转换为 Map。此外，在案例 8 中，它似乎甚至因为转换而将 Map 更改为 Function。

问题：

CastTo 的第一个通用参数没有被删除，我的理解是否正确，或者还有其他我没有看到的东西？一些隐式操作或什么？
是否有描述此行为的文档？
我有什么理由想要这种行为吗？我发现它有点违反直觉，但也许只有我一个人，我用错了工具...

感谢阅读。

编辑：在类似的示例中四处寻找发现了 2.12.4 编译器的问题（参见下面我自己的 "answer"），但这是一个单独的问题.

Answer 1

好的，这可能不是预期的行为。

这是上面的随机版本：

ignoreException {
  class Foo[X, Y]
  class Bar[X, Y]

  val castToMap = new CastTo[Map, Int, Long]
  val madderCast = if (math.random() > 0.5) {
    println("Foo")
    castToMap.asInstanceOf[CastTo[Foo, Float, Double]]
  } else {
    println("Bar")
    castToMap.asInstanceOf[CastTo[Bar, String, Any]]
  }
  val v9 = madderCast.cast("crash 'dis serva, awww yeah!")
  // crashes the compile server for scala 2.12.4
  // dotty 0.4.0-RC1 doesn't complain at all (as expected!)
}

2.12.4 编译器崩溃。 Dotty 没有问题。

编辑更短、独立的示例：

进口scala.language.higherKinds

class Foo[X]
class Bar[X]
class CrashIt[A[_]] {
  def cast(a: Any): A[Int] = a.asInstanceOf[A[Int]]
}
val c = if (true) new CrashIt[Foo] else new CrashIt[Bar]
val x = c.cast("")

崩溃：

java.util.NoSuchElementException: head of empty list
    at scala.collection.immutable.Nil$.head(List.scala:428)
    at scala.collection.immutable.Nil$.head(List.scala:425)
    at scala.tools.nsc.typechecker.ContextErrors$InferencerContextErrors$InferErrorGen$.
      NotWithinBoundsErrorMessage(ContextErrors.scala:1045)
    at scala.tools.nsc.typechecker.ContextErrors$InferencerContextErrors$InferErrorGen$.
      NotWithinBoundsContextErrors.scala:1052)
    at scala.tools.nsc.typechecker.Infer$Inferencer.issueBoundsError(Infer.scala:881)
    at scala.tools.nsc.typechecker.Infer$Inferencer.check(Infer.scala:887)
    at scala.tools.nsc.typechecker.Infer$Inferencer.checkBounds(Infer.scala:891)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.checkBounds(RefChecks.scala:1203)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.checkTypeRef(RefChecks.scala:1384)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.$anonfun$transform(RefChecks.scala:1668)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.$anonfun$transform$adapted(RefChecks.scala:1659)
    at scala.reflect.internal.tpe.TypeMaps$ForEachTypeTraverser.traverse(TypeMaps.scala:1102)
    at scala.reflect.internal.Types$Type.foreach(Types.scala:787)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transform(RefChecks.scala:1407)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transform(RefChecks.scala:107)
    at scala.reflect.internal.Trees.$anonfun$itransform(Trees.scala:1369)
    at scala.reflect.api.Trees$Transformer.atOwner(Trees.scala:2600)
    at scala.reflect.internal.Trees.itransform(Trees.scala:1368)
    at scala.reflect.internal.Trees.itransform$(Trees.scala:1348)
    at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
    at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
    at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transform(RefChecks.scala:1751)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transformStat(RefChecks.scala:1186)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.$anonfun$transformStats(RefChecks.scala:1169)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transformStats(RefChecks.scala:1169)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transformStats(RefChecks.scala:107)
    at scala.reflect.internal.Trees.itransform(Trees.scala:1416)
    at scala.reflect.internal.Trees.itransform$(Trees.scala:1348)
    at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
    at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
    at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transform(RefChecks.scala:1751)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transform(RefChecks.scala:107)
    at scala.reflect.api.Trees$Transformer.transformTemplate(Trees.scala:2563)
    at scala.reflect.internal.Trees.$anonfun$itransform(Trees.scala:1420)
    at scala.reflect.api.Trees$Transformer.atOwner(Trees.scala:2600)
    at scala.reflect.internal.Trees.itransform(Trees.scala:1419)
    at scala.reflect.internal.Trees.itransform$(Trees.scala:1348)
    at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
    at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
    at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transform(RefChecks.scala:1751)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transformStat(RefChecks.scala:1198)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.$anonfun$transformStats(RefChecks.scala:1169)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transformStats(RefChecks.scala:1169)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transformStats(RefChecks.scala:107)
    at scala.reflect.internal.Trees.itransform(Trees.scala:1378)
    at scala.reflect.internal.Trees.itransform$(Trees.scala:1348)
    at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
    at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
    at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transform(RefChecks.scala:1751)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transform(RefChecks.scala:107)
    at scala.reflect.internal.Trees.$anonfun$itransform(Trees.scala:1375)
    at scala.reflect.api.Trees$Transformer.atOwner(Trees.scala:2600)
    at scala.reflect.internal.Trees.itransform(Trees.scala:1373)
    at scala.reflect.internal.Trees.itransform$(Trees.scala:1348)
    at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
    at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
    at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transform(RefChecks.scala:1751)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transformStat(RefChecks.scala:1198)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.$anonfun$transformStats(RefChecks.scala:1169)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transformStats(RefChecks.scala:1169)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transformStats(RefChecks.scala:107)
    at scala.reflect.internal.Trees.itransform(Trees.scala:1416)
    at scala.reflect.internal.Trees.itransform$(Trees.scala:1348)
    at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
    at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
    at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transform(RefChecks.scala:1751)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transform(RefChecks.scala:107)
    at scala.reflect.api.Trees$Transformer.transformTemplate(Trees.scala:2563)
    at scala.reflect.internal.Trees.$anonfun$itransform(Trees.scala:1425)
    at scala.reflect.api.Trees$Transformer.atOwner(Trees.scala:2600)
    at scala.reflect.internal.Trees.itransform(Trees.scala:1424)
    at scala.reflect.internal.Trees.itransform$(Trees.scala:1348)
    at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
    at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
    at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transform(RefChecks.scala:1751)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transformStat(RefChecks.scala:1198)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.$anonfun$transformStats(RefChecks.scala:1169)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transformStats(RefChecks.scala:1169)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transformStats(RefChecks.scala:107)
    at scala.reflect.internal.Trees.$anonfun$itransform(Trees.scala:1438)
    at scala.reflect.api.Trees$Transformer.atOwner(Trees.scala:2600)
    at scala.reflect.internal.Trees.itransform(Trees.scala:1438)
    at scala.reflect.internal.Trees.itransform$(Trees.scala:1348)
    at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
    at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
    at scala.reflect.api.Trees$Transformer.transform(Trees.scala:2555)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transform(RefChecks.scala:1751)
    at scala.tools.nsc.typechecker.RefChecks$RefCheckTransformer.transform(RefChecks.scala:107)
    at scala.tools.nsc.ast.Trees$Transformer.transformUnit(Trees.scala:140)
    at scala.tools.nsc.transform.Transform$Phase.apply(Transform.scala:30)
    at scala.tools.nsc.Global$GlobalPhase.$anonfun$applyPhase(Global.scala:436)
    at scala.tools.nsc.Global$GlobalPhase.applyPhase(Global.scala:429)
    at scala.tools.nsc.Global$GlobalPhase.$anonfun$run(Global.scala:400)
    at scala.tools.nsc.Global$GlobalPhase.$anonfun$run$adapted(Global.scala:400)
    at scala.collection.Iterator.foreach(Iterator.scala:929)
    at scala.collection.Iterator.foreach$(Iterator.scala:929)
    at scala.collection.AbstractIterator.foreach(Iterator.scala:1417)
    at scala.tools.nsc.Global$GlobalPhase.run(Global.scala:400)
    at scala.tools.nsc.Global$Run.compileUnitsInternal(Global.scala:1452)
    at scala.tools.nsc.Global$Run.compileUnits(Global.scala:1436)
    at scala.tools.nsc.Global$Run.compileSources(Global.scala:1429)
    at scala.tools.nsc.Global$Run.compile(Global.scala:1545)
    at scala.tools.nsc.StandardCompileServer.session(CompileServer.scala:150)
    at scala.tools.util.SocketServer.$anonfun$doSession(SocketServer.scala:80)
    at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
    at scala.util.DynamicVariable.withValue(DynamicVariable.scala:58)
    at scala.Console$.withOut(Console.scala:163)
    at scala.tools.util.SocketServer.$anonfun$doSession(SocketServer.scala:80)
    at scala.tools.util.SocketServer.$anonfun$doSession$adapted(SocketServer.scala:75)
    at scala.tools.nsc.io.Socket.applyReaderAndWriter(Socket.scala:49)
    at scala.tools.util.SocketServer.doSession(SocketServer.scala:75)
    at scala.tools.util.SocketServer.loop(SocketServer.scala:91)
    at scala.tools.util.SocketServer.run(SocketServer.scala:103)
    at scala.tools.nsc.CompileServer$.$anonfun$execute(CompileServer.scala:216)
    at scala.runtime.java8.JFunction0$mcZ$sp.apply(JFunction0$mcZ$sp.java:12)
    at scala.util.DynamicVariable.withValue(DynamicVariable.scala:58)
    at scala.Console$.withOut(Console.scala:163)
    at scala.tools.nsc.CompileServer$.$anonfun$execute(CompileServer.scala:211)
    at scala.runtime.java8.JFunction0$mcZ$sp.apply(JFunction0$mcZ$sp.java:12)
    at scala.util.DynamicVariable.withValue(DynamicVariable.scala:58)
    at scala.Console$.withErr(Console.scala:192)
    at scala.tools.nsc.CompileServer$.main(CompileServer.scala:211)
    at scala.tools.nsc.CompileServer.main(CompileServer.scala)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at scala.reflect.internal.util.ScalaClassLoader.$anonfun$run(ScalaClassLoader.scala:99)
    at scala.reflect.internal.util.ScalaClassLoader.asContext(ScalaClassLoader.scala:34)
    at scala.reflect.internal.util.ScalaClassLoader.asContext$(ScalaClassLoader.scala:30)
    at scala.reflect.internal.util.ScalaClassLoader$URLClassLoader.asContext(ScalaClassLoader.scala:125)
    at scala.reflect.internal.util.ScalaClassLoader.run(ScalaClassLoader.scala:99)
    at scala.reflect.internal.util.ScalaClassLoader.run$(ScalaClassLoader.scala:91)
    at scala.reflect.internal.util.ScalaClassLoader$URLClassLoader.run(ScalaClassLoader.scala:125)
    at scala.tools.nsc.CommonRunner.run(ObjectRunner.scala:22)
    at scala.tools.nsc.CommonRunner.run$(ObjectRunner.scala:21)
    at scala.tools.nsc.ObjectRunner$.run(ObjectRunner.scala:39)
    at scala.tools.nsc.CommonRunner.runAndCatch(ObjectRunner.scala:29)
    at scala.tools.nsc.CommonRunner.runAndCatch$(ObjectRunner.scala:28)
    at scala.tools.nsc.ObjectRunner$.runAndCatch(ObjectRunner.scala:39)
    at scala.tools.nsc.MainGenericRunner.runTarget(MainGenericRunner.scala:66)
    at scala.tools.nsc.MainGenericRunner.run(MainGenericRunner.scala:85)
    at scala.tools.nsc.MainGenericRunner.process(MainGenericRunner.scala:96)
    at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:101)
    at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)

铸造步骤似乎很关键。如果删除 cast 的调用，则不会发生任何事情。

如果参数 A 不是更高级的，则不会发生这种情况，因此例如对于 class CrashIt[A] 和 new CrashIt[Int] 编译器不会报错。

EDIT-2：找不到这个确切的错误（或明显的概括），在此处报告：https://github.com/scala/bug/issues/10700 .

Answer 2

我认为你混淆了一些东西。

对通用类型的转换被推迟到类型变得具体的时候。例如，拿这段代码：

class CastTo[H[_, _], A, B] {
  def cast(x: Any): H[A, B] = x.asInstanceOf[H[A, B]]
}

在字节码中你只能转换为真正的 class，因为它对泛型一无所知。所以上面的内容，在字节码中，大致等同于：

class CastTo {
  def cast(x: Object): Object = x
}

然后在后面的代码中你给方法cast一个String，编译器可以看到根据它拥有的类型信息，一个Map[Int, Long]会出来。但是在字节码 cast 中有一个被擦除的 return 类型的 Object，所以编译器必须在 use-site 的cast 方法。此代码

val castToMap = new CastTo[Map, Int, Long]
val v7 = castToMap.cast("how can it detect this?")

将在字节码中大致等同于以下（伪）代码：

val castToMap = new CastTo
val v7 = castToMap.cast("how can it detect this?").asInstanceOf[Map]

关于您的其他问题：

不是我立即知道的。
你为什么不想要它？您正在将 String 转换为 Map[Int, Long]。那最终肯定会崩溃。使用 ClassCastException 快速（相对）失败可能是最安全、对用户最友好的选择。

Scala 的类型擦除如何用于更高种类的类型参数？

How does Scala's type erasure work for higher kinded type parameters?

generics

scala

type-erasure

higher-kinded-types

scala-compiler