RHS 上的模式绑定变量类型与 LHS 上的匹配模式不对应

Question

为什么偏函数

val warpedEngineers: PartialFunction[Warped[Crewmember], Warped[Engineer]] = {
  case v@Warped(Engineer(name: String)) => v.asInstanceOf[Warped[Engineer]]
}

似乎需要 asInstanceOf 在 RHS 上强制转换，而以下不需要

val engineers: PartialFunction[Crewmember, Engineer] = {
  case v@Engineer(name) => v
}

given

sealed trait Crewmember
case class Engineer(name: String) extends Crewmember
case class Commander(name: String) extends Crewmember

case class Warped[+A <: Crewmember](v: A)

val engineers: PartialFunction[Crewmember, Engineer] = {
  case v@Engineer(name) => v
}

val warpedEngineers: PartialFunction[Warped[Crewmember], Warped[Engineer]] = {
  case v@Warped(Engineer(name: String)) => v.asInstanceOf[Warped[Engineer]]
}

val crew: List[Crewmember] = 
  List(Engineer("Geordi"), Commander("Picard"), Engineer("Scott"), Commander("Kirk"))

val warpedCrew: List[Warped[Crewmember]] = 
  List(Warped(Engineer("Geordi")), Warped(Commander("Picard")), Warped(Engineer("Scott")), Warped(Commander("Kirk")))

crew collect engineers
// res0: List[Engineer] = List(Engineer(Geordi), Engineer(Scott))

warpedCrew collect warpedEngineers
// res1: List[Warped[Engineer]] = List(Warped(Engineer(Geordi)), Warped(Engineer(Scott)))

可以像这样避免使用asInstanceOf 进行强制转换

case Warped(eng: Engineer) => Warped(eng) 

但我想知道为什么编译器不插入隐式asInstanceOf 而是将v 键入到Warped[Crewmember]

val warpedEngineers: PartialFunction[Warped[Crewmember], Warped[Engineer]] = {
  case v@Warped(Engineer(name: String)) => v 
}

Error: type mismatch;
 found   : Warped[Crewmember]
 required: Warped[Engineer]
      case v@Warped(Engineer(name: String)) => v

根据SLS 8.1.3: Pattern Binders

图案活页夹????@????由模式变量组成？？？？和一个图案 ????。变量的类型？？？是静态类型？？？暗示的 图案 ？？？？。此模式匹配任何值 ????与模式匹配 ??????，并将变量名绑定到该值。

一个模式？？？？暗示一种类型？？？？如果模式只匹配 类型？？？？？？。

Answer 1

Warped(Engineer(name))在左边

case v@Warped(Engineer(name: String)) => v

有静态类型 Warped[Crewmember] 因为那是你在类型签名中写的

val warpedEngineers: PartialFunction[Warped[Crewmember], ...

所以如果你在右边只写v，那就是类型不匹配。

Warped(Engineer(name))在右边和左边

case Warped(Engineer(name)) => Warped(Engineer(name))

看起来相似但不同，因为它们有不同的类型。它们实际上是Warped[Crewmember](Engineer(name)) 和Warped[Engineer](Engineer(name))。因为协方差Warped[Engineer] 是Warped[Crewmember]，但反之则不然。

编译器应该如何猜测它应该在此处插入 asInstanceOf 而不应该在例如 val x: Int = "a" 中插入？

如果你更改签名

val warpedEngineers: PartialFunction[Warped[Engineer], Warped[Engineer]] = {
  case v@Warped(Engineer(name)) => v
}

那么v 的静态类型将是Warped[Engineer] 并且代码将编译。

同样，如果你使用类型化模式

val warpedEngineers: PartialFunction[Warped[Crewmember], Warped[Engineer]] = {
  case v: Warped[Engineer] => v
}

那么v 的静态类型将是Warped[Engineer] 并且代码将编译。

似乎在规范模式v@Warped(Engineer(name))中

val warpedEngineers: PartialFunction[Warped[Crewmember], Warped[Engineer]] = {
  case v@Warped(Engineer(name)) => ...
}

“暗示”类型Warped[Crewmember]（因为签名）。