斯卡拉的虚无与部分统一 [英] Scala's Nothing vs partial unification

问题描述

我希望以下代码能够正常编译:

 特征Widen [M [_]] {def widen [A，B> ;: A](ma:M [A]):M [B]}物件扩大{隐式类Ops [M [_]，A](ma:M [A]){def widen [B> ;: A](隐式ev:Widen [M]):M [B] = ev.widen [A，B](ma)}//隐式类OpsNothing [M [_]](ma:M [Nothing]){//def widen [B](隐式ev:Widen [M]):M [B] = ev.widen(ma)//}隐式val WidenList = new Widen [List] {def widen [A，B> ;: A](l:List [A]):List [B] = l}}导入Widen._List.empty [Some [Int]].widen [Option [Int]]List.empty [Nothing] .widen [Int]//在取消注释OpsNothing之前不会编译 

但是最后一行不编译.这似乎与部分统一有关，因为 new Widen.Ops [List，Nothing](List.empty [Nothing]).widen [Int] 确实可以编译.

现在真正奇怪的是，如果我取消注释 Nothing 的特殊情况，则所有内容都会编译.

我不知道发生了什么事...

(这使用的是Scala 2.13.3)

解决方案

编译器不希望在解析隐式时推断 Nothing

对于<:< 的无"失败的隐式解析失败>

除了 OpsNothing 之外的另一种解决方法是键入 Bottom

  type底部< ;:无List.empty [Bottom] .widen [Int]//编译 

I would expect the following code to compile just fine:

trait Widen[M[_]] { def widen[A, B >: A](ma: M[A]): M[B] }

object Widen {
  implicit class Ops[M[_], A](ma: M[A]) {
    def widen[B >: A](implicit ev: Widen[M]): M[B] = ev.widen[A, B](ma)
  }
  // implicit class OpsNothing[M[_]](ma: M[Nothing]) {
  //   def widen[B](implicit ev: Widen[M]): M[B] = ev.widen(ma)
  // }
  implicit val WidenList = new Widen[List] { def widen[A, B >: A](l: List[A]): List[B] = l }
}

import Widen._

List.empty[Some[Int]].widen[Option[Int]]

List.empty[Nothing].widen[Int] // does not compile until uncommenting OpsNothing

But the last line does not compile. It seems to be related to partial unification because new Widen.Ops[List, Nothing](List.empty[Nothing]).widen[Int] does compile.

Now what's really weird is that if I uncomment the special case for Nothing then everything compiles.

I have no idea what's going on...

(This is using Scala 2.13.3)

解决方案

Compiler doesn't like to infer Nothing while resolving implicits

Failed implicit resolution for Nothing with <:<

Another workaround besides OpsNothing is type Bottom

type Bottom <: Nothing
List.empty[Bottom].widen[Int] // compiles

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