如何在dotty中用缩小的字符串类型头编写匹配类型模式？

Question

目前，我正在尝试了解Scala 3/dotty中的新特性。所以我试着重新做一些我以前尝试过的不成形的东西。考虑到缩小字符串类型的异构列表(在无形状字符串中将是"a" :: "c" :: "f" :: HNil，而在dotty中，元组可以使用("a", "c", "f"))，我想根据某些映射替换这些类型。例如，考虑以下伪代码：

type MyListOfNames = ("a", "c", "f")
type Mapping = ("a" -> "b", "c" -> "d")
// somehow apply the mapping/replacements as the new type alias `MyListOfRenamedNames`
type MyListOfRenamedNames = ("b", "d", "f")

为此，我提出了以下代码。重新映射单个缩小的字符串类型正在工作。但我也无法让它与元组一起工作：

object A:
  trait Remapping
  case object ReEmpty extends Remapping
  case class ReCons[N1 <: String, N2 <: String, R <: Remapping](n1: N1, n2: N2, rest: R) extends Remapping

  type Remapped[X <: String, R <: Remapping] <: String = R match
    case ReEmpty.type     => X
    case ReCons[X, n, _]  => n
    case ReCons[_, _, rr] => Remapped[X, rr]

  type AllRemapped[T <: Tuple, R <: Remapping] <: Tuple = T match
    case Unit      => Unit
    case s *: rest => s match
      case String => Remapped[s, R] *: AllRemapped[rest, R]
  //this part doesn't compile, giving following compile error:
  //type s doesn't satisfy upper bound String

  @main def main: Unit =
    type RemapAtoBAdCtoD = ReCons["a", "b", ReCons["c", "d", ReEmpty.type]]
    val expectedToCompile1: Remapped["a", RemapAtoBAdCtoD] = "b"
    val expectedToCompile2: Remapped["c", RemapAtoBAdCtoD] = "d"
    val expectedToCompile3: Remapped["f", RemapAtoBAdCtoD] = "f"
    val expectedToCompile4: Remapped["a", ReEmpty.type] = "a"
    //above examples compile as expected

    // val expectedNotToCompile: Remapped["a", RemapAtoBAdCtoD] = "a"
    //above example doesn't compile as expected

    //I am trying to get following:
    type MyList = ("a", "c", "f")
    val remapped: AllRemapped[MyList, RemapAtoBAdCtoD] = ("b", "d", "f")
  end main

end A

我得到的编译错误是以下行中的Type argument s does not conform to upper bound String：

      s match
        case String => Remapped[s, R] *: AllRemapped[rest, R]

我使用了dotty版本的0.18.1-RC1，因为它是Scastie上最新的可用版本。下面是一个您可以实验的链接：https://scastie.scala-lang.org/BKzhEV7PRiKyfQ3CE2vjww

这不受支持吗?是否有一种方法可以实现这一点，即如何进一步限制匹配类型内的类型类型(我尝试过case (s <: String) *: rest =>，但编译器由于错误：scala.MatchError: Parens(Ident(s)) (of class dotty.tools.dotc.ast.untpd$Parens)而失败)？另外，是否有更好的方法来实现我所尝试的总体目标(在dotty当前的能力范围内，比如erased和inline)？

Answer 1

尝试引入助手类型，并将其用作类型模式。

type Hlp[X <: String, Rest <: Tuple] = X *: Rest

type AllRemapped[T <: Tuple, R <: Remapping] <: Tuple = T match {
  case Unit         => Unit
  case Hlp[s, rest] => Remapped[s, R] *: AllRemapped[rest, R]
}

inline和erased不适用于type。

实际上，对于在元组上的映射，有标准类型的Tuple.Map，尽管目前在0.18.1-RC1中，我无法使它工作。

type AllRemapped[T <: Tuple, R <: Remapping] = Tuple.Map[T, [X <: String] =>> Remapped[X, R]]

//Type argument [X <: String] => A.Remapped[X, R] does not conform to upper bound [_$22] => Any 

有了inline你就能做到

inline def g(x: "a" | "c" | "f") <: String = inline x match {
  case "a" => "b"
  case "c" => "d"
  case "f" => "f"
}

g("a"): "b"
g("c"): "d"
g("f"): "f"
// g("x") // doesn't compile

试一试

sealed trait Remapping
case object ReEmpty extends Remapping
case class ReCons[N1 <: String, N2 <: String, R <: Remapping](n1: N1, n2: N2, rest: R) extends Remapping

type Remapped[X <: String, R <: Remapping] <: String = R match {
  case ReEmpty.type     => X
  case ReCons[X, n, _]  => n
  case ReCons[_, _, rr] => Remapped[X, rr]
}

inline def getRemapped[X <: String & Singleton, R <: Remapping] erased (x: X, r: R) <: String = inline r match {
  case ReEmpty             => x
  case rc: ReCons[X, _, _] => rc.n2
  case rc: ReCons[_, _, _] => getRemapped(x, rc.rest).asInstanceOf[Remapped[X, R]]
}

type RemapAtoBAndCtoD = ReCons["a", "b", ReCons["c", "d", ReEmpty.type]]
val remapping: RemapAtoBAndCtoD = ReCons("a", "b", ReCons("c", "d", ReEmpty))
val remapped2: ("b", "d", "f") = (
  getRemapped("a", remapping),
  getRemapped("c", remapping),
  getRemapped("f", remapping)
)  // (b,d,f)

//myList.map[[X <: String] =>> Remapped[X, RemapAtoBAndCtoD]]([X <: String] => (x: X) => getRemapped(x, remapping))
//[error]    |Found:    Object with PolyFunction {...}
//[error]    |Required: PolyFunction{apply: [t](x$1: t): A.Remapped[t, A.RemapAtoBAndCtoD]}
//https://github.com/milessabin/shapeless/pull/901