How do I write a match type pattern with a narrowed String type head of tuple case in dotty?

Question

I am currently try to get a feeling of the new features in Scala 3/dotty. So I am trying to redo something I tried with shapeless before. Given a heterogenous list of narrowed string types (in shapeless it would be "a" :: "c" :: "f" :: HNil and in dotty as I understand, the tuples can be utilized ("a", "c", "f")), I'd like to replace the types according to some mapping. For example consider following pseudocode:

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")

For this, I came up with following code. Remapping a single narrowed String type is working. But I couldn't get it to work with tuples as well:

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

The compilation error I get is Type argument s does not conform to upper bound String in following line:

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

I used dotty version 0.18.1-RC1 as it is the latest available one on Scastie. Here is a link that you can experiment on: https://scastie.scala-lang.org/BKzhEV7PRiKyfQ3CE2vjww

Is this not supported, is there a way to achieve this, i.e. how to further restrict the type in type patterns inside a match type (I tried case (s <: String) *: rest =>, but compiler failed with an error: scala.MatchError: Parens(Ident(s)) (of class dotty.tools.dotc.ast.untpd$Parens))? Also is there better way to achieve what I try to do overall (within current capabilities of dotty, like with erased and inline)?

Answer 1

Try to introduce helper type and use it as a type pattern

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 and erased do not work with type.

Actually for mapping over a tuple there is standard type Tuple.Map although currently in 0.18.1-RC1 I can't make it work

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 

With inline you can do

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

---

Try

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