Trouble with using match-types

Question
1

Hi. Can anyone who know match types tell me why compiler says that it cannot find b?

Code 
trait InfiniteSequence {
  type L; type R;
  val left: L; val right: R
}
class Cons [A, B <: InfiniteSequence](var a:A, var b:B) extends InfiniteSequence {
  type L = A; type R = B; val left = a; val right = b;
}
object End extends InfiniteSequence { 
  type L = End.type; type R = End.type; val left = this; val right = this; }
type End = End.type
type _AdditionHelper[A <: InfiniteSequence, B <: InfiniteSequence] = B match {
  case End => A
  case Cons[a, b] => _AdditionHelper[Cons[a, B], b.R]
}
type Add[A <: InfiniteSequence, B <: InfiniteSequence] = _AdditionHelper[A, B]
type _RemoveHelper[Value, Checked <: InfiniteSequence, Remaining <: InfiniteSequence] = Remaining match {
  case End => Checked
  case Cons[t, p] => _RemoveHelper[Value, Cons[t, p], p.R]
  case Cons[Value, p] => _RemoveHelper[Value, Checked, p.R]
}
type Remove[Value, Target <: InfiniteSequence] = _RemoveHelper[Value, End, Target]
type Z4 = Add[Cons[Nothing, Cons[Nothing, End]], Cons[Nothing, Cons[Nothing, End]]]
2

I guess because in b.R, b is always a value. While in case Cons[a, b], b is a type. So there is a type b, but the compiler cannot find a value b.

1 Like
3

So I assume that I should use # to get the type, and I did but there is another error p is not a legal path since it is not a concrete type which suggests that I cannot do recursion? Is there a way to make it work, I don’t know scala 3 well.

4

p is an abstract type “variable” in the case clause.

5

It turns out that it was actually trivial. I could just use it like this:

trait InfiniteSequence {
  type L; type R;
  val left: L; val right: R
}
class Cons [A, B <: InfiniteSequence](var a:A, var b:B) extends InfiniteSequence {
  type L = A; type R = B; val left = a; val right = b;
}
object End extends InfiniteSequence { 
  type L = End.type; type R = End.type; val left = this; val right = this; }
type End = End.type
type _AdditionHelper[A <: InfiniteSequence, B <: InfiniteSequence] = B match {
  case End => A
  case Cons[a, b] => _AdditionHelper[Cons[a, B], b]
}
type Add[A <: InfiniteSequence, B <: InfiniteSequence] = _AdditionHelper[A, B]
type _RemoveHelper[Value, Checked <: InfiniteSequence, Remaining <: InfiniteSequence] = Remaining match {
  case End => Checked
  case Cons[t, p] => _RemoveHelper[Value, Cons[t, p], p]
  case Cons[Value, p] => _RemoveHelper[Value, Checked, p]
}
type Remove[Value, Target <: InfiniteSequence] = _RemoveHelper[Value, End, Target]
type Z4 = Add[Cons[Unit, Cons[Unit, End]], Cons[Unit, Cons[Unit, End]]]