ShallowUnique.scala

package huset

sealed class CanMutate {
  type C
}

final class ShallowUnique[T >: Null <: AnyRef] private[huset] (private var instance: T) {
  final def borrow(body: ((x: T) => (CanMutate {type C = x.type } => T))): Unit = {
    val x = instance
    given perm as CanMutate {type C = x.type}
    instance = body(x)(perm)
  }
}

object Test {
  final case class Atom[T](private var instance: T) {
    self =>
    def get = instance
    def set(x: T)(using CanMutate {type C = self.type}): Unit = {
      instance = x
    }
  }

  def main(args: Array[String]): Unit = {
    val a = new Atom(1)
    val boxed: ShallowUnique[Atom[Int]] = new ShallowUnique[Atom[Int]](a)
    println(a)
    boxed.borrow { atom => { case (perm: CanMutate {type C = atom.type}) =>
      atom.set(2)(using perm)
      atom
    }}
    println(a)
  }
}