Constraining an operation by matching a type parameter to an argument's path-dependent type

Question

I would like to exploit Scala\'s type system to constrain operations in a system where there are versioned references to some values. This is all happening in some transacti

Answer 1

FYI, and to close the question, here is another idea that I like because the client code is fairly clutter free:

trait System[A <: Access[_]] {
  def in[T](v: Version)(fun: A => T): T
}

trait Access[Repr] {
  def version: Version
  def meld[R[_]](v: Version)(fun: Repr => Ref[_, R]): R[this.type]
}

trait Version

trait Ref[A, Repr[_]] {
  def sub[B](b: B): Repr[B]
}

object MyRef {
  def apply[A <: MyAccess](implicit a: A): MyRef[A] = new Impl[A](a)

  private class Impl[A](a: A) extends MyRef[A] {
    def sub[B](b: B) = new Impl[B](b)
    def schnuppi(implicit ev: A <:< MyAccess) = a.gagaism
  }
}
trait MyRef[A] extends Ref[A, MyRef] {
  // this is how we get MyAccess specific functionality
  // in here without getting trapped in more type parameters
  // in all the traits
  def schnuppi(implicit ev: A <:< MyAccess): Int
}

trait MyAccess extends Access[MyAccess] {
  var head: MyRef[this.type]
  var tail: MyRef[this.type]
  def gagaism: Int
}

def test(sys: System[MyAccess], v0: Version, v1: Version): Unit = {
  val v2 = sys.in(v0) { a => a.tail = a.meld(v1)(_.head); a.version }
  val a3 = sys.in(v2) { a => a }
  val (v4, a4) = sys.in(v1) { a =>
    a.head = a.head
    println(a.head.schnuppi) // yes!
    (a.version, a)
  }
  // a3.head = a4.head // forbidden
}

Answer 2

The following seems to work:

trait Version

trait Ctx[+V1 <: Version] {
  type V = V1
}

type AnyCtx   = Ctx[_ <: Version]
type AnyRf[T] = Ref[_ <: Version, T]

object Ref {
  implicit def access[C <: AnyCtx, R, T](r: R)(
    implicit c: C, view: R => Ref[C#V, T]): T = view(r).access(c)

  implicit def substitute[C <: AnyCtx, T](r: AnyRf[T])(implicit c: C): Ref[C#V, T] =
    r.substitute( c )
}
trait Ref[V1 <: Version, T] {
  def access(implicit c: Ctx[V1]): T
  def substitute[C <: AnyCtx](implicit c: C): Ref[C#V, T]
}

trait Factory {
  def makeVar[C <: AnyCtx, T](init: T)(implicit c: C): Ref[C#V, T]
}

// def shouldCompile1(r: AnyRf[String])(implicit c: AnyCtx): String = r

def shouldCompile2(r: AnyRf[String])(implicit c: AnyCtx): String = {
  val r1 = Ref.substitute(r)
  r1.access(c)
}

// def shouldFail(r: AnyRf[String])(implicit c: AnyCtx): String = r.access(c)

So the follow-up questions are

1. why I need a redundancy of the type parameter for Ctx to achieve this. I hate that these type parameters accumulate like rabbits in my code.
2. why shouldCompile1 doesn't compile —can i get the implicits to work as planned?

EDIT:

This is wrong, too. The variance annotation is wrong. Because now the following compiles although it shouldn't:

def versionStep(c: AnyCtx): AnyCtx = c // no importa

def shouldFail3[C <: AnyCtx](f: Factory, c: C): String = {
  val r    = f.makeVar("Hallo")(c)
  val c2   = versionStep(c)
  r.access(c2)
}