gistfile1.scala

// *** What we want to do ***
// Write a mini-cake that would take and return path-dependent universe artifacts
// retaining precise path-dependent types when returning values

trait Universe {
  type Tree
}

object ru extends Universe {
  class Tree
}

// Attempt #1: Use refinements

object Helper1 {
  def apply(u0: Universe): Helper1 { val u: u0.type } = new { val u: u0.type = u0 } with Helper1
}

trait Helper1 {
  val u: Universe
  def foo: u.Tree = ???
}

object Test1 {
  val foo1 = Helper1(ru).foo // type of foo1: _1.u.Tree forSome { val _1: Helper1{val u: ru.type} }
}

// Approach #2: Use a singleton type parameter

object Helper2 {
  def apply[U <: Universe with Singleton](u0: U): Helper2[U] = new { val u = u0 } with Helper2[U]
}

trait Helper2[U <: Universe with Singleton] {
  val u: U
  def foo: u.Tree = ???
}

object Test2 {
  val foo2 = Helper2(ru).foo // type of foo2: _2.u.Tree forSome { val _2: Helper2[ru.type] }
}

// Approach #3: Use a singleton type parameter and a type projection

object Helper3 {
  def apply[U <: Universe with Singleton](u0: U): Helper3[U] = new { val u = u0 } with Helper3[U]
}

trait Helper3[U <: Universe with Singleton] {
  val u: U
  def foo: U#Tree = ???
}

object Test3 {
  val foo3 = Helper3(ru).foo // type of foo3: ru.Tree
}

Please reply at https://groups.google.com/forum/#!topic/scala-internals/kMpBWLkPz38. I'm not getting notifications about comments to gists, so I will most likely overlook them.