Attempting to model F-bounded polymorphism as a type member in Scala

Question

I wanted to try writing a type whose methods can be homogeneous and return values of the same type:

object SimpleTest {
  trait Foo extends Product with Serializable {
    type Self <: Foo
    def bar: Self
  }

  case class X() extends Foo {
    type Self = X
    def bar = this
  }

  case class Y() extends Foo {
    type Self = Y
    def bar = this
  }


  trait TC[A]

  implicit val tc: TC[Foo] = new TC[Foo] { }

  def tester[A: TC](x: Seq[A]) = "foo"

  // tester(Seq(X(), Y()))
}

Unfortunately, the commented-out line calling tester fails with the following error (Scala 2.10):

Error: could not find implicit value for evidence parameter of type
SimpleTest.TC[SimpleTest.Foo{type Self >: SimpleTest.Y with SimpleTest.X <: SimpleTest.Foo}]
tester(Seq(X(), Y()))
      ^

Basically, I'm confused as to why X and Y don't unify to Foo, which seems like a clear LUB for the two of them. Clearly the type member is complicating matters but its bounds appear to be respected.

At the higher level, I'm looking for a lightweight way to get the equivalent of F-bounded polymorphism without the overhead of pervasive type parameters. This mostly seems to work, but I need to add annotations that force X and Y to unify to Foo.

Answer 1

I think this is an example of what you are looking for:

sealed trait Event { self =>
  type E >: self.type <: Event
  def instance: E = self
}

case class UserJoined() extends Event {
  type E = UserJoined
}

case class UserLeft() extends Event {
  type E = UserLeft
}

If you would like to read more, this snippet is from a recent post that covers related concepts.

Edit: To complete the answer, it would be:

scala> trait Foo extends Product with Serializable with Event{}
defined trait Foo

scala> case class X() extends Foo {
     |     type Self = X
     |     def bar = this
     |   }
defined class X

scala> case class Y() extends Foo {
     |     type Self = Y
     |     def bar = this
     |   }
defined class Y

scala> List(X(),Y())
res9: List[Foo] = List(X(), Y())

scala>   def tester[A: TC](x: Seq[A]) = "foo"
tester: [A](x: Seq[A])(implicit evidence$1: TC[A])String

scala>  tester(Seq(X(), Y()))
res10: String = foo