Unpacking tuple types in Scala

Question

I was just wondering, can I decompose a tuple type into its components' types in Scala?

I mean, something like this

trait Container {
  type Element
}

trait AssociativeContainer extends Container {
  type Element <: (Unit, Unit)
  def get(x : Element#First) : Element#Second
}

Answer 1

You can't unpack, per se, but maybe this achieves what you want:

  type First
  type Second
  type Element = (First, Second)
  def get(x: First): Second

Answer 2

I'm a bit later to this, but what about using pattern matching? Doesn't have quite the correct return type, and my syntax might be a bit off, but here goes:

def get[K](key: K): Iterable[Any] {
  for ((key, x) <- elements) yield x
}

Answer 3

This doesn't unpack the types, but it does constrain the types A and B when calling get.

trait Container {
  type Element
}

trait AssociativeContainer extends Container {
  type Element <: Tuple2[_, _]

  def get[A, B](x: A)(implicit ev: (A, B) =:= Element): B
}

This looks promising, but is cheating -- it doesn't work if Element is abstract.

def Unpack[T<:Tuple2[_, _]] = new {
  def apply[A, B](implicit ev: T <:< (A, B)) = new {
    type AA = A
    type BB = B
  }
}

trait AssociativeContainer {
  type Element = (Int, String)
  val unpacked = Unpack[Element].apply
  type A = unpacked.AA
  type B = unpacked.BB

  1: A
  "": B
  def get(x: A): B
}