Scala 3 - type parameters and type members

Language Design
1

There was mention in the past about some form of unification between type parameters and type members in Scala 3, as a means to simplify the language. I have not seen any recent mentions about this effort, nor does it seem featured in the Scala 3 roadmap.

I’m just wondering if this is still an ongoing research or whether it is something that was abandoned or turned out to be impractical, unsound or otherwise undesirable.

Thank you.

(Sorry, I looked for a reference to this effort but came up short with Google; I believe this was previously covered in one of Martin’s talks about Scala 3 / Dotty.)

4 Likes
2

Hello, type parameters are indeed encoded as abstract type members (with name mangling), you can read more in this paper:

http://guillaume.martres.me/publications/dotty-hk.pdf,

Although this may be out of date again

3

We went back having type parameters as first class feature. We learned a lot trying to do the encoding but in the end it was too brittle and too hard to make 100% backwards compatible, so we abandoned the effort.

10 Likes
4

There are still 2 problems though:

  1. The old design doc is still on the latest language reference (Higher-Kinded Types in Dotty), and ranked pretty high on Google search.

  2. Does the old design proposed a way to encode covariant/contravariant modifiers into type members? I was trying to manually convert it using type members:

before:

  object AsArg {

    trait P[+TT] {
      val vv: TT
    }

    trait P1 extends P[Product] {
      val vv: Product
    }
    trait P2 extends P1 with P[Tuple1[Int]] {
      val vv: Tuple1[Int]
    }
  }

after:

  object AsDependentType {

    trait P {
      type TT

      val vv: TT
    }

    trait P1 extends P {
      type TT <: Product

      val vv: Product
    }

    trait P2 extends P1 with P {
      type TT <: Tuple1[Int]

      val vv: Tuple1[Int]
    }
  }

But it never works and apparently is incompatible with the new type logic.

(main discussion: scala - In Scala3, if generic type argument(s) is mapped to dependent type, how are covariant & contravariant modifiers mapped? - Stack Overflow)

5

In case somebody would find it helpful, here are slides that explain the history behind encoding type parameters as type members and why it hasn’t worked out in the end:

(sadly it seems like we don’t have recording from the talk)

7 Likes
6

Yeah its very helpful, never thought it would be tied to the predicativeness of the language.