Abstract parametric types in Scala

Question

I have:

trait A[B[_]]

and I have:

trait Base {
    type AImpl <: ???
    def foo: AImpl
}

so that the classes extending Base would define A and implement methods and fields. ie:

trait BB[T]
trait AA extends A[BB]

trait Child extends Base {
  override type AImpl = AA
}

My problem is with declaring the abstract type AImpl. I tried:

type AImpl <: A[B[_]] forSome {type B[_]}

but I am getting the following compile error:

B[_] takes no type parameters, expected: one
type AImpl <: A[B[_],_] forSome {type B[_]}

What is the right way of declaring this abstract type?

this can hardly be what you want. to be specific, the chances are you also want to make AImpl higher kinded. — Jason Hu
– Jason Hu, Commented Feb 21, 2018 at 17:18

Jason Hu · Accepted Answer · 2018-02-21 19:35:32Z

1

This code does what you are asking but I don't think you mean to do that:

trait A[B[_]]

trait Base {
  type AImpl <: A[B] forSome { type B[_] }
  def foo: AImpl
}

if you can be more specific I can probably pinpoint your problem and suggest another way of doing so.

As I previously insisted, there are many problems in your intention. It's very clear to me that you are trying to do something you are not supposed to.

To explain a couple issue, you first need to understand existential types. Here, you are quantifying AImpl with certain constraint:

type AImpl <: A[B] forSome { type B[_] }

This asks its realization to comply to such type. However, this type cannot possibly have any realization, because of

B is hidden inside of existential type, therefore it's unknown outside;
A is invariant, so it forces realization to be subtype of A[B] for that exact hidden B.

these two together forbids AImpl from realizing. The way to fix it is to turn A covariant:

trait A[+B[_]]

trait Base {
  type AImpl <: A[B] forSome { type B[_] }
  def foo: AImpl
}

trait BB[T]
trait AA extends A[BB]

object Child extends Base {
  override type AImpl = A[BB]

  def foo = ???
}

this code compiles with no problem.

However, again, I have to say existentially quantifying B is a fundamentally flawed idea, because in the given environment, there is no type safe way to recover B anymore, if you ever need it, not to mention higher kinded types in scala is insufficient and malformed.

edited Feb 21, 2018 at 19:35

answered Feb 21, 2018 at 17:21

Jason Hu

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8 Comments

Maths noob Over a year ago

thanks. your solution almost works but it still gives me errors. I have updated my expected usage. Basically, I want to have "type families".

Jason Hu Over a year ago

@Mathsnoob this sample code compiled in my IDE. the chances are you need to turn on higher kinded types in compiler option. anyways, let me check what you want.

Jason Hu Over a year ago

why you can't drop A[B[_],_]? it's clear that A here is of kind (*, *) -> *, which cannot possibly be correct. it's still unclear what you want from this aspect. also, existential type in scala is fundamentally broken, and in most occasion, it's not what you want. so what problem are you trying to solve?

Maths noob Over a year ago

sorry for the confusion. the above is a simplification of my code and I forgot to also change the error. You're right. the original error was: A[B[_]] forSome {type B[_]}. The error I am getting on your code is:

Error:(11, 18) overriding type AImpl in trait Base with bounds <: A$A32.this.A[_[_] <: Any];  type AImpl has incompatible type   override type AImpl = AA                 ^

here is a gist: gist.github.com/ShahOdin/4bde0c7844589fa7dfaf6a255dfd9a20

Jason Hu Over a year ago

@Mathsnoob i've addressed that error from my update.

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