Scala composition of generic types

Question

I have a complex use case of generics type which has been simplified below

trait A
class AB extends A{
  val v = 10
}

trait X[T<:A]{
  def request: T
}

class XY extends X[AB]{
  def request = new AB()
}
class Test extends App{

  /**
    * X[A]
    * X[AB]
    * XY[A]
    * XY[AB]
    */
  def test[C<:A, D <: X[C]](t:Int)(input: D): Unit ={
    print(input.getClass.getName)
  }
  implicit val req = new XY()
  test(2)(req)

}

test method should support the Type scenarios defined in comment section. I'm getting the below compilation error.

Error:(33, 7) inferred type arguments [XY] do not conform to method test's type parameter bounds [D <: X[Nothing]] test(2)(req)

Is this syntactically legal? Thanks in advance.

Unless it's a part of simplification, if `D` is used in signature only once it can be removed: `test[C<:A](t:Int)(input: X[C])`. — Alexey Romanov, Jun 27 '19 at 16:27

Dmytro Mitin · Answer 1 · 2019-06-27T12:27:39.530

2

Nothing in compile error usually means that some type wasn't inferred.

Try to specify type parameters explicitly

test[AB, XY](2)(req)

Generic nested type inference works with arity-2 but not with currying

edited Jun 27 '19 at 12:27

answered Jun 27 '19 at 12:20

Dmytro Mitin

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score 2 · Accepted Answer · answered Jun 27 '19 at 12:48

The compiler can't infer the type of C in 2 steps with a definition like this.

So either have the compiler do it in 1 step, by having both D and C in the definition of the input argument:

def test[C <: A, D <: X[C]](t: Int)(input: D with X[C]): Unit

Or have an implicit evidence for D <: X[C], that will help the compiler to infer C in 2 steps:

def test[C <: A, D <: X[_]](t: Int)(input: D)(implicit ev: D <:< X[C]): Unit

Scala composition of generic types

2 Answers2