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scalagenericsscala-cats

What does the `*` mean in a generic type?

I was learning Cats library and found * as a generic type, like that:

implicit def catsDataSemigroupKForValidated[A](implicit A: Semigroup[A]): SemigroupK[Validated[A, *]] =
  new SemigroupK[Validated[A, *]] {
    def combineK[B](x: Validated[A, B], y: Validated[A, B]): Validated[A, B] = x match {
      case v @ Valid(_) => v
      case Invalid(ix) =>
        y match {
          case Invalid(iy)  => Invalid(A.combine(ix, iy))
          case v @ Valid(_) => v
        }
    }
  }

My guess is that the * is used, because the combineK method return Validated[A, B] so there is no need to specify generic type. Or it could be Any type (like Inteliij is suggested). I would be very glad for your explanations.

Solution

  • The cats code you are looking at is master branch, which is for Dotty (Scala 3). * is a type parameter placeholder in 3.0:

    https://dotty.epfl.ch/docs/reference/changed-features/wildcards.html

    Note that it is already deprecated in 3.2 and removed in 3.3 in favor for _.

    EDIT

    It's also used in kind projector plugin