Question

I have the following scala class definition (found in a paper), modeling categories:

trait Category[~>[_, _]] {
    def compose[A, B, C]
     (f: B ~> C)
     (g: A ~> B)
     : A ~> C
    def id[A]: A ~> A
}

can someone explain me what the ‘~>’ means in the Category type parameter, and in the methods return type?
Or direct me to a resource that explains it…
I’m new to Scala (coming from Java), so forgive me if that’s something a scala user should have known…
Thank you in advance

Answer 1

~> is just the placeholder-name for the type-parameter of Category. Like the T in class Option[T].

Additionally, Scala syntax allows you to write B ~> C as a shorthand for ~>[B, C].

Maybe things get clearer, if you rename it:

trait Category[Mapping[_, _]] {
  def compose[A, B, C](f: Mapping[B, C])(g: Mapping[A, B]): Mapping[A, C]
  def id[A]: Mapping[A, A]
}