Scala ~> (tilde greater than) operator

This is also used in Akka streams as the edge operator.

~> 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]
}