Question

Where can I find the reference in Lang specification or any blog how to read these constructions:

trait GenericFunctor[->>[_, _ ], ->>>[_, _ ], F[_]] { 
  def fmap[A, B](f: A ->> B): F[A] ->>> F[B]
}

or

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

if it’s a Type name – then the code F[A] ->>> F[B] shouldn’t be compilable, but it is

Answer 1

trait GenericFunctor[->>[_, _], ->>>[_, _], F[_]] { 
  def fmap[A, B](f: A ->> B): F[A] ->>> F[B]
}

Scala allows symbols in identifiers. Let’s replace the symbolic identifiers with alphanumeric ones.

trait GenericFunctor[M[_, _], N[_, _], F[_]] { 
  def fmap[A, B](f: A M B): F[A] N F[B]
}

Scala provides infix type notation for binary type constructors. In other words, A M B is same as M[A, B]. Let’s rewrite above code without that sugar.

trait GenericFunctor[M[_, _], N[_, _], F[_]] { 
  def fmap[A, B](f: M[A, B]): N[F[A], F[B]]
}

Similary, the Category definition in your question can be rewritten as:

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

Is it clear now?