Confusion about type refinement syntax

Question

On Type Level, i stumble upon the following:

sealed abstract class StSource[A] {
  type S
  def init: S            // create the initial state
  def emit(s: S): (         


        

Answer 1

StSource[A] {type S = S0} is a refined type. {type S = S0} is a type refinement.

From one side, StSource[A] {type S = S0} is a subtype of StSource[A].

From the other side, StSource[A] is also an existential type with respect to StSource[A] {type S = S0}, namely StSource[A] is StSource.Aux[A, _] (aka StSource.Aux[A, X] forSome {type X}).

def test[A, S] = {
  implicitly[StSource.Aux[A, S] <:< StSource[A]]
  implicitly[StSource.Aux[A, _] =:= StSource[A]]
  implicitly[StSource[A] =:= StSource.Aux[A, _]]
}

https://scala-lang.org/files/archive/spec/2.13/03-types.html#compound-types

A compound type