stsourceTypeMember.scala

sealed abstract class StSource[A] {
  type S
  def init: S
  def emit(s: S): (A, S)
}

object StSource {
  type Aux[S0, A] = StSource[A] { type S = S0 }
  def apply[A, S0](i: S0)(f: S0 => (A, S0)): Aux[S0, A] =
    new StSource[A] {
      type S = S0
      def init = i
      def emit(s: S0): (A, S0) = f(s)
    }
}

def runStSource[A](ss: StSource[A])(s: ss.S): (A, ss.S) =
  ss.emit(s)

val stSource = StSource(100.0)(i => (i / 10, i))

runStSource(stSource)(stSource.init)