¯\_(ツ)_/¯

gistfile1.scala

// this is really "category" but omg math words
trait Stackable[S[_, _]] {
  def compose[A, B, C](lhs: S[A, B], rhs: S[B, C]): S[A, C]
  def id[A]: S[A, A]
}

object Stackable {
  // for example ...
  implicit val function1stackable = new Stackable[Function1] {
    def compose[A, B, C](lhs: A => B, rhs: B => C): A => C = lhs andThen rhs
    def id[A]: A => A = identity
  }
}

class Stack[S[_, _], A, B](implicit stackable: Stackable[S]) {
  val impl = // a heterogeneous collection of S[_, _] elements, where the first elem is S[A, _] and the last is S[_, B]
  def build: S[A, B] = impl.foldLeft(stackable.id)(stackable.compose)
}