dropE.scala

def mdropFirstE[T0](xs: MList { type T = T0 })
  : MList { type T = T0 } =
  xs.uncons match {
    case None => MNil()
    case Some(ml) => ml.tail
  }

def pdropFirst[T](xs: PList[T]): PList[T] = xs match {
  case PNil() => PNil()
  case PCons(_, t) => t
}

def toMlist[T0](xs: PList[T0]): MList { type T = T0 } =
  xs match {
    case PNil() => MNil[T0]()
    case PCons(el, t) => MCons(el, toMlist(t))
  }

def toPlist[T0](xs: MList { type T = T0 }): PList[T0] =
  xs.uncons match {
    case None => PNil[T0]()
    case Some(cons) => PCons(cons.head, toPlist(cons.tail))
  }

def pdropFirstWithE[T](xs: PList[T]): PList[T] =
  toPlist[T](mdropFirstE(toMlist[T](xs)))

def mdropFirstEwithP(xs: MList): MList =
  toMlist(pdropFirst[xs.T](toPlist(xs)))

val plist = PCons(10, PCons(20, PNil()))
val mlist = MCons[Int](10, MCons(20, MNil()))

mdropFirstEwithP(mlist)
pdropFirstWithE(plist)