S99_P26.scala

object S99_P26 {

  def combinations[T](count: Int, ts: Seq[T]): Seq[Seq[T]] = {
    def addHeadToSeqs[T](tss: Seq[Seq[T]], elt: T): Seq[Seq[T]] =
      tss.map(ts => elt +: ts)

    ts match {
      case h +: tail if count == 1 =>
        ts.flatMap(e => Seq(Seq(e)))

      case h +: tail =>
        combinations(count, tail) ++ addHeadToSeqs(combinations(count - 1, tail), h)
      
      case _ => Nil
    }
  }
}

Thank you for nice solution, I've made a simplified version.

object S99_P26 {
def combinations[T](count: Int, ts: Seq[T]): Seq[Seq[T]] = {
    if (count == 1)
        ts.map(Seq(_))

    else ts match {
        case h +: tail => combinations(count - 1, tail).map(h +: _) ++ combinations(count, tail)
        case _ => Nil
        }
    }
}