Creating List with elements are of type A and B consecutively

gistfile1.sbt

class MyA[A, B](val head: A, val tail: Option[MyA[B, A]]) {
     override def toString() =  head + tail.map( "," + _.toString).getOrElse("")
     def ::( x: B) = new MyA(x, Some(this))
}

object MyA {
  def apply[A,B](value: A) : MyA[A, B] = new MyA(value, None)
}
 
// examples:
 5 :: "help" :: 6 :: "kjl" :: 9 :: MyA("d")  // 5,help,6,kjl,9,d
6:: "six" :: 7 :: "seven" :: 8 :: "eight" :: MyA(9) // 6,six,7,seven,8,eight,9
6:: "six" :: 7 :: "seven" :: "eight" :: MyA(9) // does not compile