A rudimentary implementation of List collection in Scala.

MyList.scala

  describe("自前のListを定義する"){
    object test {
      abstract class MyList[+T] {
        def isEmpty: Boolean
        def head: T
        def tail: MyList[T]
        def ::[U>:T](item: U): MyList[U]
      }
      case class MyListImpl[T](val head: T, val tail: MyList[T]) extends MyList[T] {
        def ::[U>:T](item: U): MyList[U] = new MyListImpl(item, this)
        def isEmpty = false
      }
      case object MyNil extends MyList[Nothing] {
        def ::[U>:Nothing](item: U): MyList[U] = MyListImpl(item, MyNil)
        override def isEmpty = true
        def head: Nothing = throw new NoSuchElementException("no head in empty list")
        def tail: MyList[Nothing] = throw new NoSuchElementException("no tail in empty list")
      }
    }
    it("MyListを使う"){
      import test._
      val mylist =  "ABC" :: "XYZ" :: "123" :: MyNil
      mylist.head should equal("ABC")
      mylist.tail should equal("XYZ" :: "123" :: MyNil)

    }
  }