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How to call a constructor for tree leaves of algebraic data type in Scala?

I am creating some basic abstract data types and algorithms to refresh my CS fundamentals and learn Scala along the way. I'm having problems with my BinarySearchTree data type, which is an implementation of the more abstract BinaryTree:

abstract class BinaryTree[T](stored_value: T) { 
  var contents = stored_value
  var l: this.type = _
  var r: this.type = _
  ...
}

class BinarySearchTree[T <: Ordered[T]](stored_value: T) extends BinaryTree(stored_value) {  
  def insert(newval: T) {
    if (newval <= contents) {
      if (l == null) {
        throw new NotDefinedError("Still trying to work around type erasure.")
      } else {
        l.insert(newval)
      }
    } else {
      if (r == null) {
        throw new NotDefinedError("Still trying to work around type erasure.")
      } else {
        r.insert(newval)
      }
    }
  }

In the blocks that throw NotDefinedErrors, I tried several approaches, such as l = new this.type(newval)replacing this.typewith BinarySearchTree[T]and everything I can think of. Depending on how I try to express this type, I either get something like:

class type required but BinarySearchTree.this.type found

or

type mismatch;  found   : trees.BinarySearchTree[T]  required: BinarySearchTree.this.type

Do I need to override l and r with a different type in the BinarySearchTree definition? Or just calling another type of constructor when I bind a new value to them? Or some other option?

+3
2

@dhg, . , , , ...

, this.type BinaryTree[T] , , . , .. , this . , ,

scala> class Foo { def self : this.type = this /* OK */ }
defined class Foo

scala> class Bar { def self : this.type = new Bar /* Does not compile */ }
<console>:7: error: type mismatch;
 found   : Bar
 required: Bar.this.type
       class Bar { def self : this.type = new Bar /* Does not compile */ }
                                          ^

, , , , .

, l r BinaryTree[T], @dhg. , , , this.type, , BinarySearchTree [T] . , Java , , ,

abstract class BinaryTree[T] {
  type Self <: BinaryTree[T]
  var contents : T
  var l: Self = _
  var r: Self = _
}

class BinarySearchTree[T <: Ordered[T]](stored_value : T) extends BinaryTree[T] {
  type Self = BinarySearchTree[T]
    var contents = stored_value
    def insert(newval: T) {
      if (newval <= contents) {
        if (l == null) {
          new BinarySearchTree(newval)
        } else {
          l.insert(newval)
        }
      } else {
        if (r == null) {
          new BinarySearchTree(newval)
        } else {
          r.insert(newval)
        }
      }
  }
}
+7

:

class BinaryTree[T](
  val item: T,
  val left: Option[BinaryTree[T]] = None,
  val right: Option[BinaryTree[T]] = None) {

  override def toString() = "Tree(%s,%s,%s)".format(item,left,right)
}

class BinarySearchTree[T: Ordering](
  override val item: T,
  override val left: Option[BinarySearchTree[T]] = None,
  override val right: Option[BinarySearchTree[T]] = None) extends BinaryTree[T](item, left, right) {

  def insert(newval: T): BinarySearchTree[T] = {
    val (newLeft, newRight) =
      if (implicitly[Ordering[T]].lteq(newval, item))
        (insertSubtree(newval, left), right)
      else
        (left, insertSubtree(newval, right))
    new BinarySearchTree(item, newLeft, newRight)
  }

  private def insertSubtree(newval: T, subtree: Option[BinarySearchTree[T]]) =
    Option(subtree
      .map(_.insert(newval))
      .getOrElse(new BinarySearchTree(newval, None, None)))
}

, , Scala -like:

  • , val. insert , . () , .
  • null Option. , left right Option[Binary(Search)Tree[T]], , .
  • map , Option. insertSubtree : " , . , ".

:

scala> var t = new BinarySearchTree(5, None, None)
t: BinarySearchTree[Int] = Tree(5,None,None)

scala> t = t.insert(3)
t: BinarySearchTree[Int] = Tree(5,Some(Tree(3,None,None)),None)

scala> t = t.insert(4)
t: BinarySearchTree[Int] = Tree(5,Some(Tree(3,None,Some(Tree(4,None,None)))),None)

scala> t = t.insert(7)
t: BinarySearchTree[Int] = Tree(5,Some(Tree(3,None,Some(Tree(4,None,None)))),Some(Tree(7,None,None)))
+6

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