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POWhy doesn't this code compile? Is this really unsafe?
 

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  1. POWhy doesn't this code compile? Is this really unsafe?
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    <p>Consider this simple binary tree type:</p> <pre><code>abstract class BinaryTree[T] { type Repr &lt;: BinaryTree[T] def left: Repr def value: T def right: Repr def height: Int def add(elem: T): Repr = if ( left.height &lt;= right.height ) copy( left.add( elem ), right) else copy( left, right.add(elem)) def copy( left: Repr, right: Repr ) : Repr } abstract class BinarySearchTree[T] extends BinaryTree[T] { type Repr &lt;: BinarySearchTree[T] def ordering: Ordering[T] override def add(elem: T) = if ( ordering.equiv( elem, value ) ) this.asInstanceOf[Repr] else if ( ordering.lt(elem, value) ) copy( left.add( elem ), right ) else copy( left, right.add( elem ) ) } class ScapegoatTree[T]( val value: T, val left: ScapegoatTree[T], val right: ScapegoatTree[T] )(implicit val ordering: Ordering[T]) extends BinarySearchTree[T] { type Repr = ScapegoatTree[T] def add = this /* snip */ def copy( left: ScapegoatTree[T], right: ScapegoatTree[T] ) = new ScapegoatTree( value, left, right ) } class BalancedBinaryTree[T]( val value: T, val left: BalancedBinaryTree[T], val right: BalancedBinaryTree[T] )(implicit val ordering: Ordering[T]) extends BinarySearchTree[T] { type Repr = BalancedBinaryTree[T] def add = this /* snip */ def copy( left: BalancedBinaryTree[T], right: BalancedBinaryTree[T] ) = new BalancedBinaryTree( value, left, right ) } </code></pre> <p>Using Scala 2.9 in Scala-IDE, the add method fails to compile because copy takes a <code>Repr</code> while <code>left.add(elem)</code> and <code>right.add(elem)</code> return a <code>Repr#Repr</code>. As far as I can tell, <code>Repr#Repr</code> must always be a subtype of <code>Repr</code>, and therefore it should be safe. Have I failed to consider some case, or is this one of those fortunately rare instances where the type system prevents you from doing something that ought to work?</p> <p>Also, is there any way I could change my type definition so this would be legal, or assert to the compiler that <code>Repr#Repr</code> must be a subtype of <code>Repr</code> (aside from a cast, I mean). I'm still getting used to Scala and I'm not sure I've fully comprehended the type system yet. I did get it to compile by writing an implicit method that does a cast, but that feels like more of a workaround than a solution.</p> <p>Thanks in advance for your help.</p> <p>Edit: I should add that in my actual implementation of this I have subclasses that further restrict the type of Repr, if that makes a difference.</p> <p>Edit 2: Added more code, as requested. This issue appears in the <code>BinarySearchTree</code> type as well, but not in concrete implementations such as <code>ScapegoatTree</code> where <code>Repr</code> is specified. There is also a concrete implementation of the basic <code>BinaryTree</code>, but I'm trying to keep it minimal so you don't have to wade through too much unnecessary code.</p>
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