在Scala中，為什么下限在這里不能正常工作？

Question

這些代碼可以很好地編譯：

  abstract class CodeTree

  case class Fork(left: CodeTree, right: CodeTree, chars: List[Char], weight: Int) extends CodeTree

  case class Leaf(char: Char, weight: Int) extends CodeTree


  def weight(tree: CodeTree): Int = tree match {
    case Fork(left, right, chars, weight) => weight
    case Leaf(char, weight) => weight
  }

  def insert_orderedCodeTreeList[T >: CodeTree](leaf: CodeTree, orderedCodeTrees: List[CodeTree]): List[T] = orderedCodeTrees match {
    case Nil => List(leaf)
    case head :: tail => {
      if (weight(leaf) <= weight(head)) leaf :: orderedCodeTrees
      else head :: insert_orderedCodeTreeList(leaf, tail)
    }
  }

但是當我這樣使用它時會拋出錯誤：

insert_orderedCodeTreeList[Leaf](Leaf(theChar, theInt), makeOrderedLeafList(tail))

type arguments [patmat.Huffman.Leaf] do not conform to method insert_orderedCodeTreeList's type parameter bounds [T >: patmat.Huffman.CodeTree]
[error]     case (theChar, theInt) :: tail => insert_orderedCodeTreeList[Leaf](Leaf(theChar, theInt), makeOrderedLeafList(tail))
[error]                                                                 ^
[error] one error found

可以看出，類型參數T使用CodeTree作為下限，因此，我認為可以在insert_orderedCodeTreeList的參數列表中用T替換insert_orderedCodeTreeList ：

  def insert_orderedCodeTreeList[T >: CodeTree](leaf: T, orderedCodeTrees: List[T]): List[T] = orderedCodeTrees match {
    case Nil => List(leaf)
    case head :: tail => {
      if (weight(leaf) <= weight(head)) leaf :: orderedCodeTrees
      else head :: insert_orderedCodeTreeList(leaf, tail)
    }
  }

但是，編譯器抱怨：

[error]  found   : T
[error]  required: patmat.Huffman.CodeTree
[error]       if (weight(leaf) <= weight(head)) leaf :: orderedCodeTrees
[error]                                  ^

有人對此有想法嗎？

Answer 1

閱讀有關邊界的文檔后，我發現正確的方法是這樣寫：

  def insert_orderedCodeTreeList[T <: CodeTree](leaf: T, orderedCodeTrees: List[T]): List[T] = orderedCodeTrees match {
    case Nil => List(leaf)
    case head :: tail => {
      if (weight(leaf) <= weight(head)) leaf :: orderedCodeTrees
      else head :: insert_orderedCodeTreeList(leaf, tail)
    }
  }