在 Scala 中，为什么下限在这里不能很好地工作？

Question

这些代码编译得很好：

  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)
    }
  }

但是我这样用会报错:

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，类型参数T使用CodeTree作为下界，因此，我认为将参数列表中的CodeTree替换为T可能就可以了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，编译器会抱怨：

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

有人对此有想法吗？

Answer 1

看完the document about bounds，我发现正确的写法是这样写的：

  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)
    }
  }

在 Scala 中，为什么下限在这里不能很好地工作？

In scala, why doesn't lower bound work well here?

generics

types

functional-programming

scala

type-inference