找到单位：必填[SomethingElse]

Question

我有以下scala函数，我不会编译，因为它发现Unit甚至很难它需要[SomethingElse]

def combine(trees: List[CodeTree]): List[CodeTree] = {

  if(trees.length < 2) trees
  else isortForTrees(trees.+:(new Fork(trees.head, trees.tail.head, chars(trees.head).:::(chars(trees.tail.head)), weight(trees.head) + weight(trees.tail.head)))) 

  def isortForTrees(myList: List[CodeTree]): List[CodeTree] = {
    if(myList.isEmpty) Nil
    else insertForTrees(myList.head, isortForTrees(myList.tail))
  }

  def insertForTrees(toBeInserted: CodeTree, lisToBe: List[CodeTree]): List[CodeTree] = {
    if(lisToBe.isEmpty || weight(toBeInserted) < weight(lisToBe.head)) toBeInserted :: lisToBe
    else lisToBe.head :: insertForTrees(toBeInserted, lisToBe.tail)
  }

}

我无法理解为什么要归还一个单位？ if语句的两端都返回一个List [CodeTree]。这可能是我犯的一个愚蠢的错误，但我根本找不到它。我在这里缺少什么？

Answer 1

方法中的最后一个表达式是返回Unit的方法定义。只需移动

if(trees.length < 2) trees
else isortForTrees(trees.+:(new Fork(trees.head, trees.tail.head, chars(trees.head).:::(chars(trees.tail.head)), weight(trees.head) + weight(trees.tail.head))))

到方法的底部，它应该可以工作。