scala:实现一个通用的递归 max 函数

Question

我正在尝试将此 haskell max 函数实现移植到 Scala

maximum' :: (Ord a) => [a] -> a  
maximum' [] = error "maximum of empty list"  
maximum' [x] = x  
maximum' (x:xs) = max x (maximum' xs) 

这是我的第一次尝试:

def max[T <: Ordered[T]](list: List[T]): T = list match {

  case Nil => throw new Error("maximum of empty list")
  case head :: Nil => head
  case list => {
    val maxTail = max(list.tail)
    if (list.head > maxTail) list.head else maxTail
  }
}


max(List[Int](3,4))

但我收到以下错误:

inferred type arguments [Int] do not conform to method max's type parameter bounds [T <: Ordered[T]]

我尝试订购，可比较等，结果类似......

知道缺少什么吗？

Answer 1

也许你想要 Ordering类型类？

def max[T: Ordering](list: List[T]): T = list match {
  case Nil => throw new RuntimeException("maximum of empty list")
  case head :: Nil => head
  case list =>
    val maxTail = max(list.tail)
    if (implicitly[Ordering[T]].gt(list.head, maxTail)) list.head else maxTail
}

这毕竟是如何内置 max方法有效:

// From GenTraversableOnce
def max[A1 >: A](implicit ord: Ordering[A1]): A

如果你这样做，你可以清理很多东西:

def max[T](list: List[T])(implicit ord: Ordering[T]): T = list match {
  case Nil => throw new RuntimeException("maximum of empty list")
  case head :: Nil => head
  case head :: tail => ord.max(head, max(tail))
}

或者，您可以将其设为尾递归以提高效率(因为编译器会对其进行优化):

def max[T](list: List[T])(implicit ord: Ordering[T]): T = {
  if (list.isEmpty)
    throw new RuntimeException("maximum of empty list")

  @tailrec
  def inner(list: List[T], currMax: T): T =
    list match {
      case Nil => currMax
      case head :: tail => inner(tail, ord.max(head, currMax))
    }
  inner(list.tail, list.head)
}

另外，你应该扔 RuntimeException或者它的一个子类，而不是 Error .