scala – 找不到有序类型的证据参数的隐含值[T]

以下不起作用(大概)但仅仅编译.代码审查的其他事项将是额外的括号和类似的东西.

package evident

object Test extends App {

  def partition[T](pred: (T)=>Boolean,pair._2))
  }

  def bestN[U,V<%Ordered[V]](list:List[(U,V)],n:Int): List[(U,V)] = {
    list match {
      case pivot::other => {
        println(s"pivot: $pivot and rest ${other mkString ","}")
        def cmp(a: (U,V),b: (U,V)) = (a: OrderedPair[U,V]) < (b: OrderedPair[U,V])
        val (smaller,bigger) = partition(((x:(U,V)) => cmp(x,pivot)),list)
        //val (smaller,bigger) = list partition ((x:(U,pivot))
        println(s"smaller: ${smaller mkString ","} and bigger ${bigger mkString ","}")
        val s = smaller.size
        if (s == n) smaller
        else if (s+1 == n) pivot::smaller
        else if (s < n) bestN(bigger,n-s-1)
        else bestN(smaller,n)
      }
      case Nil => Nil
    }
  }

  implicit class OrderedPair[T,V <% Ordered[V]](tv: (T,V)) extends Pair(tv._1,tv._2) with Ordered[OrderedPair[T,V]] {
    override def compare(that:OrderedPair[T,V]) : Int = this._2.compare(that._2)
  }

  val z = List(Pair("alfred",3))
  println(bestN(z,3))
}

我发现分区功能难以阅读;你需要一个功能来分割所有的parens.这里有几个公式,它们也使用过滤器接受的结果左边的约定,拒绝正确.

def partition[T](p: T => Boolean,list: List[T]) = 
  ((List.empty[T],List.empty[T]) /: list) { (s,t) =>
    if (p(t)) (t :: s._1,s._2) else (s._1,t :: s._2)
  }
def partition2[T](p: T => Boolean,list: List[T]) =
  ((List.empty[T],List.empty[T]) /: list) {
    case ((is,not),t) if p(t) => (t :: is,not)
    case ((is,t)         => (is,t :: not)
  }
// like List.partition
def partition3[T](p: T => Boolean,list: List[T]) = {
  import collection.mutable.ListBuffer
  val is,not = new ListBuffer[T]
  for (t <- list) (if (p(t)) is else not) += t
  (is.toList,not.toList)
}

这可能更接近原始代码的意图：

def bestN[U,V <% Ordered[V]](list: List[(U,n: Int): List[(U,V)] = {
  require(n >= 0)
  require(n <= list.length)
  if (n == 0) Nil
  else if (n == list.length) list
  else list match {
    case pivot :: other =>
      println(s"pivot: $pivot and rest ${other mkString ","}")
      def cmp(x: (U,V)) = x._2 < pivot._2
      val (smaller,bigger) = partition(cmp,other)     // other partition cmp
      println(s"smaller: ${smaller mkString ","}")
      val s = smaller.size
      if (s == n) smaller
      else if (s == 0) pivot :: bestN(bigger,n - 1)
      else if (s < n) smaller ::: bestN(pivot :: bigger,n - s)
      else bestN(smaller,n)
    case Nil => Nil
  }
}

箭头符号更常见：

val z = List(
    "alfred" -> 1,"peter" -> 4,"Xaver" -> 1,"Ulf" -> 2,"Alfons" -> 6,"Gulliver" -> 3
  )