scala - 理解时类型不匹配 : getting "Product with Serializable"

Question

我正在编写一个函数，该函数将获取字符串中出现的字符列表 (List[(Char, Int)])，并生成该出现列表的所有子集。

所以，给定

List(('a', 2), ('b', 2))

它将产生

List(
  List(),
  List(('a', 1)),
  List(('a', 2)),
  List(('b', 1)),
  List(('a', 1), ('b', 1)),
  List(('a', 2), ('b', 1)),
  List(('b', 2)),
  List(('a', 1), ('b', 2)),
  List(('a', 2), ('b', 2))
)

我是这样实现的:

type Occurrences = List[(Char, Int)]

def combinations(occurrences: Occurrences): List[Occurrences] =
  if (occurrences.isEmpty) List(List())
  else for {
    (c, n) <- occurrences
    i <- n to 1 by -1
  } yield (c, i) :: combinations(occurrences.tail)

我收到此错误:

type mismatch;
 found   : List[List[Product with Serializable]]
 required: List[Occurrences]
    (which expands to)  List[List[(Char, Int)]]

请帮助我理解为什么会发生这种情况，以及如何解决它？

我尝试将其重写为 flatMap...，使用 Intellij 的“解释 Scala 代码”等。

Answer 1

实际上缺少括号:

type Occurrences = List[(Char, Int)]

  def combinations(occurrences: Occurrences): List[Occurrences] =
    if (occurrences.isEmpty) List(List())
    else (for {
      (c, n) <- occurrences
      i <- n to 1 by -1
    } yield (c, i)) :: combinations(occurrences.tail)

在您的原始理解代码中尝试生成 (c, i)::Combinations(occurrences.tail) ，它是 List 和 Any 里面的元素(Tuple/另一个List)。

更新:

实现所需魔法的正确方法:

type Occurrences = List[(Char, Int)]


  /**
    * Returns the list of all subsets of the occurrence list.
    *  This includes the occurrence itself, i.e. `List(('k', 1), ('o', 1))`
    *  is a subset of `List(('k', 1), ('o', 1))`.
    *  It also include the empty subset `List()`.
    *
    *  Example: the subsets of the occurrence list `List(('a', 2), ('b', 2))` are:
    *
    *    List(
    *      List(),
    *      List(('a', 1)),
    *      List(('a', 2)),
    *      List(('b', 1)),
    *      List(('a', 1), ('b', 1)),
    *      List(('a', 2), ('b', 1)),
    *      List(('b', 2)),
    *      List(('a', 1), ('b', 2)),
    *      List(('a', 2), ('b', 2))
    *    )
    *
    *  Note that the order of the occurrence list subsets does not matter -- the subsets
    *  in the example above could have been displayed in some other order.
    */
  def combinations(occurrences: Occurrences): List[Occurrences] =
    occurrences.foldRight(List[Occurrences](Nil)) {
      case ((ltr, cnt), acc) =>
        acc ::: (for {
          comb <- acc
          ltrNum <- 1 to cnt
        } yield (ltr, ltrNum) :: comb)
    }

向 this code 的作者致敬.